onevcs/

publish.rs

//! Publication: verifying a branch and landing it under its repository's policy.
//!
//! One path, two endings. `local-direct` builds the branch-to-base squash in a
//! **detached scratch worktree** and pushes that exact tree, so the registered
//! publication checkout is never worked in and only ever fast-forwarded. Every
//! `change-*` policy pushes the branch, opens (or adopts) a change request, and
//! then differs only in what it asks the host to do with it.
//!
//! Everything automated goes through the FIFO merge queue keyed by the publication
//! checkout's git common directory, which is the one thing every worktree and alias
//! of an identity shares.

use std::path::{Path, PathBuf};

use serde::{Deserialize, Serialize};
use serde_json::json;

use crate::error::{Error, Result};
use crate::event::{EventKind, Phase};
use url::Url;

use std::collections::BTreeSet;

use crate::host::{
    ChangeRequest, ChangeSpec, Check, CheckState, Hosting, MergeOutcome, Mergeability, RemoteHost,
    Sha,
};
use crate::releases::TargetName;
use crate::rules::{MergePolicy, Policy};
use crate::session::{Lifecycle, Provenance, SessionToken};
use crate::store::Resolution;
use crate::stream::{self, Stream};
use crate::workspace::{object, Ref};
use crate::{
    gh, git, guidance, home, ids, lock, merge_path, policy, provenance, queue, store, vcs,
    workspace,
};

/// How many times a base that moved under a publication is re-merged before the
/// attempt is abandoned. Bounded, because a base advancing on every retry is a
/// conflict that resolving again will not settle.
pub const SYNC_ATTEMPTS: usize = 3;

/// What one publication was asked for beyond the session it publishes.
///
/// The options `onevcs publish` takes, and nothing else: everything else about a
/// publication comes from the session and from the repository's rules.
#[derive(Debug, Clone, Default, PartialEq, Eq, Serialize, Deserialize)]
pub struct PublishRequest {
    /// A per-run policy. It may narrow the repository's but never widen it.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub policy: Option<MergePolicy>,
    /// An explicit title, which replaces the subject synthesized from the branch.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub title: Option<Subject>,
    /// The body the change request is opened with, verbatim.
    ///
    /// Absent opens it with no body at all. Nothing here composes one: every
    /// change request this crate opened used to carry the same three lines — the
    /// branch's own subject echoed back under `## What`, and `Published by
    /// onevcs.` under `## Why` — which told a reviewer nothing the title had not.
    /// Unvalidated, because a body is prose and a host places no shape on it; a
    /// title is a [`Subject`] for the opposite reason.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub body: Option<String>,
    /// Open the change request as a **draft**, and why it is not ready.
    ///
    /// Absent is a publication giving no reason, which takes the draft lifecycle: its
    /// change request opens as a draft while the required checks run and is lifted or
    /// kept for review on their verdict — or opens ready, where the rules file says
    /// `drafts: {disabled: true}`. Present is one of two things, and the reason
    /// says which: a change whose work is as far along as it can go while something
    /// outside this repository has not happened yet, or a change the session that
    /// opened it is still making. The reason travels with it, because a draft
    /// nobody can read the reason for is a change request nobody knows how to
    /// finish.
    ///
    /// A draft is unmergeable in that state, and this crate keeps it so: nothing
    /// merges it, arms the host's own merge on it, or advances a base from it while
    /// the draft stands, under any `drafts:` setting. A later publication of the same
    /// branch carrying **no** `DraftReason` adopts it as awaiting its checks and lifts
    /// it only on their verdict, or keeps it for review; `onevcs change ready` lifts it
    /// at once, for a session that wants its draft lifted without landing anything.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub draft: Option<DraftReason>,
}

/// Why a change request is a draft, in the shape a machine reads it.
///
/// A draft is a change request that must not land yet, and there are two reasons one
/// is held. A change that has gone as far as it can and stopped short of the one step
/// that would make something temporary permanent — a dependency pinned to a branch
/// rather than to a release — is [`AwaitingRelease`](DraftReason::AwaitingRelease),
/// and what it is waiting for is the whole of the reason: which repository, which of
/// that repository's release targets, and the reference the change is pinned to in
/// the meantime. A change whose work is *still being made* — evidence gathered, a
/// description started — is [`Held`](DraftReason::Held) by the session that opened
/// it, and the one line a person reads is the whole of that reason. The `kind` tag
/// is what a consumer routes on; `because` is the same fact as a sentence, for
/// whoever reads it rather than routes on it.
///
/// It is recorded on the session's own event stream — the publication record — and
/// nowhere else. **Nothing is written into the change request's body**, under a
/// marker heading or anywhere: a body is prose a reviewer reads and a drafting
/// caller may rewrite, and deciding a control action from it would turn an editorial
/// act into one. The cost of that is carried openly: somebody looking at the draft
/// change request without access to this host sees that it **is** a draft and not
/// why.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
#[serde(tag = "kind", rename_all = "kebab-case")]
pub enum DraftReason {
    /// A change pinned to a dependency's branch until that dependency releases —
    /// what a fast-adoption publication carries.
    AwaitingRelease {
        /// The repository identity whose release is awaited, as the registry keys one.
        ///
        /// A `String` rather than a newtype because an identity key is one everywhere
        /// this crate spells one — `Identity::origin`, `Recoverable::identity`, the
        /// registry document's own map key — so a narrower type here would disagree
        /// with every type it is compared against. What must not be representable is
        /// a value that renders as something *other* than itself in a refusal or an
        /// event payload, and `checked` refuses exactly that at every boundary one
        /// arrives at.
        // llmlint: ignore[invalid_states_unrepresentable] the doc above: the key's own type, checked.
        awaiting: String,
        /// Which release target of it is awaited.
        target: TargetName,
        /// The reference this change is pinned to until that release arrives — the
        /// branch of the awaited repository the pin names.
        ///
        /// A `String` for the reason `awaiting` above is, and one more of its own:
        /// this is the *awaited* repository's branch name rather than one this
        /// repository's git could be asked about, so the parser that would decide it
        /// is not this host's. `checked` refuses the values that would render as
        /// something else.
        // llmlint: ignore[invalid_states_unrepresentable] the doc above: another host's name, checked.
        reference: String,
        /// One line a person reads, saying why the change is not ready.
        ///
        /// Prose, so there is nothing for a type to narrow. The one shape that matters
        /// is where it lands — a single rendered line — and `checked` is what holds it
        /// to that.
        // llmlint: ignore[invalid_states_unrepresentable] the doc above: prose, checked where it lands.
        because: String,
    },
    /// A change request the session that opened it is holding open as a draft while
    /// its work is still being made: evidence gathered, description started. Lifted
    /// by `onevcs change ready`, or taken by a later publication of the same session
    /// carrying no reason into the draft lifecycle — which keeps it a draft when green
    /// on a `change-open` identity whose approvals are required.
    Held {
        /// One line a person reads, saying why the session is holding it. Prose,
        /// held to the one line it is printed on by `checked`, as the line above is.
        // llmlint: ignore[invalid_states_unrepresentable] the doc above: prose, checked where it lands.
        because: String,
    },
}

impl DraftReason {
    /// The rule a publication holds a reason to, or the refusal that this is not one.
    ///
    /// Every field here is printed: into a refusal, into an event payload a consumer
    /// reads back, and — through `--draft` — at a host. So the check is the one that
    /// decides whether a value renders as itself: nothing empty, and nothing carrying
    /// a control character, which is what turns one line of a record into two.
    ///
    /// Public for the reason [`MergePolicy::narrow`] and [`FailureKind::of`] are: it
    /// belongs to publication rather than to any one implementation of it, so a
    /// supplied [`Vcs`](crate::Vcs) applies *this* rule at its own boundary rather
    /// than a restatement of it that could accept what the real one refuses. It is a
    /// method rather than a conversion because the contract fixes the fields as
    /// public and settable, so there is no constructor for a check to live in.
    ///
    /// # Errors
    ///
    /// [`Error::Invalid`] naming the field that would not render as itself.
    pub fn checked(&self) -> Result<()> {
        let printed: Vec<(&str, &String)> = match self {
            DraftReason::AwaitingRelease {
                awaiting,
                reference,
                because,
                ..
            } => vec![
                ("the repository whose release is awaited", awaiting),
                ("the reference the change is pinned to", reference),
                ("the reason the change is not ready", because),
            ],
            DraftReason::Held { because } => {
                vec![("the reason the session is holding the change", because)]
            }
        };
        for (what, value) in printed {
            if value.is_empty() {
                return Err(crate::error::invalid(format!(
                    "a draft change request must say {what}, and this one names none: a draft \
                     whose reason cannot be read is a change request nobody knows how to finish"
                )));
            }
            if value.chars().any(char::is_control) {
                return Err(crate::error::invalid(format!(
                    "{what} is {value:?}, which carries a control character: every field of a \
                     draft's reason is printed on one line, in a refusal and in the publication \
                     record, and a value that renders as something other than itself is not one"
                )));
            }
        }
        Ok(())
    }

    /// The one line a person reads, whichever kind of reason this is.
    pub(crate) fn because(&self) -> &str {
        match self {
            DraftReason::AwaitingRelease { because, .. } | DraftReason::Held { because } => because,
        }
    }

    /// The word the `kind` tag travels as: `awaiting-release` or `held`.
    ///
    /// Read off the serialized form rather than restated, so a rendering cannot
    /// spell a kind the record does not.
    pub(crate) fn kind(&self) -> String {
        self.fields()["kind"]
            .as_str()
            .expect("a tagged enum serializes its kind as a string")
            .to_owned()
    }

    /// The reason as the fields an event payload and a refusal both name it by:
    /// the `kind` tag and every field of that kind, which is exactly the serialized
    /// form.
    fn fields(&self) -> serde_json::Value {
        serde_json::to_value(self).expect("a draft reason serializes: strings and a name")
    }

    /// The reason as a refusal spells what was asked for — "a draft awaiting … " or
    /// "a draft held by the session that opened it" — so the two refusals a draft
    /// can meet name the same thing the same way whichever kind of reason it was.
    pub(crate) fn asked_for(&self) -> String {
        match self {
            DraftReason::AwaitingRelease {
                awaiting, target, ..
            } => format!("a draft awaiting {awaiting} {target}"),
            DraftReason::Held { because } => {
                format!("a draft held by the session that opened it ({because})")
            }
        }
    }
}

/// A title that can be the subject of the commit a publication lands.
///
/// The check is in the conversion, for the reason every other validated name in
/// this crate has one there: a publication commits the session's work and merges
/// its base before it composes a message, so a title rejected where the message is
/// composed is rejected *after* those. A [`PublishRequest`] carrying an unusable
/// title is therefore unrepresentable rather than representable-and-refused-later
/// — and a caller embedding this crate meets the refusal where it built the
/// request, not after a commit it cannot undo.
#[derive(Clone, PartialEq, Eq, Serialize, Deserialize)]
#[serde(try_from = "String", into = "String")]
pub struct Subject(String);

/// The title itself, quoted — never the wrapper. A refusal that names one writes
/// it with `{:?}`, and a derived `Debug` would spell it `Subject("feat: …")` at
/// the operator rather than the title they typed.
impl std::fmt::Debug for Subject {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        std::fmt::Debug::fmt(&self.0, f)
    }
}

impl TryFrom<String> for Subject {
    type Error = String;

    fn try_from(value: String) -> std::result::Result<Self, String> {
        provenance::checked_subject(&value).map(Subject)
    }
}

/// So a title arriving on a command line is checked by the same conversion a title
/// arriving through the library is.
impl std::str::FromStr for Subject {
    type Err = String;

    fn from_str(value: &str) -> std::result::Result<Self, String> {
        Subject::try_from(value.to_owned())
    }
}

impl From<Subject> for String {
    fn from(subject: Subject) -> Self {
        subject.0
    }
}

impl std::ops::Deref for Subject {
    type Target = str;

    fn deref(&self) -> &str {
        &self.0
    }
}

impl std::fmt::Display for Subject {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.write_str(&self.0)
    }
}

/// What one publication did.
///
/// The value `onevcs publish` renders and a caller embedding this crate branches
/// on. Every ending is a case of [`PublishOutcome`] rather than a sentence to
/// match on, because the sentence is what a consumer read wrongly.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct Publication {
    /// The session that was published.
    pub session: SessionToken,
    /// The branch that carried the change.
    // llmlint: ignore[invalid_states_unrepresentable] this is the session's own branch,
    // and the contract declares that field verbatim as `pub branch: String` on `Session`;
    // spelling it as a validated ref here would disagree with the type the contract fixes
    // and add a public item it does not name. The validated `workspace::Ref` is what the
    // publication path carries internally, and every value written here is one — it is
    // `record.branch`, which git's own parser accepted before the session was opened.
    pub branch: String,
    /// The policy it was published under, after the repository's rules and any
    /// per-run narrowing have both had their say.
    pub policy: MergePolicy,
    /// What happened.
    pub outcome: PublishOutcome,
}

/// How a publication ended.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "kebab-case")]
pub enum PublishOutcome {
    /// The change reached its base, at this commit.
    Merged(Sha),
    /// A change request is open, which the policy asked for.
    ChangeOpen(Url),
    /// A change request is open **as a draft**, carrying the reason it is not ready.
    ///
    /// Its own case rather than a shade of [`ChangeOpen`](PublishOutcome::ChangeOpen),
    /// deliberately: the two differ in the one thing a caller acts on — whether this
    /// change can land — and folding them together would make every exhaustive match
    /// on this enum go on compiling while meaning something else.
    ChangeDraft(Url),
    /// A change request whose required checks came back green, kept **as a draft**
    /// for its own user's review: what the draft lifecycle ends in under `change-open`
    /// with approvals required.
    ///
    /// Its own case rather than a shade of [`ChangeDraft`](PublishOutcome::ChangeDraft)
    /// or [`ChangeOpen`](PublishOutcome::ChangeOpen), for the reason `ChangeDraft` is
    /// one: a consumer acts on the difference. This work is done — green, and waiting
    /// for the person who dispatched it — while a `ChangeDraft` is held back by a
    /// reason, and a `ChangeOpen` is already asking the team for review.
    ChangeReviewDraft(Url),
    /// The host queued the merge and will land it once its checks pass.
    Queued(Url),
    /// The branch had nothing the base did not already carry.
    NothingToPublish,
    /// The change did not land, and this is why.
    ///
    /// A case rather than an `Err`, in exactly the places the CLI reports a
    /// non-zero exit rather than a refusal — so the two surfaces cannot disagree
    /// about which failures are which.
    Failed {
        /// Which failure it is, which is what a caller branches on.
        kind: FailureKind,
        /// The failure as it reads, which is what the CLI writes to stderr.
        reason: String,
        /// What became of the branch. The work is the only record of itself, so
        /// whether it outlived the session is the first thing a caller asks.
        #[serde(default, skip_serializing_if = "Option::is_none")]
        retained: Option<Retention>,
    },
}

impl PublishOutcome {
    /// How the publication is reported to a human.
    #[must_use]
    pub fn describe(&self) -> String {
        match self {
            PublishOutcome::Merged(sha) => format!("merged at {}", sha.0),
            PublishOutcome::ChangeOpen(url) => format!("change request open at {url}"),
            PublishOutcome::ChangeDraft(url) => {
                format!(
                    "change request open as a draft at {url}, which cannot land while it is one"
                )
            }
            PublishOutcome::ChangeReviewDraft(url) => format!(
                "change request's checks are green at {url}, and it is kept as a draft for its \
                 user's review"
            ),
            PublishOutcome::Queued(url) => format!("merge queued for {url}"),
            PublishOutcome::NothingToPublish => {
                "nothing to publish: the base already carries this branch's content".to_owned()
            }
            PublishOutcome::Failed { reason, .. } => reason.clone(),
        }
    }
}

/// Which failure ended a publication, and therefore which exit code says so.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "kebab-case")]
pub enum FailureKind {
    /// A publication was refused by something that judged it, where no narrower
    /// kind says which — the repository's own `commit-msg` hook turning down the
    /// subject, or a host that took a merge and then reported it unperformed. What
    /// the *host's checks* reported is [`ChecksFailed`](FailureKind::ChecksFailed)
    /// or [`ChecksUnsettled`](FailureKind::ChecksUnsettled), what its `pre-push`
    /// hook said is [`PushRejected`](FailureKind::PushRejected), and a push that
    /// landed with the merge path unreadable behind it is
    /// [`PushedUnverified`](FailureKind::PushedUnverified).
    ///
    /// Named for the tier this crate used to run itself and kept under that name:
    /// the contract fixes this vocabulary across the three libraries that route on
    /// it, so a variant is not renamed because the tier behind it went away.
    // llmlint: ignore[names_match_behavior] the approved contract fixes the spelling.
    Gate,
    /// Input was rejected at a trust boundary.
    Invalid,
    /// The base moved under the publication and the bounded resolve-and-requeue
    /// did not converge.
    SyncConflict,
    /// The request was well-formed and the seam behind it has no implementation.
    NotImplemented,
    /// A required check the host reports concluded red. The reason names it.
    ChecksFailed,
    /// The bound on watching the host elapsed with the change still outstanding.
    /// The reason names what was: the required checks that had not settled, that
    /// the host declared none, or that it never performed the merge it took.
    // The vocabulary is fixed across the three libraries that route on it, and it
    // gives the bound one kind whichever of its endings this was; `reason` says which.
    // llmlint: ignore[names_match_behavior] one fixed kind for the bound's three endings.
    ChecksUnsettled,
    /// The publishing push was refused by the merge path. The reason carries git's
    /// own per-ref refusal.
    PushRejected,
    /// The publishing push **reached the remote**, and the merge path could not
    /// then be read. The reason names both: where the push landed, and what stopped
    /// the read.
    ///
    /// Widening the vocabulary rather than adding a clause to the three above is
    /// the decision recorded on [`Error::PushedUnverified`], and the reasoning is
    /// there: a router branches on the kind, so only a kind can stop work that is
    /// on the remote reading as work that never landed.
    PushedUnverified,
    /// The publishing push was refused by the merge path because this host is
    /// missing a tool or a credential a hook needs, which the hook said on the line
    /// [`HOST_PREREQUISITE_MARKER`] begins. The reason carries what it said is
    /// missing and how to install it, and the same evidence pointers
    /// [`PushRejected`](FailureKind::PushRejected) does.
    ///
    /// A kind rather than a clause on `PushRejected`, for the reason
    /// [`PushedUnverified`](FailureKind::PushedUnverified) is one: a router branches
    /// on the kind, and this is a refusal no change to the work can clear.
    HostPrerequisite,
    /// The caller cancelled the publication while it waited — on the host's checks,
    /// on a merge the host was armed to perform, or on the identity's merge queue —
    /// through [`publish_with_cancellation`](crate::publish_with_cancellation). The
    /// reason names what it was waiting on.
    ///
    /// Nothing is undone: the branch stays on its remote, any change request stays
    /// open, and the session is left so that publishing it again continues the same
    /// branch. A kind of its own, with an exit code of its own, because it is neither a
    /// verdict on the work nor a merge path that could not be read, and a router that
    /// read it as either would retry or abandon work nobody judged.
    Cancelled,
}

impl FailureKind {
    /// The exit code the contract fixes for this failure.
    ///
    /// The one statement of it: `onevcs publish` reports this, and a caller
    /// embedding the crate reads the same mapping rather than rediscovering it.
    #[must_use]
    pub fn exit_code(self) -> u8 {
        match self {
            // The contract fixes `1` for every verification failure and the codes are
            // the published surface, so which one it was travels on the kind instead.
            // Widening the set is an amendment, reported rather than taken here.
            // llmlint: ignore-block[cli_output_contract] the approved contract fixes this code.
            FailureKind::Gate
            | FailureKind::ChecksFailed
            | FailureKind::ChecksUnsettled
            | FailureKind::PushRejected
            | FailureKind::PushedUnverified
            | FailureKind::HostPrerequisite => 1,
            // llmlint: ignore-end[cli_output_contract]
            FailureKind::Invalid => 2,
            FailureKind::SyncConflict => 3,
            FailureKind::NotImplemented => 70,
            // `EX_TEMPFAIL`: nothing failed, and the same publication can be asked for
            // again. Distinct from every other kind's, so a caller reading `$?` alone
            // cannot take a cancellation for a verdict.
            FailureKind::Cancelled => 75,
        }
    }

    /// Which failure an error is.
    ///
    /// Public because a supplied implementation of [`Vcs`](crate::Vcs) has to
    /// report the same kind for the same failure — a publication that started and
    /// did not land is an outcome on every backend, and which one it is cannot be
    /// left to a restatement of this match.
    #[must_use]
    pub fn of(error: &Error) -> Self {
        match error {
            Error::GateFailed { .. } => FailureKind::Gate,
            Error::SyncConflict { .. } => FailureKind::SyncConflict,
            Error::NotImplemented { .. } => FailureKind::NotImplemented,
            Error::ChecksFailed { .. } => FailureKind::ChecksFailed,
            Error::ChecksUnsettled { .. } => FailureKind::ChecksUnsettled,
            Error::PushRejected { .. } => FailureKind::PushRejected,
            Error::PushedUnverified { .. } => FailureKind::PushedUnverified,
            Error::HostPrerequisite { .. } => FailureKind::HostPrerequisite,
            Error::Cancelled { .. } => FailureKind::Cancelled,
            _ => FailureKind::Invalid,
        }
    }
}

/// What became of a branch whose publication did not land.
///
/// The session's clone is disposable, so a branch that is not handed back goes
/// with it. Which of these happened decides whether there is anything left to
/// recover, and the CLI says so on stderr for the same reason.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "kebab-case")]
pub enum Retention {
    /// The branch was handed back to this checkout, which now carries the work.
    HandedBack(PathBuf),
    /// This checkout refused the branch, so nothing outside the session carries it.
    Refused(PathBuf),
}

/// Whether the caller of a publication has asked it to stop.
///
/// Handed to [`publish_with_cancellation`](crate::publish_with_cancellation) and asked,
/// at least once a second, by every phase of a publication that waits: the watch of the
/// host's checks, a draft's settle and the one after an early lift, the watch of a
/// merge the host was armed to perform, and the wait for the identity's merge queue.
/// A cancelled publication ends as [`FailureKind::Cancelled`] and undoes nothing.
///
/// A git command already running — the publishing push and its `pre-push` hook
/// included — is never interrupted: the answer is asked again once it returns.
pub trait PublicationCancellation {
    /// Whether the publication should stop at the next point it is asked.
    fn is_cancelled(&self) -> bool;
}

/// The cancellation nobody can trigger, which is every publication [`crate::publish`]
/// and the command line run.
pub(crate) struct NeverCancelled;

impl PublicationCancellation for NeverCancelled {
    fn is_cancelled(&self) -> bool {
        false
    }
}

/// The longest a waiting phase sleeps between two askings of its cancellation, so a
/// cancellation is observed within it whatever `ONEVCS_CHECKS_POLL_SECONDS` says.
pub(crate) const CANCELLATION_GRAIN: std::time::Duration = std::time::Duration::from_millis(100);

/// Sleep for `span`, asking `cancellation` at least every [`CANCELLATION_GRAIN`], and
/// answer `cancelled()` the first time it says so.
pub(crate) fn pause_unless_cancelled(
    span: std::time::Duration,
    cancellation: &dyn PublicationCancellation,
    cancelled: impl Fn() -> Error,
) -> Result<()> {
    let until = std::time::Instant::now() + span;
    loop {
        if cancellation.is_cancelled() {
            return Err(cancelled());
        }
        let left = until.saturating_duration_since(std::time::Instant::now());
        if left.is_zero() {
            return Ok(());
        }
        std::thread::sleep(left.min(CANCELLATION_GRAIN));
    }
}

/// The failure a cancelled publication ends with, saying what it was doing and that
/// nothing was undone.
pub(crate) fn cancelled_while(doing: &str) -> Error {
    Error::Cancelled {
        reason: format!(
            "the caller cancelled the publication while {doing}. Nothing was undone: the \
             branch stays where it is, any change request stays open, and publishing the \
             session again continues it"
        ),
    }
}

/// Publish one session's branch, as the git implementation does it.
///
/// The whole of `onevcs publish` behind the interface, which is where it had to
/// move: it starts from the session record, and a record only this implementation
/// writes is what made a session opened through the seam unpublishable.
///
/// The split between `Err` and [`PublishOutcome::Failed`] is the CLI's own, kept
/// exactly: a publication that could not *start* — an unknown token, an unreadable
/// registry, a `--policy` that widens — is a refusal, and one that started and did
/// not land is an outcome carrying what became of the branch.
///
/// `cancellation` is asked by every phase that waits, and a publication it stops ends
/// as [`FailureKind::Cancelled`] with the record left open, exactly as every other
/// failure leaves it — so publishing the session again continues the same branch.
pub fn run_for_session(
    token: &SessionToken,
    request: &PublishRequest,
    hosting: &dyn Hosting,
    cancellation: &dyn PublicationCancellation,
) -> Result<Publication> {
    let record = workspace::load(&token.0)?;
    // Before anything is read, locked or committed: a session opened over a conflicted
    // merge is one whose worker has not concluded it yet, and the preserve below would
    // conclude it for them, markers and all.
    vcs::refuse_unfinished_merge(
        &record.worktree,
        &record.branch,
        &format!(
            "publish it again with `{}`",
            crate::guidance::command(["onevcs", "publish", &record.token])
        ),
    )?;
    let registry = store::load()?;
    let resolution = store::resolve(&registry, &record.identity)?;
    let (file, source) = policy::load(&registry)?;
    let normalized = store::normalize(&resolution.identity.origin);
    let resolved = policy::resolve(&file, &source, &normalized, &resolution.publication);
    let effective = effective_policy(&resolved.policy, request.policy)?;

    let mut stream = Stream::open(&record.token)?;
    stream.label("identity", &record.identity);

    // On the stream this publication is already writing, deliberately: a second
    // stream over the same file resumes the same sequence number, and the two would
    // then emit one `seq` twice — which is exactly the gap a consumer reads as loss.
    if git::is_dirty(&record.worktree)? {
        vcs::preserve_into(&record, &mut stream, Provenance::Complete)?;
    }
    let target = recorded_target(&record, &resolution.publication)?;
    let context = Context {
        resolution,
        policy: resolved.policy.clone(),
        effective,
        repo: record.clone.clone(),
        worktree: record.worktree.clone(),
        branch: record.branch.clone(),
        target,
        // Nothing has rewritten this branch yet; a replay below decides otherwise.
        push: Push::Forward,
        run_root: record.run_root.clone(),
        preserved_into: record.execution_checkout.clone(),
        title: request.title.clone(),
        body: request.body.clone(),
        draft: request.draft.clone(),
        drafts: resolved.drafts.clone(),
        trailers: Vec::new(),
        provenance: provenance::from_rules(&file),
        hosting,
        cancellation,
    };
    let branch = record.branch.to_string();
    let outcome = match run_resolving(&context, &mut stream) {
        Ok((outcome, landed)) => {
            // A stacked change the root carried was replayed onto the root, and the
            // record learns that: every later read and publication of this session
            // then resolves the root directly rather than a stack the replayed
            // branch no longer shows — the tip it was cut from is out of its history
            // now, so there is no stack left to record. Recorded rather than
            // inferred, exactly as the stack itself was: this publication is the one
            // that moved it.
            let moved = landed != context.target;
            // A publication is the end of a session's work, so the record closes
            // with it — with one exception. A draft the session itself is holding
            // is a session still being worked in: its worktree is still the
            // dispatch's, and `session close` is what ends it. That matters because
            // `workspace::reclaim` keeps a run root only while an *open* record names
            // it, so a held draft that closed the record would let the next sibling
            // `session open` reap the worktree the worker is committing in. A draft
            // held for a release is the other thing: the work is finished, and the
            // next worker opens a session of its own.
            //
            // Written through `workspace::update`, which re-reads the record under its
            // lock: the copy loaded when this publication began is minutes old by now,
            // and a session opened over the same branch meanwhile has recorded on it
            // which session continued the work.
            let still_working = matches!(outcome, PublishOutcome::ChangeDraft(_))
                && matches!(context.draft, Some(DraftReason::Held { .. }));
            if !still_working || moved {
                workspace::update(&record.token, |stored| {
                    if moved {
                        stored.change_base = Some(landed.base().clone());
                        stored.stack_tip = None;
                    }
                    if !still_working {
                        stored.state = Lifecycle::Closed;
                    }
                })?;
            }
            outcome
        }
        Err(error) => {
            // The branch is the only record of the work, so it is handed back to the
            // execution checkout whatever refused it — the alternative is a rejected
            // tree that exists only in a run root about to be reclaimed.
            let checkout = record.execution_checkout.clone();
            let retained = match git::copy_branch(&record.clone, &checkout, &record.branch) {
                Ok(true) => Retention::HandedBack(checkout),
                _ => Retention::Refused(checkout),
            };
            PublishOutcome::Failed {
                kind: FailureKind::of(&error),
                reason: error.to_string(),
                retained: Some(retained),
            }
        }
    };
    Ok(Publication {
        session: token.clone(),
        branch,
        policy: effective,
        outcome,
    })
}

/// Everything one publication needs to know about itself.
pub struct Context<'a> {
    /// The identity and checkouts the branch belongs to.
    pub resolution: Resolution,
    /// The resolved policy, before any per-run narrowing.
    pub policy: Policy,
    /// The policy actually being published under.
    pub effective: MergePolicy,
    /// The repository the branch lives in — a session clone, or a recovery clone.
    pub repo: PathBuf,
    /// The tree this publication is built in.
    pub worktree: PathBuf,
    /// The branch carrying the change.
    pub branch: Ref,
    /// Where this publication lands, and what it is compared against on the way.
    pub target: Target,
    /// How its branch reaches the host, for a caller that rewrote the branch before
    /// handing it here. A publication that replays during its own run decides this
    /// for itself.
    pub push: Push,
    /// Where preserved merge-path logs are written.
    pub run_root: PathBuf,
    /// The checkout that keeps this branch once the run root is gone — a session's
    /// execution checkout, or the checkout a branch-keyed verb read the branch out
    /// of.
    ///
    /// Written to for one thing only: the landing record. Everything else this
    /// publication does happens in [`repo`](Context::repo), which is disposable.
    pub preserved_into: PathBuf,
    /// An explicit title, which replaces the synthesized subject. Checked where it
    /// was built, so nothing here can compose a message from one that is not a
    /// subject.
    pub title: Option<Subject>,
    /// The body the change request is opened with, verbatim, or none at all. A
    /// branch-keyed verb has no caller to take one from and publishes without one.
    pub body: Option<String>,
    /// Why this change request is opened as a draft, or none at all — which is both
    /// an ordinary publication and the thing that *lifts* a draft one.
    ///
    /// A branch-keyed verb has no caller to take one from and publishes without one,
    /// as it does with a body: landing a branch somebody else drafted is exactly the
    /// call that says the reason no longer holds.
    pub draft: Option<DraftReason>,
    /// How the draft lifecycle resolved for this identity: whether a reasonless
    /// publication opens a draft while its checks run, and whether an early lift
    /// warns.
    pub drafts: policy::DraftLifecycle,
    /// Trailers the publication commit must carry.
    pub trailers: Vec<String>,
    /// The provenance trailer keys this host reads and writes, which decide which
    /// of the branch's commits describe the change and which record the session.
    pub provenance: provenance::Trailers,
    /// Where the host that lands a change request comes from. The seam, carried on
    /// the context rather than reached for at the call site, so every publication —
    /// a session's and a recovery's — goes through the one a caller supplied.
    pub hosting: &'a dyn Hosting,
    /// Whether the caller has asked this publication to stop, asked by every phase
    /// that waits. A branch-keyed verb has no caller to ask and is never cancelled.
    pub cancellation: &'a dyn PublicationCancellation,
}

/// How a publication's branch reaches the host.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum Push {
    /// Forward or not at all, which is every publication that rewrote nothing.
    Forward,
    /// Replacing one commit and no other, because this publication replayed the
    /// branch's own commits onto the root and what it pushes is therefore no
    /// descendant of what the host has.
    Replacing {
        /// The commit this repository last saw the host's copy of the branch at,
        /// which is the only one this may replace. Read before the fetch rather than
        /// after: a host that moved *since* is the thing this exists to catch, and a
        /// value taken from a later fetch would name that move and allow it.
        // llmlint: ignore[invalid_states_unrepresentable] `git::tip` answered it.
        replaced: String,
    },
}

/// Where a publication lands, and what it is compared against on the way there.
///
/// One or the other and never a mixture: everything that follows from being stacked
/// — what the branch is compared against, what its change request targets, where it
/// lands, and which commit its own work begins after — comes from this one value, so
/// a publication cannot hold a stack and a root that disagree about any of it.
///
/// Stacked is *recorded*, never inferred: a branch that carries the change below it
/// commit for commit reads exactly like a branch that wrote those commits itself, so
/// a stack read off content would rewrite branches nobody stacked.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum Target {
    /// Onto one branch, which is also what the change is compared against: the base
    /// a session was opened with, or the root a stacked change moved onto once the
    /// change below it landed.
    Base(Ref),
    /// Onto the change below this one, until the root carries that change.
    Stacked {
        /// The branch below: what this change targets and is compared against.
        below: Ref,
        /// The identity's root base, which this moves onto once the root carries
        /// the change below.
        root: Ref,
        /// The commit the branch was cut from, after which its own work begins.
        // llmlint: ignore[invalid_states_unrepresentable] `git::tip` answered it.
        tip: String,
    },
}

impl Target {
    /// The branch this change is compared against and lands on.
    pub fn base(&self) -> &Ref {
        match self {
            Target::Base(base) => base,
            Target::Stacked { below, .. } => below,
        }
    }
}

/// The stack a session's record wrote down, or `None` for the branch every ordinary
/// session cuts.
///
/// Two things have to be there: the tip the session was cut at, which `session open`
/// records only for a base that is not the identity's root, and a root to land on
/// once the change below has landed.
///
/// The recorded tip is resolved through git before it is trusted — a record is a file
/// under the state root — and one this session's clone does not have is refused rather
/// than read as no stack: answering "no stack" from a value nothing could read is the
/// merge this whole path exists to stop.
///
/// A root nobody can name is different and is no stack: nothing here fails, there is
/// simply nowhere to move the change to, and the publication is the one it always
/// was.
fn recorded_target(record: &workspace::Record, publication: &Path) -> Result<Target> {
    let base = preserved_change_base(&record.base, record.change_base.as_ref());
    let Some(recorded) = record.stack_tip.as_deref() else {
        return Ok(Target::Base(base));
    };
    let Some(tip) = git::tip(&record.clone, recorded) else {
        return Err(Error::Invalid {
            reason: format!(
                "the record for session {token} names {recorded:?} as the commit branch {branch:?} \
                 was cut from, and the clone at {clone} does not have it, so nothing can tell \
                 which of that branch's commits belong to the change below it. The branch is \
                 whole — publish it by name with `{command}`, which reads the stack off the \
                 branch instead",
                token = record.token,
                branch = record.branch,
                clone = record.clone.display(),
                command = guidance::command([
                    "onevcs",
                    "publish-branch",
                    &record.branch,
                    "--repo",
                    &record.publication_checkout.to_string_lossy(),
                ]),
            ),
        });
    };
    let Some(root) = git::default_branch(publication, "origin").ok() else {
        return Ok(Target::Base(base));
    };
    let root = Ref::from_git(root);
    if root == base {
        return Ok(Target::Base(base));
    }
    Ok(Target::Stacked {
        below: base,
        root,
        tip,
    })
}

/// Whether the root base already carries everything the change below this one
/// contributed, while carrying none of the commits that contributed it.
///
/// Three things past the record: the branch was cut from that tip, the root does not
/// hold the tip's commits under their own names, and the root does hold what they
/// changed. What that establishes is exactly its name and no more — it is what a
/// squash-merge of the change below leaves, and a root that came by the same content
/// some other way is indistinguishable from one here, because content equality is all
/// git can be asked. Both are answered the same way deliberately: the commits a
/// replay drops are commits whose content the root already has, so the branch's own
/// work is what is left either way. A change still open whose content has *not*
/// reached the root fails the last test, and one merged as its own commits fails the
/// second.
pub(crate) fn root_is_known_to_carry_the_stack(
    repo: &Path,
    branch: &str,
    root: &Ref,
    tip: &str,
) -> Result<bool> {
    let root = vcs::base_ref(repo, root);
    if !git::ref_exists(repo, &format!("refs/remotes/{root}")) && !git::branch_exists(repo, &root) {
        return Ok(false);
    }
    if !git::is_ancestor(repo, tip, branch)? {
        return Ok(false);
    }
    // Still in the root as the commits it was written as: merging the root brings
    // them in the way it always has, and there is nothing to replay past.
    if git::is_ancestor(repo, tip, &root)? {
        return Ok(false);
    }
    let Some(fork) = git::merge_base(repo, &root, tip)? else {
        return Ok(false);
    };
    git::known_to_carry_changes(repo, &root, &fork, tip)
}

/// Where a publication lands once the root has been consulted: the target it was
/// handed, or the root once the root carries the change below — and, in that case,
/// the tip the branch's own work begins after, which is what a replay onto the root
/// starts from.
///
/// Asked after a fetch, deliberately: the root is what decides it, and a clone's own
/// copy of the root is only as current as its last fetch. Every target that is not a
/// stacked one is handed back as it was, with nothing to replay.
pub(crate) fn landed_target(
    repo: &Path,
    branch: &Ref,
    target: &Target,
) -> Result<(Target, Option<String>)> {
    if let Target::Stacked { root, tip, .. } = target {
        if root_is_known_to_carry_the_stack(repo, branch, root, tip)? {
            return Ok((Target::Base(root.clone()), Some(tip.clone())));
        }
    }
    Ok((target.clone(), None))
}

/// The change request a session's publication opens or adopts: the branch it pushes,
/// into the base it resolves for it, stacked or not.
///
/// One computation for `publish` and for the `change` verbs that address the same
/// change request afterwards, so no session can have the two naming different ones.
/// It is the resolution [`run_for_session`] and [`run`] make between them — the
/// target the record wrote down, then the root once the root carries the change
/// below — made after the same fetch a publication makes, because the root decides
/// the second step and the clone's own copy of it is only as current as its last
/// fetch. A `change show` on a stacked session therefore answers the change request
/// the stack publishes onto, which is the one its next `publish` adopts.
pub(crate) fn session_target(record: &workspace::Record) -> Result<Target> {
    if git::has_remote(&record.clone, "origin") {
        git::fetch(&record.clone, "origin")?;
    }
    standing_target(record)
}

/// The same resolution as [`session_target`], from what the session's clone already
/// holds, without asking the remote.
///
/// What a *read* of the session judges its branch against: the base its publication
/// would resolve from the clone's own view, which is the branch below until the
/// clone has seen the root carry it. A stacked session whose branch below has
/// landed and gone loses that name from the clone at the next fetch — its
/// publication's own fetch prunes — and a read that kept comparing against the
/// recorded base would then fail on a branch nothing has any more, for a session
/// whose work is exactly where its publication put it.
pub(crate) fn standing_target(record: &workspace::Record) -> Result<Target> {
    let recorded = recorded_target(record, &record.publication_checkout)?;
    let (target, _replay) = landed_target(&record.clone, &record.branch, &recorded)?;
    Ok(target)
}

impl<'a> Context<'a> {
    /// The same publication, landing where `target` says instead.
    fn onto(&self, target: Target) -> Context<'a> {
        Context {
            resolution: self.resolution.clone(),
            policy: self.policy.clone(),
            effective: self.effective,
            repo: self.repo.clone(),
            worktree: self.worktree.clone(),
            branch: self.branch.clone(),
            target,
            push: self.push.clone(),
            run_root: self.run_root.clone(),
            preserved_into: self.preserved_into.clone(),
            title: self.title.clone(),
            body: self.body.clone(),
            draft: self.draft.clone(),
            drafts: self.drafts.clone(),
            trailers: self.trailers.clone(),
            provenance: self.provenance.clone(),
            hosting: self.hosting,
            cancellation: self.cancellation,
        }
    }
}

/// Verify and publish a branch.
pub fn run(context: &Context<'_>, stream: &mut Stream) -> Result<PublishOutcome> {
    run_resolving(context, stream).map(|(outcome, _)| outcome)
}

/// [`run`], also answering where the branch landed: the target it was handed, or
/// the root a stacked change was replayed onto because the root already carried the
/// change below.
///
/// The second is what a session's own record has to learn, because a replay changes
/// the branch's shape: the tip it was cut from is no longer in its history, so the
/// record's stack cannot be recognised on the branch again — and a later read or
/// publication of the same session that still resolved the recorded stack would
/// compare the branch against a branch below it that has landed and gone.
fn run_resolving(context: &Context<'_>, stream: &mut Stream) -> Result<(PublishOutcome, Target)> {
    // Input, rejected at its boundary: before the fetch, before the sync, and before
    // anything reaches a remote. A draft is a state of a *change request*, so a
    // publication that opens none cannot be in it, and a reason nobody can read is
    // not a reason.
    if let Some(reason) = &context.draft {
        reason.checked()?;
        if context.effective == MergePolicy::LocalDirect {
            return Err(crate::error::invalid(format!(
                "{branch:?} was asked to publish as {asked}, and this identity publishes with \
                 local-direct, which squashes the branch onto {base:?} and opens no change \
                 request at all — so there is nothing to draft and {lands}. Publish it under a \
                 change-* policy, or publish it without a draft",
                branch = context.branch,
                asked = reason.asked_for(),
                base = context.target.base(),
                lands = match reason {
                    DraftReason::AwaitingRelease { .. } => {
                        "the work would land with the very pin the draft exists to hold back"
                    }
                    DraftReason::Held { .. } => {
                        "the work would land while the session is still making it"
                    }
                },
            )));
        }
    }
    // Before anything is compared, because what it may establish is that this branch
    // already landed: a change request a `checks-unsettled` close stopped watching
    // merges on the host's clock, and a publication that met it again republished the
    // work the base already carried.
    take_up_a_late_merge(context);
    // Where this repository last saw the host's copy of the branch, read before the
    // fetch that is about to update it: a publication that replays its own commits
    // pushes over that copy, and what it may replace is what it had already seen —
    // never whatever arrived in between, which is the thing worth refusing over.
    let last_seen = git::tip(&context.repo, &format!("origin/{}", context.branch));
    if git::has_remote(&context.repo, "origin") {
        // Outside every exclusive section, deliberately.
        git::fetch(&context.repo, "origin")?;
        stream.emit(
            EventKind::Fetch,
            object(json!({"remote": "origin", "checkout": context.repo.display().to_string()})),
        );
    }
    // Asked after the fetch and before anything else: a change whose stack the root
    // already carries is a change onto the root base, and everything below — what it is compared
    // against, what its change request targets — follows from that rather than from
    // the branch it was opened against.
    let (target, replay) = landed_target(&context.repo, &context.branch, &context.target)?;
    let landed;
    let mut context = context;
    if replay.is_some() {
        landed = context.onto(target.clone());
        context = &landed;
    }
    let remote_base = format!("origin/{}", context.target.base());
    let compared = if git::ref_exists(&context.repo, &format!("refs/remotes/{remote_base}")) {
        remote_base.clone()
    } else {
        context.target.base().to_string()
    };

    sync(context, stream, &compared, replay.as_deref())?;

    if nothing_to_publish(context, &compared)? {
        return Ok((PublishOutcome::NothingToPublish, target));
    }

    let push = match (&replay, &last_seen) {
        (Some(_), Some(replaced)) => Push::Replacing {
            replaced: replaced.clone(),
        },
        // Otherwise it is whatever the caller knows: a branch-keyed verb replays
        // before it hands a publication here, and its branch is rewritten just the
        // same.
        _ => context.push.clone(),
    };
    // The trailers are the publication *commit*'s, so only the local path takes them
    // here — and it re-describes the branch after the queue anyway. Asking now is
    // still what refuses a branch with no usable subject before anything is pushed.
    let (subject, _) = describe(context, &compared)?;
    let environment = merge_path::comparison_env("origin", context.target.base());

    let outcome = match context.effective {
        MergePolicy::LocalDirect => publish_locally(context, stream, &compared, &environment)?,
        _ => publish_as_change(context, stream, &subject, &environment, push)?,
    };
    Ok((outcome, target))
}

/// The subject a publication of one branch would carry, or the refusal that none
/// of its commits can supply one.
///
/// Public to this crate because it is also a *precondition*: a branch-keyed verb
/// asks it before it writes anything to the branch, so a refusal an explicit
/// `--title` answers is met before the work rather than after a commit the operator
/// then has to reason about.
pub(crate) fn subject_for(
    repo: &Path,
    compared: &str,
    branch: &str,
    title: Option<&str>,
    trailers: &provenance::Trailers,
) -> Result<String> {
    let subject = match provenance::publication_subject(repo, compared, branch, title, trailers)? {
        Ok(subject) => subject,
        Err(reason) => {
            return Err(Error::Invalid {
                reason: format!(
                    "cannot publish {branch:?}: {reason}. A subject that names no change would \
                     make the base branch a worse record than this refusal does."
                ),
            })
        }
    };
    hold_to_repository_policy(repo, branch, &subject)?;
    Ok(subject)
}

/// Put the composed subject to the repository's own `commit-msg` hook, and refuse
/// the publication when it turns the subject down.
///
/// This crate states no subject policy of its own and never will: a squash-merge
/// subject decides whether a release cuts, and *which* subjects release is a fact
/// about the repository rather than about publishing. So the question is asked of
/// the repository, in the one place it can be asked at all — the subject a merge
/// lands under comes from a change request's title, which no local hook ever sees.
///
/// A repository with no `commit-msg` hook expresses no policy and acquires none by
/// being published through here: [`git::MessagePolicy::Unstated`] passes silently.
pub(crate) fn hold_to_repository_policy(repo: &Path, branch: &str, subject: &str) -> Result<()> {
    let git::MessagePolicy::Rejected { status, output } = git::message_policy(repo, subject)?
    else {
        return Ok(());
    };
    let said = guidance::quoted_output(output.trim());
    let said = if said.is_empty() {
        "<no output>".to_owned()
    } else {
        said
    };
    Err(Error::GateFailed {
        reason: format!(
            "the repository's own commit-msg hook rejected the subject publishing {branch:?} \
             would land, {subject:?} (exit {status}). Reword the commit the subject comes from, \
             or publish with an explicit title that satisfies it. The hook said:\n{said}"
        ),
    })
}

/// Whether the base already carries everything this branch has, once the base has
/// been merged into it.
///
/// Two shapes, and only one of them is "no commits": a branch squash-merged under
/// somebody else's change request keeps every commit it had and adds nothing to the
/// tree, so the tree is what decides. Opening a change request for one produces an
/// empty diff, which every path-filtered required check skips rather than runs and
/// the host then blocks forever.
///
/// Asked here, on the one path both policies go through and before anything is
/// pushed, so every caller of a publication gets it.
fn nothing_to_publish(context: &Context<'_>, compared: &str) -> Result<bool> {
    if git::log_messages(&context.repo, compared, &context.branch)?.is_empty() {
        return Ok(true);
    }
    Ok(!git::trees_differ(
        &context.repo,
        compared,
        &context.branch,
    )?)
}

/// The subject one publication commit carries, and what it must carry forward.
fn describe(context: &Context<'_>, compared: &str) -> Result<(String, Vec<String>)> {
    let subject = subject_for(
        &context.repo,
        compared,
        &context.branch,
        context.title.as_deref(),
        &context.provenance,
    )?;
    let mut trailers = context.trailers.clone();
    trailers.extend(provenance::attestation_trailers(
        &context.repo,
        compared,
        &context.branch,
        &context.provenance,
    )?);
    // What this commit lands, named on the base itself. A squash leaves the base
    // carrying the branch's content under no name it kept, so whether the work
    // reached the base could only ever be *inferred* from content afterwards — and
    // that inference stops being true the moment anything edits those paths. This is
    // the record that answers it instead, and it is written here because this path is
    // the one landing that opens no change request: everything the host lands carries
    // the change request's own number.
    if let Some(tip) = git::tip(&context.repo, &context.branch) {
        trailers.push(format!("{} {tip}", context.provenance.landed()));
    }
    Ok((subject, trailers))
}

/// What bringing a branch level with what it lands on did.
pub(crate) enum Reconciled {
    /// The branch carries what it lands on now.
    Settled,
    /// It conflicted, doing this — which is what a refusal about it has to name,
    /// since the resolution an operator is sent to is the shape that was attempted
    /// — and *what* conflicted, which is what makes the refusal actionable rather
    /// than merely true.
    Conflicted(Reconciliation, git::Conflict),
}

/// Which shape bringing a branch level with what it lands on takes.
pub(crate) enum Reconciliation {
    /// The base is merged into the branch.
    Merge,
    /// Only the branch's own commits are replayed onto the base, because the base
    /// already carries its history up to this commit.
    Replay {
        /// The commit where the branch's own work begins, as `git` spelled it.
        // llmlint: ignore[invalid_states_unrepresentable] `git::tip` answered it.
        from: String,
    },
}

/// Bring a branch level with the base it is published onto, once.
///
/// Two shapes, and the record decides which rather than this: ordinarily the base is
/// merged into the branch, and a branch whose recorded stack has landed replays only
/// its own commits onto the base instead — `git rebase --onto <base> <stack tip>
/// <branch>`, which is what an operator ends up running by hand. Merging there would
/// replay the change below against its own squashed equivalent, which conflicts in
/// every file both touched and conflicts again on every bounded retry.
///
/// Shared by `publish` and by the branch-keyed verbs, so the two cannot come to
/// disagree about what a sync does.
pub(crate) fn reconcile(
    worktree: &Path,
    compared: &str,
    branch: &str,
    replay_from: Option<&str>,
) -> Result<Reconciled> {
    let (shape, integrated) = match replay_from {
        Some(tip) => (
            Reconciliation::Replay {
                from: tip.to_owned(),
            },
            git::rebase_onto(worktree, compared, tip, branch)?,
        ),
        None => (
            Reconciliation::Merge,
            git::merge_into_branch(worktree, compared, &merge_message(compared, branch))?,
        ),
    };
    Ok(match integrated {
        git::Integrated::Settled => Reconciled::Settled,
        git::Integrated::Conflicted(conflict) => Reconciled::Conflicted(shape, conflict),
    })
}

/// The subject of the commit a sync's merge writes, so a session's opening merge
/// and a publication's read the same in `git log`.
pub(crate) fn merge_message(compared: &str, branch: &str) -> String {
    format!("Merge {compared} into {branch}")
}

/// Report the conflict that stopped a publication: which paths, and the hunks.
///
/// The paths go in the payload because they are a short list an operator acts on
/// directly — "what conflicts" is the question a refusal that only said *that*
/// something conflicted left unanswered. The hunks go beside it as an artifact,
/// because a diff is evidence rather than a field, and the envelope's own rule is
/// that large evidence is referenced by id.
///
/// Shared with the branch-keyed verbs, which conflict at the same place for the
/// same reasons: a second emitter would be a second answer to what conflicts.
///
/// A hunk artifact that could not be stored is a warning on stderr and no artifact,
/// never a failure: the conflict is the answer, and reporting a filesystem problem
/// in place of it would lose the diagnosis this exists to keep.
pub(crate) fn report_conflict(
    stream: &mut Stream,
    branch: &Ref,
    base: &Ref,
    conflict: &git::Conflict,
    attempts: Option<usize>,
) {
    let artifacts = match stream::store_artifact("diff", conflict.hunks()) {
        Ok(artifact) => vec![artifact],
        Err(error) => {
            eprintln!(
                "onevcs: warning: the conflict on {branch:?} is recorded without its \
                 hunks: {error}"
            );
            Vec::new()
        }
    };
    let mut payload = object(json!({
        "branch": branch,
        "base": base,
        "paths": conflict.paths(),
    }));
    if let Some(attempts) = attempts {
        payload.insert("attempts".to_owned(), json!(attempts));
    }
    stream.emit_with(EventKind::SyncConflict, payload, artifacts);
}

/// Sync the branch with the current base, bounded, before anything is published.
///
/// `replay_from` is the recorded stack tip when the change below this one has
/// landed, and `None` — every publication that is not a stacked one — is the merge
/// this has always been.
fn sync(
    context: &Context<'_>,
    stream: &mut Stream,
    compared: &str,
    replay_from: Option<&str>,
) -> Result<()> {
    if !git::ref_exists(&context.repo, &format!("refs/remotes/{compared}"))
        && !git::branch_exists(&context.repo, compared)
    {
        return Ok(());
    }
    let mut last: Option<(Reconciliation, git::Conflict)> = None;
    for attempt in 1..=SYNC_ATTEMPTS {
        match reconcile(&context.worktree, compared, &context.branch, replay_from)? {
            Reconciled::Settled => return Ok(()),
            Reconciled::Conflicted(shape, found) => last = Some((shape, found)),
        }
        if attempt < SYNC_ATTEMPTS && git::has_remote(&context.repo, "origin") {
            git::fetch(&context.repo, "origin")?;
        }
    }
    // Every path out of the loop above either returned or wrote this, and the bound
    // is at least one attempt — so the refusal below is always about a conflict this
    // run actually met, rather than about a value standing in for one.
    let Some((attempted, conflict)) = last else {
        return Ok(());
    };
    report_conflict(
        stream,
        &context.branch,
        context.target.base(),
        &conflict,
        Some(SYNC_ATTEMPTS),
    );
    let conflicting = guidance::listed(conflict.paths());
    let land = guidance::command([
        "onevcs",
        "publish-branch",
        &context.branch,
        "--repo",
        &context.resolution.publication.to_string_lossy(),
    ]);
    // The branch is retained rather than lost, and the refusal says what would
    // change the answer: another attempt resolves nothing a bounded retry has
    // already tried, and an operator told only that the two conflict is an operator
    // reaching for raw `git` to land the work. Which resolution it names follows
    // what was attempted — telling a branch whose stack parent has landed to merge
    // the base is telling it to reproduce the conflict it is refusing.
    Err(Error::SyncConflict {
        reason: match attempted {
            Reconciliation::Replay { from } => {
                format!(
                    "{compared} conflicts with {branch:?} in {conflicting} after {SYNC_ATTEMPTS} \
                 bounded attempts; the branch is retained. {compared} already carries what \
                 {branch:?} was stacked on, so only its own commits are replayed onto it. Resolve \
                 the conflict on it — replay it with `{}` — and then land it with `{land}`",
                    guidance::command([
                        "git",
                        "rebase",
                        "--onto",
                        compared,
                        &from,
                        &context.branch
                    ]),
                    branch = context.branch,
                )
            }
            Reconciliation::Merge => format!(
                "{compared} conflicts with {branch:?} in {conflicting} after {SYNC_ATTEMPTS} \
                 bounded attempts; the branch is retained. Resolve the conflict on it — merge \
                 {compared} into {branch} — and then land it with `{land}`",
                branch = context.branch,
            ),
        },
    })
}

/// Land the branch as one squashed commit on the base, built detached.
fn publish_locally(
    context: &Context<'_>,
    stream: &mut Stream,
    compared: &str,
    environment: &[(String, String)],
) -> Result<PublishOutcome> {
    let publication = &context.resolution.publication;
    require_publication_checkout_ready(publication, context.target.base())?;
    let synced = git::tip(&context.repo, compared);

    let identity = lock::git_identity(&git::common_dir(publication)?);
    let turn = queue::turn(&identity, context.cancellation)?;
    stream.emit(
        EventKind::LockWait,
        object(json!({
            "identity": identity,
            "elapsed": turn.waited.as_secs_f64(),
            "queue_position": turn.position,
        })),
    );
    stream.emit(
        EventKind::LockAcquired,
        object(json!({"identity": identity})),
    );
    stream.emit(
        EventKind::MergeQueued,
        object(json!({"identity": identity, "queue_position": turn.position})),
    );

    let outcome = (|| -> Result<PublishOutcome> {
        // The base can have advanced while this turn was queued — the writer ahead
        // of it in the queue is the usual reason. The tree that lands is therefore
        // re-synced here rather than silently reconciled: what was brought level with
        // the base is no longer what would be published, and the merge path is about
        // to rule on whatever this pushes.
        if git::has_remote(&context.repo, "origin") {
            git::fetch(&context.repo, "origin")?;
        }
        if git::tip(&context.repo, compared) != synced {
            sync(context, stream, compared, None)?;
        }
        let (subject, trailers) = describe(context, compared)?;
        let message = compose_squash(&subject, &closing_lines(context, compared)?, &trailers);

        let scratch_parent = context.run_root.join(format!("publish-{}", ids::unique()));
        home::ensure_dir(&scratch_parent)?;
        let scratch = scratch_parent.join("worktree");
        git::worktree_add_detached(&context.repo, &scratch, compared)?;
        let landed = (|| -> Result<PublishOutcome> {
            let Some(sha) = git::merge_squash(&scratch, &context.branch, &message)? else {
                return Ok(PublishOutcome::NothingToPublish);
            };
            let pushed = git::push(
                &scratch,
                &format!("HEAD:refs/heads/{}", context.target.base()),
                "origin",
                environment,
            )?;
            // The squash goes onto the base rather than onto the branch it landed,
            // so this push is the work being integrated whatever branch the payload
            // names.
            let kept = record_push(
                stream,
                &context.branch,
                &pushed,
                Some(&context.run_root),
                Phase::Integrate,
                None,
            )?;
            if !pushed.accepted() {
                // The tree this push was built in goes when this closure returns,
                // so the refusal must not send anybody to a path inside it.
                return Err(rejected(
                    &publishing(&context.branch),
                    &pushed,
                    &kept,
                    Some(&scratch_parent),
                ));
            }
            fast_forward_publication(publication, context.target.base())?;
            stream.emit(
                EventKind::MergeCompleted,
                object(json!({"identity": identity, "sha": sha, "base": context.target.base()})),
            );
            record_baselines(context, &sha, stream);
            Ok(PublishOutcome::Merged(Sha(sha)))
        })();
        git::worktree_remove(&context.repo, &scratch)?;
        let _ = std::fs::remove_dir_all(&scratch_parent);
        landed
    })();

    drop(turn);
    outcome
}

/// How a publication names the push it was refused, so the one refusal builder is
/// handed the same phrase from both of its call sites here.
fn publishing(branch: &Ref) -> String {
    format!("the publishing push of {branch:?}")
}

/// A push the merge path refused: what git said it was, everywhere the merge path's
/// own account of it can be read, and the end of that account.
///
/// [`Error::PushRejected`] rather than [`Error::GateFailed`]: the same exit code, and a
/// kind a caller can route on.
///
/// **The pointers are the point.** What the hook or the remote wrote is a run of the
/// repository's whole verification and does not belong inline, so `record_push`
/// stored it a moment earlier — as an artifact, and where the caller had a run root,
/// as a file that outlives the tree the publication was built in. This used to name
/// neither, and a refusal that reports git's three generic lines while the diagnosis
/// sits in two places nothing points at is a refusal an operator has to go searching
/// behind: one landing here cost an hour that way, to find four redundant comment
/// lines. So the failure names both, and each only where it exists.
///
/// It quotes an excerpt rather than the whole, because the whole is what the
/// pointers above are for; [`excerpt`] is where the bound and the end it is taken
/// from are reasoned about.
///
/// **A refusal that captured nothing is a different sentence.** There is no excerpt
/// to quote and no per-ref summary to name, so the two clauses above collapse into
/// prose that describes nothing — `rejected by the merge path: .` is what one
/// seventeen-minute publishing push was reported as, and its caller had to run the
/// whole gate again to learn anything. What is said instead is that the merge path
/// wrote nothing, and what the run itself reported: the exit status or the signal
/// that terminated the push, and any read of its pipes that stopped short. A
/// refusal that *did* capture output is reported exactly as it always was.
///
/// This is also what an *unclassified* rejection is reported as, deliberately: it
/// names the push and hands over git's own per-ref summary without deciding what
/// produced it. The fallback below it is for a failure that never reached a ref at
/// all — no credential, no remote — where git's last line is the whole answer.
///
/// `what` is the push, named as its own caller names it: the branch a publication
/// was landing, or the base a merge train advanced.
///
/// `removed` is the tree the push was built in where its caller removes that tree
/// before returning, and is what [`outliving`] scrubs the message of: every path
/// this refusal names has to be openable by the person reading it.
pub(crate) fn rejected(
    what: &str,
    pushed: &git::Pushed,
    kept: &Kept,
    removed: Option<&Path>,
) -> Error {
    let summary = pushed
        .refusal()
        .unwrap_or_else(|| pushed.output().lines().next_back().unwrap_or("").trim());
    // Said rather than left to be inferred. A refusal that names nowhere reads as one
    // that simply did not bother to; "there is nothing to go and read" and "there is,
    // and this failure has lost the address of it" are the same sentence to an
    // operator and different next moves. `record_push` has already said on stderr why
    // each store refused the bytes.
    let where_it_is = if kept.anywhere() {
        kept.evidence()
    } else {
        " What it wrote could not be stored anywhere, so the excerpt below is all that \
         survives of it."
            .to_owned()
    };
    let wrote = pushed.output().trim();
    let missing = host_prerequisite(wrote);
    let said = if wrote.is_empty() {
        String::new()
    } else {
        format!(
            " It said:\n{}",
            guidance::quoted_output(&excerpt(
                wrote,
                "the whole of it is where this refusal says"
            ))
        )
    };
    // The one case the sentence above cannot describe: nothing was captured, so
    // `summary` is empty and `said` is empty, and what an operator is handed reads
    // `rejected by the merge path: .` — which is how a seventeen-minute push was
    // reported, and why the whole gate had to be run again to learn anything about
    // it. The run's own account of itself is what is left, so it is said outright.
    if wrote.is_empty() && missing.is_none() {
        return Error::PushRejected {
            reason: outliving(
                &format!(
                    "{what} was rejected by the merge path, and the merge path wrote nothing: \
                     {diagnosis}.{where_it_is}",
                    diagnosis = diagnosis(pushed),
                ),
                removed,
            ),
        };
    }
    match missing {
        Some(missing) => Error::HostPrerequisite {
            reason: outliving(
                &format!(
                    "{what} was refused by the merge path because this host is missing a \
                     prerequisite: {missing}.{where_it_is}{said}",
                    missing = missing.trim_end_matches('.'),
                ),
                removed,
            ),
        },
        None => Error::PushRejected {
            reason: outliving(
                &format!("{what} was rejected by the merge path: {summary}.{where_it_is}{said}"),
                removed,
            ),
        },
    }
}

/// What the push's own run says about itself: how it ended, and any read of its
/// pipes that stopped short.
///
/// Both, and neither inferred from the other — a push terminated by a signal and a
/// push whose output this process could not read are different failures with
/// different next moves, and a push can be both. A refusal reaching here with no
/// diagnostics at all is one no `git push` produced, and says so rather than
/// claiming an ending nobody reported.
fn diagnosis(pushed: &git::Pushed) -> String {
    let Some(diagnostics) = pushed.diagnostics() else {
        return "nothing was recorded about how the push itself ended".to_owned();
    };
    let mut said = format!("the push {}", diagnostics.ended);
    if !diagnostics.read_failures.is_empty() {
        said.push_str(&format!(
            ", and its output could not be read whole ({})",
            diagnostics.read_failures.join("; "),
        ));
    }
    said
}

/// How a merge-path hook begins the one line saying a refusal is this host's rather
/// than the work's: a tool or a credential it needs is missing, and what follows the
/// marker says which and how to install it.
///
/// The contract's spelling, stated once. A hook prints it only for a failure that does
/// not depend on the tree being pushed, because the reader on the other side settles
/// such a refusal without sending anybody to change the work.
pub const HOST_PREREQUISITE_MARKER: &str = "onevcs: host-prerequisite:";

/// What a merge path's host-prerequisite line says is missing, where its output
/// carries one.
///
/// Read line by line, and the line has to *begin* with the marker — after the
/// `remote:` git puts before what a server-side hook printed — so a hook quoting the
/// marker in prose, or a test log mentioning it, is not read as one. A marker naming
/// nothing is not the line the contract describes, and the refusal is classified as
/// though it were absent.
fn host_prerequisite(output: &str) -> Option<&str> {
    output.lines().find_map(|line| {
        let line = line.trim();
        let line = line.strip_prefix("remote:").map_or(line, str::trim_start);
        line.strip_prefix(HOST_PREREQUISITE_MARKER)
            .map(str::trim)
            .filter(|missing| !missing.is_empty())
    })
}

/// The same refusal with every path into a tree that will not outlive this command
/// replaced by the fact that it did not.
///
/// A merge path keeps its own account where it runs — `.logs/nx.log` under the tree
/// the publishing push was built from is one real spelling — and says so in the
/// output quoted above. That tree is a scratch worktree removed before the command
/// returns, so the refusal named a path an operator opened and found nothing at,
/// while the same output sat under the publication's own retained gate logs, which
/// the sentence beside it already names. A path that is not there is worse than no
/// path: it is the one thing in a failure a reader trusts enough to go and open.
///
/// What is replaced is the whole path and not its prefix, because half of one still
/// reads as somewhere to look. Nothing else is touched — the preserved log and the
/// checkouts this names are under neither this tree nor any other that is going.
fn outliving(reason: &str, removed: Option<&Path>) -> String {
    let Some(removed) = removed else {
        return reason.to_owned();
    };
    let doomed = removed.display().to_string();
    if doomed.is_empty() {
        return reason.to_owned();
    }
    let mut left = reason;
    let mut answer = String::with_capacity(reason.len());
    while let Some(at) = left.find(&doomed) {
        answer.push_str(&left[..at]);
        let rest = &left[at..];
        // To the end of the path and no further: a path ends where the prose around
        // it resumes, and the sentence-final stop of the line it sits in is prose.
        let end = rest
            .find(|c: char| c.is_whitespace() || ENDS_A_PATH.contains(&c))
            .unwrap_or(rest.len());
        let end = rest[..end].trim_end_matches('.').len().max(doomed.len());
        answer.push_str(GONE);
        left = &rest[end..];
    }
    answer.push_str(left);
    answer
}

/// What a path is quoted or punctuated with once it is over, in the prose a merge
/// path writes around one.
const ENDS_A_PATH: [char; 6] = [')', '"', '\'', '`', ',', ';'];

/// What stands where such a path stood: not a path, and the reason it is not.
const GONE: &str = "(gone with the publication worktree this ran in)";

/// Why git turned a leased push down, when the lease is what it turned down.
///
/// The two are told apart because the operator's next move differs: one branch has
/// somebody else's work on it to reconcile with, and the other has nothing on the
/// host at all.
enum Declined {
    /// The branch is on the host at a commit this run never saw.
    Moved(git::ObjectId),
    /// The host has no branch of that name any more.
    Gone,
}

/// Whether git turned this push down *because the lease was stale*, decided from
/// what git and the remote report and never from the prose beside it.
///
/// Two structural facts have to agree, because neither alone is enough. git has to
/// have declined this very ref: its `--porcelain` line for the branch carries the
/// `!` flag, which rules out a push that failed before any ref was negotiated. And
/// the lease has to be genuinely stale: `--force-with-lease=<branch>:<seen>` is
/// declined exactly when the remote's value for the branch is not `<seen>`, so the
/// remote's own answer to "where is this branch" settles whether that is what
/// happened. A remote that will not say answers `None`, leaving the rejection
/// unclassified — assuming a stale lease from an answer nobody gave is the same
/// mistake as reading it out of a message a locale could translate.
fn declined_the_lease(
    context: &Context<'_>,
    pushed: &git::Pushed,
    replaced: &str,
    environment: &[(String, String)],
) -> Result<Option<Declined>> {
    if !pushed.refused_branch(&context.branch) {
        return Ok(None);
    }
    Ok(
        match git::remote_tip(&context.worktree, "origin", &context.branch, environment)? {
            git::RemoteTip::At(tip) if tip.as_str() == replaced => None,
            git::RemoteTip::At(tip) => Some(Declined::Moved(tip)),
            // The host no longer has the branch at all, which is not where this run
            // last saw it either — the lease named a commit, and git declines it
            // against a ref that is gone just as it does against one that moved.
            git::RemoteTip::Absent => Some(Declined::Gone),
            git::RemoteTip::Unknown => None,
        },
    )
}

/// Push the branch, open or adopt its change request, and ask the host to land it.
fn publish_as_change(
    context: &Context<'_>,
    stream: &mut Stream,
    subject: &str,
    environment: &[(String, String)],
    push: Push,
) -> Result<PublishOutcome> {
    // Input, rejected at its boundary rather than half way through the watch below,
    // where the branch would already be on the remote and the refusal would read as a
    // merge path nobody could verify.
    refuse_an_unhosted_identity(&context.resolution.key)?;
    if let Some(reason) = &context.draft {
        if let Some(refusal) = refuse_a_draft_over_a_reviewed_change(context, reason) {
            return Err(refusal);
        }
    }
    // Every change policy watches its checks now, `change-open` included, so every one
    // is held to the knobs that bound the watch — before anything is pushed.
    gh::checks_timeout()?;
    gh::checks_poll()?;
    crate::host::check_source_names_a_source()?;
    if !context.drafts.disabled {
        gh::draft_grace()?;
    }

    // A branch this publication replayed is not a descendant of the one the host has
    // for it — the change below's commits are gone from it — so the push replaces one
    // commit and no other, and git refuses it if the host is anywhere else. Every
    // other publication pushes as it always has: forward or not at all.
    let replacing = match &push {
        Push::Replacing { replaced } => Some(replaced.as_str()),
        Push::Forward => None,
    };
    if context.cancellation.is_cancelled() {
        return Err(cancelled_while(&format!(
            "it prepared {:?} and before anything was pushed",
            context.branch
        )));
    }
    let pushed = git::push_replacing(
        &context.worktree,
        &context.branch,
        "origin",
        replacing,
        environment,
    )?;
    // The session's own branch, on its way to being proposed: the work being made.
    let kept = record_push(
        stream,
        &context.branch,
        &pushed,
        Some(&context.run_root),
        Phase::Development,
        git::tip(&context.worktree, &format!("refs/heads/{}", context.branch)).as_deref(),
    )?;
    if !pushed.accepted() {
        // Which refusal this is depends on what git declined, and that is decided
        // from what git and the host *report* rather than from the sentence either
        // wrote. A declined lease means somebody else's commit is on this branch
        // while the work here is a replay that would have written over it. Every
        // other rejection of the same push is what it has always been — a
        // credential, a hook, or the host's own policy — and so is every rejection
        // this cannot tell apart: reading one of those as a branch that moved would
        // send an operator to reconcile histories that never diverged.
        let declined = match replacing {
            Some(replaced) => declined_the_lease(context, &pushed, replaced, environment)?,
            None => None,
        };
        let (Some(replaced), Some(declined)) = (replacing, declined) else {
            // The session's own worktree, which outlives this command: what it
            // wrote about itself stays readable.
            return Err(rejected(&publishing(&context.branch), &pushed, &kept, None));
        };
        let branch = &context.branch;
        let base = context.target.base();
        let command = guidance::command([
            "onevcs",
            "publish-branch",
            branch,
            "--repo",
            &context.resolution.publication.to_string_lossy(),
        ]);
        return Err(Error::SyncConflict {
            // What the host has instead is what the operator has to do something
            // about, so the refusal says which of the two it is rather than one
            // sentence that is only true of the commoner one.
            reason: match declined {
                Declined::Moved(tip) => format!(
                    "{branch:?} moved on the host since this run last had it at {replaced} — it \
                     is at {tip} now — and this publication replayed it onto {base:?}, so pushing \
                     would replace whatever was pushed there in between. Nothing was overwritten \
                     and the branch is retained. Reconcile the two — fetch {branch}, and replay \
                     or merge what the host has into it — and then land it with `{command}`",
                    tip = tip.as_str(),
                ),
                Declined::Gone => format!(
                    "{branch:?} is gone from the host, which this run last had at {replaced}, and \
                     this publication replayed it onto {base:?} — so the push would put a branch \
                     back that somebody deleted, out of a history nobody there has seen. Nothing \
                     was pushed and the branch is retained. Fetch {branch} so this run sees the \
                     host as it stands — every fetch here prunes — and then land it with \
                     `{command}`"
                ),
            },
        });
    }

    // Everything past here reads the *host*, and the push above has already reached
    // the remote: from this point a failure is a merge path this build could not
    // read rather than a publication that never landed. The commit is taken from the
    // tree that was pushed rather than asked of the remote, because asking the remote
    // is one more read that can fail for the very reason being reported.
    let pushed_at = git::tip(&context.worktree, &context.branch);
    // The push — and the `pre-push` hook it ran — is never interrupted, so a caller
    // that cancelled while it ran is answered here, once it has returned and before
    // anything on the host is opened or watched.
    if context.cancellation.is_cancelled() {
        return Err(cancelled_while(&format!(
            "its publishing push ran; {:?} is on origin{}",
            context.branch,
            pushed_at
                .as_deref()
                .map_or_else(String::new, |sha| format!(" at {sha}")),
        )));
    }
    // The commit is what the watch below decides *which* of the host's checks are
    // about this publication, so a tree that will not say what it just pushed is a
    // merge path this build cannot read rather than one it reads about some other
    // commit. It reaches the operator as exactly that, through `unverified`.
    match pushed_at.as_deref() {
        Some(sha) => land_as_change(context, stream, subject, &Sha(sha.to_owned())),
        None => Err(crate::error::invalid(format!(
            "{:?} was pushed and this worktree will not say which commit it is at, so which of \
             the host's checks are about what was just pushed cannot be decided",
            context.branch
        ))),
    }
    .map_err(|unread| unverified(context, pushed_at.as_deref(), unread))
}

/// Refuse an identity that has no host at all, before anything is pushed.
///
/// Half of [`change_host`]'s question, and only that half: a **hosted** identity this
/// build has no implementation for is left until after the push, because there the
/// branch reaching the origin is not what is missing, and `edges.rs` holds that.
fn refuse_an_unhosted_identity(identity: &str) -> Result<()> {
    match change_host(identity) {
        Ok(_) | Err(Error::NotImplemented { .. }) => Ok(()),
        Err(no_host) => Err(no_host),
    }
}

/// A push that reached the remote and a merge path that could not then be read,
/// reported as the one thing it is rather than as a publication that failed.
///
/// This covers the answers nobody got. What passes through is every failure the
/// contract already fixes a kind for — [`Error::GateFailed`] included, which it fixes
/// for a host that took a merge and then reported it unperformed: that shares this
/// defect's shape, and re-pointing a meaning is an amendment somebody approves. A
/// conflict the host confirmed and a caller's cancellation pass through too: both are
/// answers, and neither is a merge path that could not be read.
fn unverified(context: &Context<'_>, pushed_at: Option<&str>, unread: Error) -> Error {
    match unread {
        verdict @ (Error::ChecksFailed { .. }
        | Error::ChecksUnsettled { .. }
        | Error::GateFailed { .. }
        | Error::NotImplemented { .. }
        | Error::SyncConflict { .. }
        | Error::Cancelled { .. }) => verdict,
        unread => Error::PushedUnverified {
            reason: format!(
                "{branch:?} is on origin{at} and the merge path could not be read: {unread}. The \
                 work reached the remote, so re-publishing it would repeat what already landed \
                 there — read the change request on the host, and land the branch with \
                 `{command}` once the host answers",
                branch = context.branch,
                at = pushed_at.map_or_else(String::new, |sha| format!(" at {sha}")),
                command = guidance::command([
                    "onevcs",
                    "publish-branch",
                    &context.branch,
                    "--repo",
                    &context.resolution.publication.to_string_lossy(),
                ]),
            ),
        },
    }
}

/// Open or adopt the change request for a branch **already on the remote**, and ask
/// the host to land it.
///
/// Separate from the push above so that what every failure in here has in common is a
/// property of the function rather than a comment somebody has to keep true.
///
/// **The draft lifecycle** is here. A publication carrying no [`DraftReason`] opens its
/// change request as a draft while the pushed commit's required checks run, records
/// `change-drafted` with `kind: "awaiting-checks"`, and once they settle lifts or keeps
/// it by the policy's row of [`policy::GreenDraft`]: every change policy watches, and
/// the lift comes **before** the merge is armed or asked for, because a host will do
/// neither to a draft. Adopting a draft is the same — it is not lifted on the spot but
/// watched into a lift or a keep. Adopting a change the host holds ready never
/// re-drafts it: the lift is one-way. `drafts: {disabled: true}` opens ready and lifts
/// an adopted draft at once, as before the lifecycle.
fn land_as_change(
    context: &Context<'_>,
    stream: &mut Stream,
    subject: &str,
    pushed: &Sha,
) -> Result<PublishOutcome> {
    let slug = change_host(&context.resolution.key)?;
    let host = context.hosting.for_repo(&slug)?;
    // Who the host believes is calling travels with the change: a change request
    // opened by an identity nobody expected is the thing an operator reads this to
    // find out.
    let author = host.authenticated_user()?;
    let lifecycle = !context.drafts.disabled;

    let existing = host.find_changes(&context.branch, context.target.base())?;
    // Whether the host already held this change request: with the lifecycle off,
    // that decides whether there can be a draft to lift, since a change this
    // publication just opened without a reason is one nobody drafted.
    let adopted = !existing.is_empty();
    let change = match existing.into_iter().next() {
        Some(change) => change,
        None => host.open_change(ChangeSpec {
            head: context.branch.to_string(),
            base: context.target.base().to_string(),
            title: subject.to_owned(),
            // Exactly what the caller passed, or nothing. The caller is the layer
            // that knows what the change is for; this one only knows the branch,
            // and everything it could say from that the title already says.
            body: context.body.clone(),
            // The reason travels to the host so that the host can open it as a
            // draft, and no further: what the host renders of a draft is the state,
            // never the reason. The reason is recorded below, on the stream.
            draft: context.draft.clone(),
            // No reason, and the lifecycle on: a draft while the checks run.
            draft_awaiting_checks: lifecycle && context.draft.is_none(),
        })?,
    };
    stream.emit(
        EventKind::ChangeOpened,
        object(json!({
            "url": change.url.to_string(),
            "host": "github",
            "id": change.id.0,
            "base": change.base,
            "author": author,
        })),
    );

    if let Some(reason) = &context.draft {
        return hold_as_draft(context, host.as_ref(), &change, reason, stream);
    }
    let drafted = if lifecycle {
        await_as_draft(host.as_ref(), &change, stream)?
    } else {
        if adopted {
            lift_any_draft(host.as_ref(), &change, stream)?;
        }
        false
    };

    let mut watcher = Watcher::new(
        host.as_ref(),
        &change,
        pushed,
        context.drafts.warn_on_early_lift,
        context.cancellation,
    )?;
    let green = policy::GreenDraft::of(context.effective, context.policy.approvals);
    if context.effective == MergePolicy::ChangeOpen {
        // No merge is asked for, so the identity's merge queue is not taken: what
        // this waits for is the verdict a person reviews against, not a turn.
        if settle(&mut watcher, stream, drafted)? {
            if green == policy::GreenDraft::KeepForReview {
                stream.emit(
                    EventKind::DraftKeptForReview,
                    object(json!({
                        "url": change.url.to_string(),
                        "id": change.id.0,
                        "base": change.base,
                    })),
                );
                return Ok(PublishOutcome::ChangeReviewDraft(change.url.clone()));
            }
            lift_draft(host.as_ref(), &change, stream)?;
        }
        return Ok(PublishOutcome::ChangeOpen(change.url.clone()));
    }

    // Everything automated is serialized against the identity, so two sessions of
    // one repository cannot ask the host to land two changes at once.
    let identity = lock::git_identity(&git::common_dir(&context.resolution.publication)?);
    let turn = queue::turn(&identity, context.cancellation)?;
    stream.emit(
        EventKind::LockWait,
        object(json!({
            "identity": identity,
            "elapsed": turn.waited.as_secs_f64(),
            "queue_position": turn.position,
        })),
    );
    stream.emit(
        EventKind::LockAcquired,
        object(json!({"identity": identity})),
    );

    let outcome = (|| -> Result<PublishOutcome> {
        // What is watched, and until when, follows the **merge policy** and nothing
        // else — least of all anything a repository *calls* its verification. A
        // change-direct publication asks for the merge itself, so it waits for the
        // checks the host says block one first; a draft waits for its checks under
        // either policy, because it is lifted on their verdict and a host will
        // neither merge a draft nor arm its own merge on one.
        if (drafted || context.effective == MergePolicy::ChangeDirect)
            && settle(&mut watcher, stream, drafted)?
        {
            lift_draft(host.as_ref(), &change, stream)?;
        }
        stream.emit(
            EventKind::MergeQueued,
            object(json!({
                "identity": identity,
                "queue_position": turn.position,
                "url": change.url.to_string(),
            })),
        );
        let sha = if context.effective == MergePolicy::ChangeAuto {
            // Arming happens inside the watch, and after its first reading of the
            // checks: a host that will not say what its checks are is refused with
            // nothing armed against it, and one loop rather than a read followed by a
            // watch reports each transition exactly once.
            watch_the_merge(&mut watcher, stream)?
        } else {
            match host.merge(&change, context.effective)? {
                MergeOutcome::Merged(sha) => sha,
                MergeOutcome::Queued => return Ok(PublishOutcome::Queued(change.url.clone())),
                MergeOutcome::Open => return Ok(PublishOutcome::ChangeOpen(change.url.clone())),
            }
        };
        stream.emit(
            EventKind::ChangeMerged,
            object(json!({"url": change.url.to_string(), "sha": sha.0})),
        );
        stream.emit(
            EventKind::MergeCompleted,
            object(json!({"identity": identity, "sha": sha.0})),
        );
        record_landing(context, &sha);
        record_baselines(context, &sha.0, stream);
        fast_forward_publication(&context.resolution.publication, context.target.base())?;
        Ok(PublishOutcome::Merged(sha))
    })();
    drop(turn);
    outcome
}

/// Whether the change a reasonless publication holds is a draft awaiting its checks,
/// recording it as one where it is.
///
/// Asked of the host rather than assumed from having asked for one, both for a change
/// this publication just opened — a host written before `draft_awaiting_checks`
/// opens it ready, and a ready change is watched as one — and for one it adopted,
/// whatever drafted it: a held draft, a release-awaiting one, or one an earlier red run
/// left awaiting its checks is watched into a lift or a keep rather than lifted on the
/// spot. A change the host holds **ready** is never asked to be a draft again, and
/// records nothing: lifting is one-way.
///
/// A host that was never taught to say whether it drafted anything has drafted
/// nothing this can lift, so it answers "not a draft" exactly as [`lift_any_draft`]
/// reads it; a host that could not say is a refusal.
fn await_as_draft(
    host: &dyn RemoteHost,
    change: &ChangeRequest,
    stream: &mut Stream,
) -> Result<bool> {
    match host.is_draft(change) {
        Ok(true) => {}
        Ok(false) | Err(Error::NotImplemented { .. }) => return Ok(false),
        Err(unreadable) => return Err(unreadable),
    }
    stream.emit(
        EventKind::ChangeDrafted,
        object(json!({
            "url": change.url.to_string(),
            "id": change.id.0,
            "base": change.base,
            "kind": AWAITING_CHECKS,
        })),
    );
    Ok(true)
}

/// The `kind` a `change-drafted` payload carries for a draft the lifecycle opened
/// while the checks run — the one kind that is no [`DraftReason`], because nobody
/// asked for it.
const AWAITING_CHECKS: &str = "awaiting-checks";

/// Hold a change request open as a draft, and record why.
///
/// The publication stops here under **every** change policy: a draft is unmergeable
/// in that state, so nothing below this asks the host to merge it, arms the host's
/// own merge on it, takes the identity's merge queue, or fast-forwards a base from
/// it. Which policy the identity publishes under decides what happens once the draft
/// is lifted, not what happens while it stands.
fn hold_as_draft(
    context: &Context<'_>,
    host: &dyn RemoteHost,
    change: &ChangeRequest,
    reason: &DraftReason,
    stream: &mut Stream,
) -> Result<PublishOutcome> {
    // Asked of the host rather than assumed from having said `--draft`, and the two
    // cases it separates are both real: a host whose `open_change` was written before
    // this field silently ignores it, and a change request already open for review is
    // one this crate will not put back into a draft — the seam has no method for it,
    // and inventing one would be a public item nobody approved. Either way the answer
    // is the same refusal, because either way the change on the host can land while
    // its caller believes it cannot.
    if !host.is_draft(change)? {
        return Err(already_open_for_review(context, change, reason));
    }
    // The publication record, and the only place the reason is written: see
    // [`DraftReason`].
    let mut payload = reason.fields();
    let fields = payload.as_object_mut().expect("the reason is an object");
    fields.insert("url".to_owned(), json!(change.url.to_string()));
    fields.insert("id".to_owned(), json!(change.id.0));
    fields.insert("base".to_owned(), json!(change.base));
    stream.emit(EventKind::ChangeDrafted, object(payload));
    Ok(PublishOutcome::ChangeDraft(change.url.clone()))
}

/// The refusal for a draft asked over a change request the host has open for review.
///
/// One sentence, two places: `hold_as_draft` reaches it once the change is adopted,
/// and the pre-push check below reaches it before anything is on the remote. A
/// refusal that read differently depending on which noticed first would be two rules
/// about one state.
fn already_open_for_review(
    context: &Context<'_>,
    change: &ChangeRequest,
    reason: &DraftReason,
) -> Error {
    crate::error::invalid(format!(
        "{url} is open for review on the host, and this publication asked for {asked}. A change \
         that is open can land, so reporting it as a draft would say the work is held back when \
         nothing is holding it. Lift nothing and publish {branch:?} without a draft, or close \
         that change request and publish again",
        url = change.url,
        asked = reason.asked_for(),
        branch = context.branch,
    ))
}

/// Refuse a draft over a change request the host already has open for review, before
/// anything reaches the remote.
///
/// The same question [`hold_as_draft`] asks after the change is adopted, asked at the
/// boundary as well — because everything past the publishing push is reported as a
/// merge path this build could not read, and this is not one of those. The merge path
/// ruled on the push perfectly well; what stopped the publication is a state the host
/// *answered*, and an operator told their push is unverified would go looking for a
/// failure that never happened.
///
/// **Only a definite answer refuses.** A host this build has no implementation for,
/// one that cannot be reached, and one that will not say whether a change is a draft
/// are each left exactly where they are met today — after the push, by the paths that
/// already handle them — so this can only ever move a refusal *earlier*, never invent
/// one. `hold_as_draft` stays the authoritative check for the same reason it existed
/// before this: a host may take `--draft` and open an ordinary change anyway.
fn refuse_a_draft_over_a_reviewed_change(
    context: &Context<'_>,
    reason: &DraftReason,
) -> Option<Error> {
    let host = change_host(&context.resolution.key)
        .ok()
        .and_then(|slug| context.hosting.for_repo(&slug).ok())?;
    let open = host
        .find_changes(&context.branch, context.target.base())
        .ok()?
        .into_iter()
        .next()?;
    match host.is_draft(&open) {
        Ok(false) => Some(already_open_for_review(context, &open, reason)),
        _ => None,
    }
}

/// Lift any draft on a change request, at once.
///
/// What `onevcs change ready` asks for, and what a publication carrying no
/// [`DraftReason`] does with a draft it adopts when the rules file says `drafts:
/// {disabled: true}` — the behaviour before the draft lifecycle. With the lifecycle
/// on, an adopted draft is watched into a lift or a keep instead (`await_as_draft`).
///
/// **Idempotent, because the host decides.** A change request that is not a draft is
/// asked for nothing, so a second publication after a lift makes no call and reports
/// exactly what the first one did.
///
/// Asked only of a change request the host already held. One this publication opened
/// moments ago, without a reason, is one nobody drafted — so a publication that
/// opens its own change request asks the host nothing extra, and every implementation
/// written against the earlier surface goes on publishing exactly as it did.
///
/// A host that cannot say is a host that was never taught to draft one, and it is
/// passed over rather than refused — `is_draft` is defaulted to
/// [`Error::NotImplemented`], so this is the answer every implementation written
/// against the earlier surface gives, and refusing it would break publications that
/// have nothing to do with drafts. The cost is bounded and safe in the one direction
/// that matters: a host that drafts a change and cannot be asked about it leaves the
/// draft standing, and a draft that stands is a change that does not land.
pub(crate) fn lift_any_draft(
    host: &dyn RemoteHost,
    change: &ChangeRequest,
    stream: &mut Stream,
) -> Result<()> {
    match host.is_draft(change) {
        Ok(true) => {}
        Ok(false) => return Ok(()),
        Err(Error::NotImplemented { .. }) => return Ok(()),
        Err(unreadable) => return Err(unreadable),
    }
    lift_draft(host, change, stream)
}

/// Ask the host to take a change it holds as a draft out of it, and record the lift.
///
/// The one place a lift is performed and recorded, whichever path decided on it: a
/// publication lifting on green checks or early, `onevcs change ready`, and a
/// publication under `drafts: {disabled: true}` lifting an adopted draft.
fn lift_draft(host: &dyn RemoteHost, change: &ChangeRequest, stream: &mut Stream) -> Result<()> {
    host.ready_for_review(change)?;
    stream.emit(
        EventKind::DraftLifted,
        object(json!({"url": change.url.to_string(), "id": change.id.0})),
    );
    Ok(())
}

/// Record the commit the host merged this change at, on the branch itself.
///
/// The most certain answer to "did this branch land": a landing that was *recorded*
/// rather than inferred from what the base happens to carry. The record is one
/// provenance trailer, `<prefix>Landed-Commit: <sha>`, under the same configured
/// prefix every other trailer this crate reads and writes uses — so a host that
/// spells its provenance differently reads its own landings and not somebody
/// else's. It goes on an otherwise empty commit, because the branch's content is
/// exactly what merged and a record that changed it would leave the branch
/// disagreeing with the base it has just reached.
///
/// Best effort, deliberately. The change has already merged by the time this runs,
/// and reporting the publication as failed because its own footnote could not be
/// written would be a worse lie than the missing line — the same rule the event
/// stream is written under. What went wrong is said on stderr, where the operator
/// running the command sees it.
fn record_landing(context: &Context<'_>, sha: &Sha) {
    if let Err(error) = write_landing(context, sha) {
        eprintln!(
            "onevcs: warning: {branch:?} merged at {merged}, and the landing was not recorded on \
             the branch: {error}",
            branch = context.branch,
            merged = sha.0,
        );
    }
}

/// Record what each of this identity's **automated** release targets had at the
/// moment this change landed.
///
/// A baseline is what makes "has this change been released" answerable later: a
/// probe answering a strictly greater version afterwards is the release that
/// carries this work. It is captured here, at the landing, because a reading taken
/// any later cannot tell a release that carries this change from one that predates
/// it.
///
/// Nothing is probed for a human-step target, because there is nothing to probe —
/// what a landing starts for one of those is a wait a person ends. And nothing at
/// all happens for an identity with no release targets, which is every identity on
/// a host with no release-targets file.
///
/// Best effort, exactly as [`record_landing`] above is and for the same reason: the
/// change has already merged, and reporting the publication as failed because its
/// own baseline could not be captured would be a worse lie than the missing record.
fn record_baselines(context: &Context<'_>, sha: &str, stream: &mut Stream) {
    match crate::store::load() {
        Ok(registry) => {
            crate::release::record_baselines(&registry, &context.resolution.key, sha, stream)
        }
        Err(failure) => eprintln!(
            "onevcs: warning: the release baselines for {identity} at landing {sha} were not \
             captured: {failure}",
            identity = context.resolution.key,
        ),
    }
}

fn write_landing(context: &Context<'_>, sha: &Sha) -> Result<()> {
    git::commit_empty(
        &context.worktree,
        &format!(
            "chore: record the landing of {branch}\n\n{key} {merged}",
            branch = context.branch,
            key = context.provenance.landed(),
            merged = sha.0,
        ),
    )?;
    // The repository this publication worked in goes with its run root, so the
    // record has to reach the checkout that keeps the branch. Fast-forward only,
    // like every other hand-back here: a checkout holding work this run does not
    // have keeps it.
    if !git::copy_branch(&context.repo, &context.preserved_into, &context.branch)? {
        return Err(crate::error::invalid(format!(
            "the checkout {} would not take the branch, so nothing outside this run carries the \
             record",
            context.preserved_into.display()
        )));
    }
    Ok(())
}

/// How much of a verifier's log travels on the failure that names it.
///
/// Well under the envelope's own 4096-byte payload limit, because this is a
/// pointer to the evidence and not a second copy of it: the whole log is the
/// artifact the event beside it already carries, fetched with `onevcs artifact
/// cat`. One bound for both verifiers this crate reports on — a host's required
/// check and the repository's own merge path — because the reason is the same for
/// each: how long a verification's log runs is the verifier's business, and neither
/// failure is the place to carry the whole of one.
const LOG_EXCERPT: usize = 2048;

/// The end of a verifier's log, bounded at a line boundary.
///
/// The *end*, because a verification prints its diagnosis last and its setup first —
/// an excerpt taken from the top of a twenty-thousand-line log is the part nobody
/// needed. That is not a guess about CI: a merge path that refused a publication
/// here put its one finding in the last twelve lines of seventy-six thousand bytes,
/// and the bounded head that travelled on the `push` event was the toolchain warming
/// up. What was cut is marked, so a reader can tell an excerpt from a short log, and
/// `whole` says where the rest of it is — which the failure has already named.
fn excerpt(log: &str, whole: &str) -> String {
    let trimmed = log.trim_end();
    if trimmed.len() <= LOG_EXCERPT {
        return trimmed.to_owned();
    }
    let mut cut = trimmed.len() - LOG_EXCERPT;
    while cut < trimmed.len() && !trimmed.is_char_boundary(cut) {
        cut += 1;
    }
    let tail = &trimmed[cut..];
    let tail = tail.find('\n').map_or(tail, |at| &tail[at + 1..]);
    format!("[…earlier output omitted; {whole}…]\n{tail}")
}

/// What a host's answer about a change request says about **one commit**.
///
/// The publication path's whole defence against a verdict that is about the wrong
/// thing, and it is a type rather than a filter so the distinction cannot be
/// dropped by accident: "the host has said nothing about this commit yet" and "the
/// host says nothing blocks it" are the same empty list and opposite answers. A
/// caller reaches the checks only by having said which of the two it is holding.
///
/// A publication watches the commit it just pushed. The host attaches that commit's
/// checks seconds to minutes later, and until it does, the change request still
/// carries the *previous* head's — a red one of which used to end the publication
/// with a verdict that predated the check it claimed to have read.
enum Reported {
    /// What this answer holds about the commit: the checks the host attached to it,
    /// and the checks the host attached to no commit at all. Empty is an answer —
    /// the one a repository that declares no check gives — and it is only ever
    /// reached when the host reported no checks whatsoever.
    About(Vec<Check>),
    /// Every check the host reported names some *other* commit, so it has said
    /// nothing about this one. What a head pushed moments ago looks like.
    NotYet,
}

impl Reported {
    /// Read one host answer as what it says about `commit`.
    fn of(answered: Vec<Check>, commit: &Sha) -> Self {
        let reported = !answered.is_empty();
        let about: Vec<Check> = answered
            .into_iter()
            .filter(|check| is_about(check, commit))
            .collect();
        if about.is_empty() && reported {
            return Self::NotYet;
        }
        Self::About(about)
    }

    /// The checks that are about the commit, which is none of them until the host
    /// has said anything about it.
    fn checks(&self) -> &[Check] {
        match self {
            Self::About(checks) => checks,
            Self::NotYet => &[],
        }
    }
}

/// Whether one check is the host's answer *about* `commit`.
///
/// Two cases, and the second is why this is not equality: a check the host attached
/// to `commit` is about it, and so is a check the host named no commit for at all.
/// A host that will not say which head its checks belong to is a host whose checks
/// are read exactly as they were before any of them carried one — declining to
/// answer is not a reason to stall a publication until its bound. Only a check
/// naming some *other* commit is set aside.
///
/// Private, and it belongs here rather than on [`Check`]: it is this path's reading
/// of the host's answer, not a property of the answer, and the approved surface
/// declares the field and no method over it.
fn is_about(check: &Check, commit: &Sha) -> bool {
    check.head.as_ref().is_none_or(|head| head == commit)
}

/// One reading of the host, narrowed to the pushed commit, with whether the answer
/// came from the host's complete rollup.
struct Reading {
    reported: Reported,
    /// Whether the checks were read from the source that sees every check — which is
    /// what makes "none of them is required" the host's own answer that nothing is.
    complete: bool,
}

/// Which checks a repository requires before a merge, as far as the host said.
///
/// Three answers, never two: "none is required" is the one a draft may act on at once,
/// and it is only ever the host's **complete** answer. An incomplete or unreadable one
/// is [`Declared::Unknown`], which waits.
#[derive(Debug, Clone, PartialEq, Eq)]
enum Declared {
    /// The host answered, completely, that nothing is required.
    Nothing,
    /// The checks the host names as required, from every source that answered.
    Names(BTreeSet<String>),
    /// The host did not say — a source it could not read, or no answer at all. What
    /// is required is then read off the checks' own `required` as they arrive.
    Unknown,
}

/// What the host says it requires before a merge into this change's base.
///
/// Asked once, before the draft's checks are watched, through the same seam the audit
/// reads. A host that was never taught to answer, or would not, has not said nothing
/// is required.
///
/// Beside a [`Declared::Unknown`] is why the declaration could not be read, which is
/// what a settlement read from the host's own per-check marking has to say it was: by
/// the manager's ruling on the amendment, such a green must never read as an ordinary
/// complete answer.
fn declared_required(host: &dyn RemoteHost, change: &ChangeRequest) -> (Declared, Option<String>) {
    match host.required_checks_on(&change.base) {
        Ok(answer) if answer.checks.is_empty() && answer.complete() => (Declared::Nothing, None),
        Ok(answer) if !answer.checks.is_empty() => (Declared::Names(answer.checks), None),
        Ok(answer) => (
            Declared::Unknown,
            Some(format!(
                "the host could not read {} for which checks a merge into {} requires",
                answer
                    .unconsulted
                    .keys()
                    .map(|source| source.describe())
                    .collect::<Vec<_>>()
                    .join(" or "),
                change.base
            )),
        ),
        Err(refused) => (
            Declared::Unknown,
            Some(format!(
                "the host would not say which checks a merge into {} requires: {refused}",
                change.base
            )),
        ),
    }
}

impl Declared {
    /// This answer, sharpened by one reading: where the host would not say what it
    /// requires, a **complete** rollup reporting checks of which none is required is
    /// that same answer given another way — `gh pr checks --required`'s "no required
    /// checks reported" — and is read as it.
    fn given(&self, reading: &Reading) -> Declared {
        match (self, &reading.reported) {
            (Declared::Unknown, Reported::About(checks))
                if reading.complete
                    && !checks.is_empty()
                    && checks.iter().all(|check| !check.required) =>
            {
                Declared::Nothing
            }
            _ => self.clone(),
        }
    }
}

/// Where one required check stands in a reading: its name, the state its entries
/// add up to — `None` where the host reported none — and the entry that decided it.
struct Standing<'c> {
    name: String,
    state: Option<CheckState>,
    check: Option<&'c Check>,
}

/// Each required check's standing in `checks`.
///
/// The names come from the host's declaration where it made one, so a required
/// check it has not reported yet is *absent* rather than overlooked; otherwise from
/// the checks' own `required`. A name reported more than once — a run re-attached
/// after a lift, a run that ended with no verdict and was re-run — stands at the most
/// decisive of its entries: a red one fails it, a running one holds it, and a passed
/// one outranks a run that ended with no verdict or was skipped, which is how a later
/// green run of a check that was cancelled or skipped proceeds.
fn standings<'c>(checks: &[&'c Check], declared: &Declared) -> Vec<Standing<'c>> {
    let names: BTreeSet<String> = match declared {
        Declared::Names(names) => names.clone(),
        Declared::Nothing => BTreeSet::new(),
        Declared::Unknown => checks
            .iter()
            .filter(|check| check.required)
            .map(|check| check.name.clone())
            .collect(),
    };
    let rank = |state: CheckState| match state {
        CheckState::Failed => 0,
        CheckState::Pending => 1,
        CheckState::Passed => 2,
        CheckState::NoVerdict => 3,
        CheckState::Skipped => 4,
    };
    names
        .into_iter()
        .map(|name| {
            let decided = checks
                .iter()
                .copied()
                .filter(|check| check.name == name)
                .min_by_key(|check| rank(check.state()));
            Standing {
                state: decided.map(Check::state),
                check: decided,
                name,
            }
        })
        .collect()
}

/// The standings' red one, where a required check concluded red.
fn red<'c>(standings: &[Standing<'c>]) -> Option<&'c Check> {
    standings
        .iter()
        .find(|standing| standing.state == Some(CheckState::Failed))
        .and_then(|standing| standing.check)
}

/// A publication watching one change request at one commit, across every phase of
/// its watch — the draft's, the one after an early lift, and the ready change's up to
/// the merge — so that each check transition is reported once and one bound covers
/// the whole of it.
///
/// Three ways out of any phase, and each one says which it was: the phase's own
/// ending, a **required** check concluding red ([`Error::ChecksFailed`], naming it and
/// quoting its log), or the bound elapsing ([`Error::ChecksUnsettled`], naming what was
/// still pending). A caller routing a failure has to be able to tell "CI said no" from
/// "nobody answered in an hour".
///
/// `pushed` is what the checks are read *about*, and it is the commit this run put on
/// the remote rather than the head the host reports for the change request — which is
/// the same value only once the host has noticed the push. [`Reported`] is where that
/// is decided, and everything past it — the stream, the red-check verdict, and the
/// bound's own sentence — sees only checks about `pushed`.
/// Where one check stood when it was last reported: its status, its conclusion, and
/// when the run started.
type Transition = (String, Option<String>, Option<String>);

struct Watcher<'a> {
    host: &'a dyn RemoteHost,
    change: &'a ChangeRequest,
    pushed: &'a Sha,
    /// Asked between readings, at least every [`CANCELLATION_GRAIN`], so a caller's
    /// cancellation ends any phase of the watch whatever the poll interval is.
    cancellation: &'a dyn PublicationCancellation,
    bound: std::time::Duration,
    poll: std::time::Duration,
    started: std::time::Instant,
    /// What each check was last reported as, so only a transition is reported. Keyed
    /// on the status, the conclusion *and* the run's start, because a run the host
    /// re-attaches after a lift can complete exactly as the skipped one had, and it is
    /// still a run of its own.
    reported: Vec<(String, Transition)>,
    /// What each settled check's log was stored as, so the refusal that names a red
    /// check can quote it without fetching it from the host a second time.
    logs: Vec<(String, crate::event::ArtifactId)>,
    /// Whether `checks-settled` has been recorded, which it is once per watch.
    settled: bool,
    /// Whether an early lift prints its warning line: the resolved
    /// `drafts.warn_on_early_lift`.
    warn_on_early_lift: bool,
}

impl<'a> Watcher<'a> {
    fn new(
        host: &'a dyn RemoteHost,
        change: &'a ChangeRequest,
        pushed: &'a Sha,
        warn_on_early_lift: bool,
        cancellation: &'a dyn PublicationCancellation,
    ) -> Result<Self> {
        Ok(Self {
            warn_on_early_lift,
            host,
            change,
            pushed,
            cancellation,
            bound: std::time::Duration::from_secs_f64(gh::checks_timeout()?),
            poll: std::time::Duration::from_secs_f64(gh::checks_poll()?),
            started: std::time::Instant::now(),
            reported: Vec::new(),
            logs: Vec::new(),
            settled: false,
        })
    }

    /// Ask the host once, narrow its answer to the pushed commit, and report every
    /// check that moved.
    fn read(&mut self, stream: &mut Stream) -> Result<Reading> {
        // What was consulted travels with the checks and is deliberately not acted
        // on here: a credential that can see only GitHub Actions still gates a merge
        // on what it *can* see, and a credential that could see nothing was a
        // refusal rather than an empty answer.
        let answered = self.host.change_checks(self.change)?;
        let complete = answered.complete();
        // Narrowed to the commit this publication pushed before anything is reported
        // or acted on, so a check the host attached to a head this run replaced
        // reaches neither the stream nor the verdict.
        let reported = Reported::of(answered.checks, self.pushed);
        for check in reported.checks() {
            let now = (
                check.status.clone(),
                check.conclusion.clone(),
                check.started_at.clone(),
            );
            let previous = self
                .reported
                .iter()
                .find(|(name, _)| name == &check.name)
                .map(|(_, was)| was.clone());
            if previous.as_ref() == Some(&now) {
                continue;
            }
            let mut artifacts = Vec::new();
            if check.settled() {
                // The log records what the check printed; `conclusion`, already read,
                // is what decides whether it blocks. So a host that will not hand
                // one over is reported the way a stream that cannot be written is —
                // on stderr, without failing the command over it.
                match self.host.check_log(self.change, check) {
                    Ok(id) => {
                        self.logs.retain(|(name, _)| name != &check.name);
                        self.logs.push((check.name.clone(), id.clone()));
                        artifacts.push(crate::event::ArtifactRef {
                            id: id.0,
                            kind: "log".to_owned(),
                            bytes: 0,
                        });
                    }
                    Err(error) => eprintln!(
                        "onevcs: warning: check {:?} on {} is recorded without its log: {error}",
                        check.name, self.change.url
                    ),
                }
            }
            stream.emit_with(
                EventKind::ChangeCheck,
                object(json!({
                    "name": check.name,
                    "required": check.required,
                    "status": check.status,
                    "from_status": previous.map(|(status, _, _)| status),
                    "conclusion": check.conclusion,
                    "state": check.state(),
                })),
                artifacts,
            );
            self.reported.retain(|(name, _)| name != &check.name);
            self.reported.push((check.name.clone(), now));
        }
        Ok(Reading { reported, complete })
    }

    /// The refusal for a required check that concluded red.
    fn failed(&self, check: &Check) -> Error {
        checks_failed(check, &self.logs)
    }

    /// Whether the bound on the whole watch has elapsed.
    fn out_of_time(&self) -> bool {
        self.started.elapsed() >= self.bound
    }

    /// Wait out one interval between readings, or end the watch as
    /// [`Error::Cancelled`] the moment the caller cancels it.
    fn pause(&self) -> Result<()> {
        pause_unless_cancelled(self.poll, self.cancellation, || {
            cancelled_while(&format!(
                "it watched {url} at {commit}",
                url = self.change.url,
                commit = self.pushed.0,
            ))
        })
    }

    /// The refusal for a change request the host confirms conflicts with its base,
    /// quoting what the host reported.
    fn conflicting(&self, reported: &str) -> Error {
        Error::SyncConflict {
            reason: format!(
                "{url} conflicts with its base {base:?}: the host reports it {reported}, so the \
                 merge it was armed to perform can never happen. The branch stays on its remote \
                 at {commit} and the change request stays open — merge {base} into the branch, \
                 resolve the conflict, and publish the session again",
                url = self.change.url,
                base = self.change.base,
                commit = self.pushed.0,
            ),
        }
    }

    /// The bound elapsed, naming what the host had not settled.
    fn unsettled(&self, reading: &Reading, awaited: &str) -> Error {
        unsettled(
            self.change,
            self.pushed,
            &reading.reported,
            self.bound,
            awaited,
        )
    }

    /// Record, once, that the required checks stopped blocking — `passed`, or
    /// `passed-with-skipped` naming the required checks that concluded skipped.
    ///
    /// `unread` is set where the draft's requirement was read from the host's own
    /// per-check `required` marking because its declaration could not be read — and
    /// then the record says so, in the payload's `requirement` and on stderr, so the
    /// settlement never reads as the host's ordinary complete answer. Omitted
    /// otherwise, so every other settlement's payload is the five fields it always was.
    fn record_settled(&mut self, stream: &mut Stream, skipped: Vec<String>, unread: Option<&str>) {
        if std::mem::replace(&mut self.settled, true) {
            return;
        }
        let verdict = if skipped.is_empty() {
            "passed"
        } else {
            "passed-with-skipped"
        };
        let mut payload = object(json!({
            "url": self.change.url.to_string(),
            "id": self.change.id.0,
            "head": self.pushed.0,
            "verdict": verdict,
            "skipped": skipped,
        }));
        if let Some(because) = unread {
            eprintln!(
                "onevcs: warning: the required checks on {url} were read from the host's own \
                 per-check marking, because the declaration could not be read: {because}. The \
                 host's merge path still rules on any merge",
                url = self.change.url,
            );
            payload.insert(
                "requirement".to_owned(),
                json!({"read_from": "host-marking", "because": because}),
            );
        }
        stream.emit(EventKind::ChecksSettled, payload);
    }
}

/// Wait until the change's required checks stop blocking, and answer whether it is
/// still a draft then.
///
/// A change that is not a draft is watched as it always was — [`settle_ready`]. A
/// draft is watched by the draft's rule, [`settle_draft`]: a skipped required check
/// on it has **not run**, and a draft none of whose required checks has run by the
/// grace window is lifted so that they can, after which it is a ready change whose
/// checks must run again. `true` means the checks came back green on the draft, which
/// is still standing for the caller to lift or keep.
fn settle(watcher: &mut Watcher<'_>, stream: &mut Stream, drafted: bool) -> Result<bool> {
    if drafted {
        settle_draft(watcher, stream)
    } else {
        settle_ready(watcher, stream).map(|()| false)
    }
}

/// Wait until every required check the host reports has settled without blocking.
///
/// What a `change-direct` publication does before it asks the host to merge, and what
/// every ready change does before its policy's ending: this run performs the merge,
/// or reports the change as open, so an answer the host's own checks have already
/// refused is one to report rather than act past.
///
/// A skipped required check satisfies this, as it satisfies the host's own merge path
/// — and the watch then records `checks-settled` as `passed-with-skipped`, naming it,
/// so the record says what the host accepted rather than calling it green. A host that
/// reports **no required check at all** is not a stall here. It has answered, and its
/// answer is that nothing blocks; `change-auto` is the policy that fails closed on
/// that, because its watch ends at a merge the host performs.
fn settle_ready(watcher: &mut Watcher<'_>, stream: &mut Stream) -> Result<()> {
    loop {
        let reading = watcher.read(stream)?;
        if let Reported::About(checks) = &reading.reported {
            let about: Vec<&Check> = checks.iter().collect();
            let standing = standings(&about, &Declared::Unknown);
            if let Some(failed) = red(&standing) {
                return Err(watcher.failed(failed));
            }
            if standing.iter().all(|standing| {
                matches!(
                    standing.state,
                    Some(CheckState::Passed | CheckState::Skipped)
                )
            }) {
                watcher.record_settled(stream, skipped_in(&standing), None);
                return Ok(());
            }
        }
        // Otherwise the host has posted nothing about the commit this publication
        // pushed, or something it requires is still running. The first is a clock and
        // not an answer — the checks it *is* reporting belong to a head this run
        // replaced — so the watch keeps waiting rather than merging on an emptiness
        // that is really somebody else's verdict having been filtered out.
        if watcher.out_of_time() {
            return Err(watcher.unsettled(&reading, "settled its required checks on"));
        }
        watcher.pause()?;
    }
}

/// The required checks that concluded skipped, by name.
fn skipped_in(standing: &[Standing<'_>]) -> Vec<String> {
    standing
        .iter()
        .filter(|standing| standing.state == Some(CheckState::Skipped))
        .map(|standing| standing.name.clone())
        .collect()
}

/// Watch a draft's required checks until they are green on it, lifting it early where
/// none of them has run by the grace window. `true` is green on the draft; `false` is
/// a draft lifted early whose checks then ran green as a ready change's.
///
/// **A skipped required check has not run** while the change is a draft: a workflow
/// that gates a job on the change not being one reports `skipped` without having
/// verified anything. So green on a draft is every required check `passed`, and a
/// draft where the only thing stopping that is checks that have not run — absent, or
/// skipped — is the draft-skipping repository: once the grace window elapses it is
/// lifted, `draft-lifted` and `draft-lifted-early` are recorded, one warning line is
/// printed unless `drafts.warn_on_early_lift` is off, and the watch goes on as
/// [`settle_after_early_lift`].
///
/// A host that says, completely, that nothing is required is green at once: no grace
/// window and no warning.
fn settle_draft(watcher: &mut Watcher<'_>, stream: &mut Stream) -> Result<bool> {
    let grace = std::time::Duration::from_secs_f64(gh::draft_grace()?);
    let (declared, unread) = declared_required(watcher.host, watcher.change);
    loop {
        let reading = watcher.read(stream)?;
        let declared = declared.given(&reading);
        if declared == Declared::Nothing {
            watcher.record_settled(stream, Vec::new(), None);
            return Ok(true);
        }
        // Whether what is required is being read off the host's marking because the
        // declaration could not be read, which a settlement then has to say.
        let marked = (declared == Declared::Unknown)
            .then_some(unread.as_deref())
            .flatten();
        let about: Vec<&Check> = reading.reported.checks().iter().collect();
        let standing = standings(&about, &declared);
        if let Some(failed) = red(&standing) {
            return Err(watcher.failed(failed));
        }
        if !standing.is_empty()
            && standing
                .iter()
                .all(|standing| standing.state == Some(CheckState::Passed))
        {
            watcher.record_settled(stream, Vec::new(), marked);
            return Ok(true);
        }
        let not_run: Vec<String> = standing
            .iter()
            .filter(|standing| matches!(standing.state, None | Some(CheckState::Skipped)))
            .map(|standing| standing.name.clone())
            .collect();
        let running = standing.iter().any(|standing| {
            matches!(
                standing.state,
                Some(CheckState::Pending | CheckState::NoVerdict)
            )
        });
        // Nothing is known to be required and nothing reported is marked required: a
        // host that will not say what a merge needs. That is never read as "none", so
        // it is not green at once — it waits the grace window. Then, where checks ran
        // on the draft and none of them was skipped, the host's own marking is the
        // answer, exactly as a ready change's watch reads it; where nothing ran, or
        // something was skipped, it is the draft-skipping repository, and only a lift
        // can show what runs.
        let unseen = declared == Declared::Unknown && standing.is_empty();
        let grace_elapsed = watcher.started.elapsed() >= grace;
        let about = reading.reported.checks();
        if unseen
            && grace_elapsed
            && !about.is_empty()
            && about
                .iter()
                .all(|check| check.state() != CheckState::Skipped)
        {
            watcher.record_settled(stream, Vec::new(), marked);
            return Ok(true);
        }
        if grace_elapsed && !running && (!not_run.is_empty() || unseen) {
            let snapshot = reading.reported.checks().to_vec();
            lift_early(watcher, stream, &not_run, grace)?;
            return settle_after_early_lift(watcher, stream, &declared, marked, &snapshot, grace)
                .map(|()| false);
        }
        if watcher.out_of_time() {
            return Err(watcher.unsettled(&reading, "settled its required checks on"));
        }
        watcher.pause()?;
    }
}

/// Lift a draft whose required checks had not run on it, and say so.
fn lift_early(
    watcher: &mut Watcher<'_>,
    stream: &mut Stream,
    awaited: &[String],
    grace: std::time::Duration,
) -> Result<()> {
    let change = watcher.change;
    lift_draft(watcher.host, change, stream)?;
    let warned = watcher.warn_on_early_lift;
    if warned {
        eprintln!(
            "onevcs: warning: {url} was lifted out of its draft before its required checks ran \
             on it ({awaited} had not run within {seconds}s), which is what a workflow that \
             skips drafts looks like. It is watched as a ready change from here; set \
             `drafts: {{warn_on_early_lift: false}}` in the rules file to silence this",
            url = change.url,
            awaited = if awaited.is_empty() {
                "the host named none".to_owned()
            } else {
                guidance::listed(awaited)
            },
            seconds = grace.as_secs_f64(),
        );
    }
    stream.emit(
        EventKind::DraftLiftedEarly,
        object(json!({
            "url": change.url.to_string(),
            "id": change.id.0,
            "base": change.base,
            "awaited": awaited,
            "grace_seconds": grace.as_secs_f64(),
            "warned": warned,
        })),
    );
    Ok(())
}

/// Whether `check` is a run the host attached after the lift, rather than the run of
/// that check it reported on the draft — `snapshot`.
///
/// Decided by the run's own identity as the host reports it, never by what it
/// concluded or where it stands: a re-run a workflow starts on `ready_for_review` can
/// conclude `skipped` exactly as the draft's run did, and it is still a run. So a check
/// the draft never reported is new, and one it did report is new only where the host
/// reports a start for it that no draft-era run of that check had — running or settled
/// alike. One reporting no start, or the draft's own start, is the draft's — which is
/// the side a wrong answer must fall on, since a draft-era skip read as a post-lift one
/// is a merge nothing verified. A re-run queued without a start yet therefore counts
/// once the host says it has started.
fn ran_after_the_lift(check: &Check, snapshot: &[Check]) -> bool {
    let earlier: Vec<&Check> = snapshot
        .iter()
        .filter(|seen| seen.name == check.name)
        .collect();
    if earlier.is_empty() {
        return true;
    }
    check.started_at.as_ref().is_some_and(|started| {
        earlier
            .iter()
            .all(|seen| seen.started_at.as_ref() != Some(started))
    })
}

/// Watch a change lifted early until a run the host attached **after** the lift
/// settles its required checks.
///
/// What was reported while it was a draft is `snapshot`, and a draft-era `skipped` in
/// it is not a verdict: nothing ran. So the watch needs a run the host attached since
/// — told from the draft's by [`ran_after_the_lift`] — and where none of the required
/// checks has one within another grace window, it ends [`Error::ChecksUnsettled`]
/// saying they did not re-run — never merging, and never lifting anything, on skips
/// from the draft. Once one has, the ordinary rule applies, under which a skipped
/// required check from a post-lift run satisfies the watch and is recorded as
/// `passed-with-skipped`.
fn settle_after_early_lift(
    watcher: &mut Watcher<'_>,
    stream: &mut Stream,
    declared: &Declared,
    unread: Option<&str>,
    snapshot: &[Check],
    grace: std::time::Duration,
) -> Result<()> {
    let lifted = std::time::Instant::now();
    let from_the_draft = |check: &Check| !ran_after_the_lift(check, snapshot);
    loop {
        let reading = watcher.read(stream)?;
        let checks = reading.reported.checks();
        // A draft-era skip is not a verdict, so it is as though it were not there.
        let counted: Vec<&Check> = checks
            .iter()
            .filter(|check| !(from_the_draft(check) && check.state() == CheckState::Skipped))
            .collect();
        let standing = standings(&counted, declared);
        if let Some(failed) = red(&standing) {
            return Err(watcher.failed(failed));
        }
        let rerun = checks.iter().any(|check| {
            !from_the_draft(check)
                && (standing.is_empty() || standing.iter().any(|s| s.name == check.name))
        });
        if rerun
            && standing.iter().all(|standing| {
                matches!(
                    standing.state,
                    Some(CheckState::Passed | CheckState::Skipped)
                )
            })
        {
            watcher.record_settled(stream, skipped_in(&standing), unread);
            return Ok(());
        }
        if !rerun && lifted.elapsed() >= grace {
            return Err(Error::ChecksUnsettled {
                reason: format!(
                    "the required checks on {url} did not re-run after its draft was lifted: no \
                     run the host attached to {commit} after the lift appeared within {seconds}s, \
                     and the runs from while it was a draft were skipped, which is not a verdict. \
                     The likely cause is a workflow that does not trigger on `ready_for_review` \
                     — add it to the workflow's pull_request types, or run the checks by hand",
                    url = watcher.change.url,
                    commit = watcher.pushed.0,
                    seconds = grace.as_secs_f64(),
                ),
            });
        }
        if watcher.out_of_time() {
            return Err(watcher.unsettled(&reading, "settled its required checks on"));
        }
        watcher.pause()?;
    }
}

/// Arm the host's own merge and watch until it performs it.
///
/// Arming happens after the first reading of the checks: a host that will not say
/// what its checks are is refused with nothing armed against it. Only required checks
/// may end it: a non-blocking check never holds or fails a merge.
///
/// A change request the host confirms **conflicts** with its base ends it too, as
/// [`Error::SyncConflict`], at the first reading that says so: the base moved under the
/// armed merge and the host can never perform it, so waiting out the bound would only
/// report later what is known now. Its branch and its change request are left as they
/// are for the session to resolve. A mergeability the host has not computed yet is
/// never read as a conflict.
fn watch_the_merge(watcher: &mut Watcher<'_>, stream: &mut Stream) -> Result<Sha> {
    let mut armed = false;
    loop {
        let reading = watcher.read(stream)?;
        if let Reported::About(checks) = &reading.reported {
            let about: Vec<&Check> = checks.iter().collect();
            let standing = standings(&about, &Declared::Unknown);
            if let Some(failed) = red(&standing) {
                return Err(watcher.failed(failed));
            }
            if standing.iter().all(|standing| {
                matches!(
                    standing.state,
                    Some(CheckState::Passed | CheckState::Skipped)
                )
            }) {
                watcher.record_settled(stream, skipped_in(&standing), None);
            }
        }
        if !armed {
            watcher
                .host
                .merge(watcher.change, MergePolicy::ChangeAuto)?;
            armed = true;
        }
        if let Some(sha) = watcher.host.merged_at(watcher.change)? {
            return Ok(sha);
        }
        // Asked after the merge, so a change the host landed is never reported as a
        // conflict it had before it landed. A host that has not computed it yet is
        // waited on like any other "not yet", and one that was never taught to answer
        // has said nothing, which is no conflict.
        match watcher.host.mergeability(watcher.change) {
            Ok(Mergeability::Conflicting { reported }) => {
                return Err(watcher.conflicting(&reported))
            }
            // Every other state, any added later included, is not a confirmed conflict.
            Ok(_) | Err(Error::NotImplemented { .. }) => {}
            Err(unreadable) => return Err(unreadable),
        }
        if watcher.out_of_time() {
            return Err(watcher.unsettled(&reading, "merged"));
        }
        watcher.pause()?;
    }
}

/// A required check that concluded red, named, with everywhere its evidence is and
/// a bounded excerpt of what it printed.
///
/// The excerpt is on the failure itself rather than only in the artifact beside it,
/// because a caller routing this failure — or a person reading stderr — otherwise
/// learns only that a check called `gate` failed and has to go and fetch the log by
/// hand to find out why. A log this crate could not read back is simply not quoted:
/// an excerpt naming the reason there is no excerpt would read as the check's own
/// output.
///
/// The other two are *pointers*, and they are here because the excerpt is bounded
/// and this failure is usually read by whatever gets dispatched next: where the
/// check is on the host, so a person can open it, and the artifact this watch
/// already stored the whole log as, with the command that prints it. A retry handed
/// the word `checks-failed` and nothing else has to rediscover both.
fn checks_failed(check: &Check, logs: &[(String, crate::event::ArtifactId)]) -> Error {
    let stored = logs.iter().find(|(name, _)| name == &check.name);
    let said = stored
        .and_then(|(_, id)| stream::read_artifact(&id.0).ok())
        .map(|log| excerpt(&log, "the whole log is the check's artifact"))
        .filter(|log| !log.trim().is_empty())
        .map(|log| format!(" It said:\n{}", guidance::quoted_output(&log)))
        .unwrap_or_default();
    let at = check
        .url
        .as_ref()
        .map(|url| format!(" It ran at {url}."))
        .unwrap_or_default();
    let evidence = stored
        .map(|(_, id)| {
            format!(
                " Its whole log is artifact {id} — `{command}`.",
                id = id.0,
                command = guidance::command(["onevcs", "artifact", "cat", &id.0]),
            )
        })
        .unwrap_or_default();
    Error::ChecksFailed {
        reason: format!(
            "required check {:?} concluded {}.{at}{evidence}{said}",
            check.name,
            check
                .conclusion
                .as_deref()
                .unwrap_or("without a conclusion"),
        ),
    }
}

/// The bound elapsed, naming what the host had not settled.
///
/// It names them because the alternative is what the bound used to do: stop after
/// an hour with a sentence that reads the same whether one check is still running,
/// the repository declares none, or the host has simply never landed the change.
/// Those are three different next moves.
fn unsettled(
    change: &ChangeRequest,
    pushed: &Sha,
    reported: &Reported,
    bound: std::time::Duration,
    awaited: &str,
) -> Error {
    let Reported::About(checks) = reported else {
        // A fourth ending, and the one that must not read as any of the three below:
        // the host answered, and everything it said was about a head this run
        // replaced. "Nothing blocks the merge" is what that used to look like.
        return Error::ChecksUnsettled {
            reason: format!(
                "the host had not {awaited} {url} within {seconds}s; it has reported no check at \
                 all on {commit}, the commit this publication pushed — every check it does report \
                 is attached to some other commit",
                url = change.url,
                seconds = bound.as_secs_f64(),
                commit = pushed.0,
            ),
        };
    };
    let required: Vec<&Check> = checks.iter().filter(|check| check.required).collect();
    let pending: Vec<&str> = required
        .iter()
        .filter(|check| check.state() == CheckState::Pending)
        .map(|check| check.name.as_str())
        .collect();
    let no_verdict: Vec<String> = required
        .iter()
        .filter(|check| check.no_verdict())
        .map(|check| {
            format!(
                "{:?}: {}",
                check.name,
                check.conclusion.as_deref().unwrap_or("unknown")
            )
        })
        .collect();
    let skipped: Vec<&str> = required
        .iter()
        .filter(|check| check.state() == CheckState::Skipped)
        .map(|check| check.name.as_str())
        .collect();
    let named = if !pending.is_empty() {
        let verdicts = if no_verdict.is_empty() {
            String::new()
        } else {
            format!("; completed with no verdict: {}", no_verdict.join(", "))
        };
        format!("still unsettled: {}{verdicts}", guidance::listed(&pending))
    } else if !no_verdict.is_empty() {
        format!("completed with no verdict: {}", no_verdict.join(", "))
    } else if !skipped.is_empty() {
        format!(
            "skipped on the draft, so not run on it: {}",
            guidance::listed(&skipped)
        )
    } else if required.is_empty() {
        "the host declared no required check on it at all".to_owned()
    } else {
        "every required check it declared had settled".to_owned()
    };
    // llmlint: ignore[names_match_behavior] one fixed kind for the bound's three endings.
    Error::ChecksUnsettled {
        reason: format!(
            "the host had not {awaited} {url} within {seconds}s; {named}",
            url = change.url,
            seconds = bound.as_secs_f64(),
        ),
    }
}

/// Record one publishing push, and what it wrote.
///
/// **Unconditionally**, which is the point of this function. The publishing push is
/// where the repository's own `pre-push` hook rules on the change, so what git and
/// that hook wrote is the verdict as well as the only account of why a push was
/// refused — and it lives in a pipe until the process ends. Preserving it only where
/// the rules named `gate: {kind: pre-push}` meant no repository on a host whose
/// rules named commands: what a policy calls its verification cannot decide whether
/// a failure is diagnosable.
///
/// Storing it is best effort: the push has already happened, so a state root that
/// would not take the bytes says so on stderr rather than turning a push git
/// accepted into a publication that failed.
///
/// Every publishing push in this crate goes through here — the merge train's push of
/// its advanced base included, because that push is where the hook rules on a train.
/// One producer, so a second one cannot come to emit a thinner `push` event than the
/// contract says a push verdict is.
///
/// `preserve_under` is the run root whose per-branch directory keeps a copy that
/// outlives the tree the push was built in, and is `None` for a caller whose scratch
/// workspace is removed when it returns — there the stored artifact is what persists,
/// and a copy under a directory about to go would only look like evidence.
///
/// `phase` is which branch this push updated, which is the one thing about a push
/// that its own payload cannot say: `branch` here names the work being published,
/// and a `local-direct` squash and a merge train both push that work onto the *base*
/// under it. So the caller that made the push says whether it was the session's own
/// branch — the work being made, [`Phase::Development`] — or another, which is that
/// work being integrated.
pub(crate) fn record_push(
    stream: &mut Stream,
    branch: &Ref,
    pushed: &git::Pushed,
    preserve_under: Option<&Path>,
    phase: Phase,
    head: Option<&str>,
) -> Result<Kept> {
    let output = pushed.output();
    let stored = stream::store_artifact("log", output);
    let kept = preserve_under.map(|root| merge_path::preserve_log(root, branch, output));
    for error in [
        stored.as_ref().err(),
        kept.as_ref().and_then(|kept| kept.as_ref().err()),
    ]
    .into_iter()
    .flatten()
    {
        eprintln!(
            "onevcs: warning: the push of {branch:?} is recorded without what it wrote: {error}"
        );
    }
    let artifact = stored.ok();
    let mut payload = object(json!({
        "branch": branch,
        "remote": "origin",
        "accepted": pushed.accepted(),
        "output": output,
    }));
    if let Some(Ok(preserved)) = &kept {
        payload.insert(
            "preserved_log".to_owned(),
            json!(preserved.display().to_string()),
        );
    }
    // The commit a push of the branch's own name put there, which is the head a change
    // request opened from it carries — and what a retirement later asks the branch's
    // content to be contained in. Only the push of the branch itself names one: a push
    // of the base under it lands a commit no change request is opened from.
    if let Some(head) = head {
        payload.insert("head".to_owned(), json!(head));
    }
    let evidence = Kept {
        artifact: artifact
            .as_ref()
            .map(|stored| crate::event::ArtifactId(stored.id.clone())),
        preserved: match kept {
            Some(Ok(preserved)) => Some(preserved),
            Some(Err(_)) | None => None,
        },
    };
    stream.emit_push(phase, payload, artifact.into_iter().collect());
    Ok(evidence)
}

/// Where a recorded push's output can be read once the push is over.
///
/// Handed back rather than only written, because the refusal that reports a push
/// the merge path turned down is read by whoever has to diagnose it — and until it
/// named these, an operator was told three lines of git's generic message while the
/// whole verification sat in two places nothing pointed at. An hour went on
/// rediscovering four redundant comment lines that way.
///
/// Either may be absent and both may be: the artifact is missing where the state
/// root would not take the bytes, and the preserved copy where the caller had no
/// run root to outlive its tree or the write failed. A refusal that has neither
/// must say there is nothing to read rather than name a place that holds nothing.
pub(crate) struct Kept {
    /// The artifact the whole output was stored as, fetched with `onevcs artifact
    /// cat`.
    artifact: Option<crate::event::ArtifactId>,
    /// The file under the run root that outlives the tree the push was built in.
    preserved: Option<PathBuf>,
}

impl Kept {
    /// Where the whole of what the push wrote can be read, as a refusal says it.
    ///
    /// Both when both are there, because they answer different questions: the
    /// artifact survives the run root being reaped and is fetched through this
    /// crate's own CLI, and the preserved file is a path an operator can open
    /// directly, beside every other attempt on the same branch.
    fn evidence(&self) -> String {
        let artifact = self.artifact.as_ref().map(|id| {
            format!(
                "artifact {id} — `{command}`",
                id = id.0,
                command = guidance::command(["onevcs", "artifact", "cat", &id.0]),
            )
        });
        let preserved = self
            .preserved
            .as_ref()
            .map(|path| format!("preserved at {}", path.display()));
        match (artifact, preserved) {
            (Some(artifact), Some(preserved)) => {
                format!(" Its whole output is {artifact}, and {preserved}.")
            }
            (Some(one), None) | (None, Some(one)) => format!(" Its whole output is {one}."),
            // Both stores refused the bytes, and each said why on stderr as it
            // happened. Naming neither is the point: a path that holds nothing sends
            // an operator to look at it.
            (None, None) => String::new(),
        }
    }

    /// Whether the output was kept anywhere at all.
    fn anywhere(&self) -> bool {
        self.artifact.is_some() || self.preserved.is_some()
    }
}

/// The publication checkout must be clean and on its root branch before anything
/// is published onto it; a safety clone never makes an arbitrary active branch the
/// fast-forward target.
fn require_publication_checkout_ready(publication: &Path, base: &str) -> Result<()> {
    let current = git::current_branch(publication)?;
    if current != base {
        return Err(Error::Invalid {
            reason: format!(
                "the publication checkout {} has {current:?} checked out, not the base {base:?}; \
                 it is never worked in and only ever fast-forwarded",
                publication.display()
            ),
        });
    }
    if git::is_dirty(publication)? {
        return Err(Error::Invalid {
            reason: format!(
                "the publication checkout {} is dirty; it is never worked in",
                publication.display()
            ),
        });
    }
    Ok(())
}

/// Advance the publication checkout, and only ever forwards.
pub fn fast_forward_publication(publication: &Path, base: &str) -> Result<()> {
    if !git::has_remote(publication, "origin") {
        return Ok(());
    }
    git::fetch(publication, "origin")?;
    if git::current_branch(publication)? != base {
        return Ok(());
    }
    git::merge_ff_only(publication, &format!("origin/{base}"))
}

/// The host slug a change request is opened against, or the reason there is none.
///
/// Two different failures, deliberately not collapsed: a local identity has no
/// host at all and is asking for the wrong policy, while a hosted identity on a
/// host this build does not speak for is asking for an implementation that has not
/// arrived. The second is the seam `Error::NotImplemented` exists for.
pub(crate) fn change_host(identity: &str) -> Result<String> {
    if let Some(slug) = gh::slug(identity) {
        return Ok(slug);
    }
    let hosted = identity.split('/').count() == 3;
    if hosted {
        return Err(Error::NotImplemented {
            operation: "RemoteHost for a host other than github.com",
        });
    }
    Err(Error::Invalid {
        reason: format!(
            "identity {identity:?} is not a hosted repository, so it cannot publish a change \
             request; a local identity publishes with local-direct"
        ),
    })
}

/// One publication commit message: the subject, then whatever it must carry
/// forward.
pub fn compose_message(subject: &str, trailers: &[String]) -> String {
    if trailers.is_empty() {
        subject.to_owned()
    } else {
        format!("{subject}\n\n{}", trailers.join("\n"))
    }
}

/// The issue-closing lines the commits a squash lands carry, each once.
///
/// Read from the branch's own commits past what it lands on, which after the sync is
/// exactly the set the squash folds into one; see [`closing`](crate::closing) for why
/// these lines, and only these, outlive the squash.
fn closing_lines(context: &Context<'_>, compared: &str) -> Result<Vec<String>> {
    let commits = git::log_messages(&context.repo, compared, &context.branch)?;
    Ok(crate::closing::lines(
        commits.iter().map(|commit| commit.message.as_str()),
    ))
}

/// A squash's message: the subject, the issue-closing lines its commits carried, then
/// the trailers — so a branch with no closing lines lands exactly as
/// [`compose_message`] composes it.
fn compose_squash(subject: &str, closing: &[String], trailers: &[String]) -> String {
    if closing.is_empty() {
        return compose_message(subject, trailers);
    }
    compose_message(&format!("{subject}\n\n{}", closing.join("\n")), trailers)
}

/// The policy a run publishes under, once the rules and any `--policy` have both
/// had their say.
pub fn effective_policy(resolved: &Policy, requested: Option<MergePolicy>) -> Result<MergePolicy> {
    match requested {
        Some(requested) => policy::narrow(resolved, requested),
        None => Ok(resolved.publication),
    }
}

/// The exit code an error reports, as the contract fixes them.
pub fn exit_code(error: &Error) -> u8 {
    FailureKind::of(error).exit_code()
}

/// A session's own record, once publication has decided what it publishes onto.
pub fn preserved_change_base(record_base: &Ref, recorded: Option<&Ref>) -> Ref {
    recorded.unwrap_or(record_base).clone()
}

/// A change request this host opened, stopped watching, and may meet again.
///
/// Everything the question below is asked from, read out of the records this host
/// already holds rather than derived a second way: the event stream's own
/// `change-opened`, and the branch as some copy of it here stands. One value rather
/// than seven arguments, for the reason [`Advance`](crate::status) is one.
pub(crate) struct Watched<'a> {
    /// The identity the work belongs to.
    pub identity: &'a str,
    /// The branch that carries it.
    pub branch: &'a str,
    /// Where a human reads the change request, as `change-opened` recorded it.
    pub url: &'a Url,
    /// The host's own identifier for it, as `change-opened` recorded it.
    pub id: &'a str,
    /// The branch it targets, as `change-opened` recorded it.
    pub base: &'a str,
    /// The commit this host last saw the branch at, which is the head the change
    /// request was opened from.
    pub head: &'a str,
    /// The stream that recorded it being opened, which is where a landing found
    /// after that publication ended is recorded.
    pub stream: Option<&'a str>,
}

/// Ask the host **once** whether a change request this host stopped watching has
/// merged since, and record the landing where it has.
///
/// A `change-auto` publication watches the host for a fixed span and closes
/// [`ChecksUnsettled`](FailureKind::ChecksUnsettled) when that span runs out. The
/// merge often happens seconds later, and nothing afterwards asks: the landing tiers
/// read stored records and the base's history, a `status` deliberately never fetches,
/// and the host is asked only which change requests are *open* — where a merged one
/// is simply absent. So a change request merged five seconds after its last required
/// check went on reading `closed without landing`, and the next attempt republished
/// work the base already carried.
///
/// The question is asked of exactly one state and asked once per read: a change
/// request this host recorded, which it asked the host to land, and for which no
/// landing is recorded — which is what a `checks-unsettled` close leaves behind.
/// [`RemoteHost::merged_at`] answers it, and its three answers are three outcomes: a
/// merge commit is recorded as the landing, `None` and every refusal leave the record
/// exactly as it stands. A change request the host reports open or closed-unmerged is
/// the `None`.
///
/// What a recorded landing then gets is what a witnessed one gets, through the one
/// function that gives it: the publication checkout fast-forwarded, each automated
/// target's baseline captured at the landing commit, and the merge recorded on the
/// stream that opened the change. The caller recomputes from that record, which is
/// what puts the landing into the same read rather than into the next one.
pub(crate) fn reconcile_late_merge(
    registry: &crate::registry::Registry,
    watched: &Watched<'_>,
    hosting: &dyn Hosting,
) -> Option<String> {
    let host = hosting
        .for_repo(&change_host(watched.identity).ok()?)
        .ok()?;
    let merged = host
        .merged_at(&ChangeRequest {
            id: crate::host::ChangeId(watched.id.to_owned()),
            url: watched.url.clone(),
            head_sha: Sha(watched.head.to_owned()),
            base: watched.base.to_owned(),
        })
        .ok()??;
    record_late_merge(registry, watched, &merged)
}

/// Record a merge the host reported for a change request this host stopped watching,
/// the way [`reconcile_late_merge`] records the one it finds — for a caller that asked
/// the host itself, because it has to tell a refusal from "not merged". Answers the
/// landing where it was recorded, and `None` where it could not be, for the reason
/// [`warn_unreconciled`] gives.
pub(crate) fn record_late_merge(
    registry: &crate::registry::Registry,
    watched: &Watched<'_>,
    merged: &Sha,
) -> Option<String> {
    let located = match crate::release::for_repository(registry, watched.identity) {
        Ok(located) => located,
        // Best effort throughout, as every part of recording a landing after the
        // fact is: the change has merged whatever this host can write about it, and
        // refusing the read that found it would be the worse answer.
        Err(failure) => return warn_unreconciled(watched, &merged.0, &failure),
    };
    let mut stream = match Stream::releases(watched.identity) {
        Ok(stream) => stream,
        Err(failure) => return warn_unreconciled(watched, &merged.0, &failure),
    };
    crate::release::reconcile_landing(
        registry,
        &located,
        watched.branch,
        watched.stream,
        &merged.0,
        watched.url,
        &mut stream,
    );
    Some(merged.0.clone())
}

/// Say on stderr that a merge the host reported could not be recorded here, and
/// answer as though it had not been found.
///
/// Answering `None` is the point: the caller goes on to decide the landing from the
/// records as they stand, which is what they still say. Reporting the merge without
/// having recorded it would leave a read answering `landed` that the next read,
/// reading the same records, answers `closed` — one landing, two answers.
fn warn_unreconciled(
    watched: &Watched<'_>,
    commit: &str,
    failure: &dyn std::fmt::Display,
) -> Option<String> {
    eprintln!(
        "onevcs: warning: the host reports {url} merged at {commit}, and this host could not \
         record that landing for {branch} ({failure}); `onevcs status {branch}` still answers \
         from what is recorded",
        url = watched.url,
        branch = watched.branch,
    );
    None
}

/// Take up a merge the host performed after a publication of this branch stopped
/// watching for it, before this one decides what there is to publish.
///
/// The third of the three reads that meet such a change request, and it asks through
/// the *same* path the other two ask through — `status`'s landing read — so a
/// publication and a status cannot come to two answers about one change request.
/// What that read does with a merge the host reports is recorded on
/// [`reconcile_late_merge`]: the landing is written, the publication checkout is
/// fast-forwarded, and each automated target's baseline is captured at the merge
/// commit. The fetch below then brings the merge into this publication's own
/// comparison, which is what answers `nothing to publish` instead of opening a
/// second change request for work already on the base.
///
/// Best effort, and deliberately silent about its own failures: everything it could
/// establish is established again by the comparison this publication is about to
/// make, and a publication refused because a question about the host could not be
/// asked would be a worse answer than one that simply asks nobody.
fn take_up_a_late_merge(context: &Context<'_>) {
    let Ok(registry) = store::load() else {
        return;
    };
    let _ = crate::status::landing_of_within(
        &registry,
        &context.branch,
        Some(&context.resolution.key),
        Some(context.hosting),
    );
}