rig_harness/host/

session.rs

//! Where this process keeps its session: the directory the launcher names,
//! its store ([`StoreOpener`]), the plain [`Scratch`] files with the text
//! log, and the log's warnings and errors as [`Logged`] data. Deleting
//! ([`DeleteSession`], crash-safe: the next start finishes it) and
//! rewriting a stopped session ([`rewrite_stopped`]) are here, and so is
//! why this app started ([`AppStart`]).

use std::collections::BTreeSet;
use std::fmt::{Debug, Write};
use std::fs::{self, File, OpenOptions};
use std::io::{self, Seek, SeekFrom};
use std::ops::Deref;
use std::path::{Path, PathBuf};
use std::sync::{Arc, Mutex, PoisonError};

use bevy_app::OnAppExitSystems;
use bevy_app::prelude::*;
use bevy_ecs::prelude::*;
use bevy_log::tracing::field::{Field, Visit};
use bevy_log::tracing::{Event, Level};
use bevy_log::tracing_subscriber::layer::{Context, Layer};
use bevy_log::tracing_subscriber::{Registry, fmt};
use bevy_log::{BoxedFmtLayer, BoxedLayer, error, warn};
use bevy_reflect::Reflect;
use crossbeam_channel::{Receiver, Sender};
use rig_harness_protocol::{Home, SessionDir, SessionId, StartReason};

use rig_core::conversation::Author;
use rig_ecs::WriteLogs;
use rig_ecs::agent::{Notice, NoticeLevel};
use rig_ecs::restore::SavedKeys;
use rig_ecs::session_log::{SessionLog, SessionStore, open_session};
use rig_memory::{
    DirFiles, DirStore, Files, Rewrite, Rewritten, Session, SessionError, SessionMeta, Store,
    rewrite, rewrite_file,
};
use rig_tools::Spill;

use super::cli::{self, Cli};
use super::exit::ExitReason;
use super::launcher::{LauncherRun, refuse};
use crate::describe::Describing;
use crate::plugin_data::PluginData;
use crate::services::{ProjectRoot, RigHome};
use crate::timings::{self, SystemTimings, TimeSystems};
use crate::workspace::ProjectWorkspace;

/// The session's directory. Its files are kept through the session's
/// store ([`StoreOpener`]): a plugin keeps its own files of the session
/// there too, under a directory of its own, such as
/// `store.write_file("notes/today.md", ..)`.
#[derive(Resource, Clone, Debug)]
pub struct SessionPaths(pub SessionDir);

impl Deref for SessionPaths {
    type Target = SessionDir;

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

/// Opens the store of a session's files: the running session's, an
/// import's, one `/resume` lists. By default a [`DirStore`] in the
/// session's directory (`logs/`, `files/`), which refuses a
/// session from before `rig-session/2` ([`SessionError::OldLayout`]). A
/// plugin replaces it while the app is built, such as with a sealed store:
/// `app.insert_resource(StoreOpener::new(|dir| ..))`.
///
/// Once every plugin is built the harness opens the running session's
/// [`SessionStore`] from the front's own store
/// ([`Front::session_store`](super::cli::Front::session_store)), else from
/// this, and reads it before writing; a run whose session does not open or
/// read is refused, unchanged. The harness marks the running session saved
/// once its log first writes, so `/resume` lists it (not a sessionless
/// run's); a program that saves a new session through this marks it
/// itself ([`SessionDir::mark_saved`]). [`Self::relist`] keeps the listing
/// of a session no rig runs current.
#[derive(Resource, Clone)]
pub struct StoreOpener {
    /// How a store is opened.
    open: Arc<OpenStore>,
    /// The keys whose values a listing holds.
    listed: Arc<BTreeSet<String>>,
}

/// How [`StoreOpener`] opens a store.
type OpenStore = dyn Fn(&SessionDir) -> Result<Arc<dyn Store>, SessionError> + Send + Sync;

impl StoreOpener {
    /// Stores that `open` opens.
    pub fn new(
        open: impl Fn(&SessionDir) -> Result<Arc<dyn Store>, SessionError> + Send + Sync + 'static,
    ) -> Self {
        Self {
            open: Arc::new(open),
            listed: Arc::default(),
        }
    }

    /// The store of the session in `dir`.
    ///
    /// # Errors
    ///
    /// When the store cannot be opened, such as without its key, or holds
    /// a session from before `rig-session/2`.
    pub fn open(&self, dir: &SessionDir) -> Result<Arc<dyn Store>, SessionError> {
        (self.open)(dir)
    }

    /// Brings the listing of the session `session` read from or saved to
    /// `store`, which no rig runs, up to it, as a running rig keeps its
    /// own: its listed values, its main agent's model and message count,
    /// and when it changed ([`SessionMeta::relist`]). Every writer of a
    /// stopped session calls it after writing; [`edit_stopped`] and
    /// [`rewrite_stopped`] do.
    ///
    /// # Errors
    ///
    /// When the listing cannot be written.
    pub fn relist(&self, store: &dyn Store, session: &Session) -> io::Result<()> {
        let mut meta = SessionMeta::read(store).unwrap_or_default();
        meta.relist(session, &self.listed);
        meta.write(store)
    }
}

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

impl Default for StoreOpener {
    fn default() -> Self {
        Self::new(|dir| Ok(Arc::new(DirStore::open(dir.path())?)))
    }
}

/// Where rig keeps plain files of the session, which are named by path
/// and so stay out of its store: tool output cut short ([`Self::spill`]),
/// pasted images ([`Self::images`]) and the text log ([`Self::log`]: tracing,
/// panics, stray output). The session's directory by default; a plugin
/// places it elsewhere, or with `None` nowhere, while the app is built:
/// tools then only cut, pasting an image is refused, and the text log and
/// stray output are dropped.
#[derive(Resource, Clone, Debug, PartialEq, Eq)]
pub struct Scratch(pub Option<PathBuf>);

impl Scratch {
    /// Where tools keep output they cut, and plugins large answers, whole,
    /// under handles the model can `read` or `search`.
    pub fn spill(&self) -> Option<Spill> {
        self.0.as_ref().map(|dir| Spill(dir.join("spill")))
    }

    /// Where images pasted into the input are kept, which messages attach
    /// by path.
    pub fn images(&self) -> Option<PathBuf> {
        self.0.as_ref().map(|dir| dir.join("images"))
    }

    /// The text log.
    pub fn log(&self) -> Option<PathBuf> {
        self.0.as_ref().map(|dir| dir.join("agent.log"))
    }
}

/// The session the launcher names, or a new one when the agent runs
/// without it, under `home`, with its directory created.
pub(crate) fn paths_in(home: &Home) -> SessionPaths {
    let id = SessionId::from_env()
        .ok()
        .flatten()
        .unwrap_or_else(SessionId::generate);
    let dir = home.session(&id);
    // A directory that cannot be created shows up as a failed log write.
    // A build describing itself keeps none.
    if crate::describe::describing().is_none() {
        fs::create_dir_all(dir.path()).ok();
    }
    SessionPaths(dir)
}

/// The session's paths in the app's home, taken from the environment the
/// first time.
fn session_paths(world: &mut World) -> SessionPaths {
    let home = RigHome::of(world);
    world
        .get_resource_or_insert_with(|| paths_in(&home))
        .clone()
}

/// Why this app started, from the launcher ([`StartReason`]), inserted
/// when the session plugin is built unless the app has one. `New` exactly
/// when no app of the session ran a whole frame before, whatever the
/// launcher said. Without a launcher, any other start is `Resumed`.
#[derive(Resource, Clone, Copy, Debug, PartialEq, Eq)]
pub struct AppStart(pub StartReason);

/// Until the end of its first frame: this app is its session's first, and
/// marks the session started then. A refused run does not.
#[derive(Resource)]
struct Unmarked;

/// Until the session's log first writes, in a run that keeps its session:
/// the session is marked saved then ([`SessionDir::mark_saved`]), so
/// `/resume` lists it. A sessionless run never has it.
#[derive(Resource)]
struct Unsaved;

/// Inserts the [`SessionPaths`] from the environment, unless the log took
/// them already, the default [`StoreOpener`] and [`Scratch`], the
/// [`SystemTimings`] (which stay empty without the log), opens the
/// [`SessionStore`] once every plugin is built and then inserts the
/// [`PluginData`], routes panics to the log,
/// says why the app started ([`AppStart`]), and deletes sessions,
/// finishing at startup the deletes a crash cut short.
pub struct SessionPlugin;

impl Plugin for SessionPlugin {
    fn build(&self, app: &mut App) {
        std::panic::set_hook(Box::new(|info| {
            error!("{info}\n{}", std::backtrace::Backtrace::capture());
        }));
        let paths = session_paths(app.world_mut());
        if !app.world().contains_resource::<Scratch>() {
            app.insert_resource(Scratch(Some(paths.path().to_owned())));
        }
        let first = !paths.started().exists();
        if !app.world().contains_resource::<AppStart>() {
            let reason = match StartReason::from_env() {
                _ if first => StartReason::New,
                None | Some(StartReason::New) => StartReason::Resumed,
                Some(reason) => reason,
            };
            app.insert_resource(AppStart(reason));
        }
        if first {
            app.insert_resource(Unmarked).add_systems(
                Last,
                mark_started
                    .run_if(resource_exists::<Unmarked>)
                    .before(discard_unsaved),
            );
        }
        app.init_resource::<StoreOpener>()
            .init_resource::<SystemTimings>()
            .add_message::<SessionDeleted>()
            .add_message::<SessionDeleteFailed>()
            .add_message::<Notice>()
            .add_observer(delete_session)
            .add_systems(PreStartup, list_keys)
            .add_systems(
                Startup,
                finish_deletes.run_if(not(resource_exists::<Describing>)),
            )
            .add_systems(First, timings::next_frame)
            .add_systems(
                Last,
                mark_saved
                    .after(WriteLogs)
                    .before(discard_unsaved)
                    .run_if(resource_exists::<Unsaved>),
            )
            .add_systems(
                Last,
                discard_unsaved
                    .after(OnAppExitSystems)
                    .run_if(on_message::<AppExit>),
            );
    }

    /// Every plugin chose the stores and the scratch place and offered its fronts
    /// by now: chooses the run's front, opens the session's store, or the one a
    /// test or a nested tool put in, and reads the session; only then moves the
    /// text log where [`Scratch`] says. A session that cannot be opened or read
    /// refuses the run, and the run writes nothing into its directory
    /// (`leave_session`). A build describing itself opens none.
    fn finish(&self, app: &mut App) {
        if !app.world().contains_resource::<Describing>() {
            match open_store(app.world_mut()) {
                Ok(()) => {
                    let wanted = app.world().get_resource::<Scratch>().and_then(Scratch::log);
                    move_text_log(app.world_mut(), wanted);
                }
                Err(failure) => {
                    refuse(
                        app,
                        format!(
                            "the session cannot be opened, so nothing of it was changed: {failure}"
                        ),
                    );
                }
            }
        }
        insert_plugin_data(app.world_mut());
    }
}

/// Tells the [`StoreOpener`] the listed keys, once every plugin said its
/// own.
fn list_keys(keys: Option<Res<SavedKeys>>, stores: Option<ResMut<StoreOpener>>) {
    if let Some(mut stores) = stores {
        let listed = keys.map(|keys| keys.listed().clone());
        stores.listed = Arc::new(listed.unwrap_or_default());
    }
}

/// Inserts the [`PluginData`] of this run, unless the app has one: in the
/// session's store when it opened, for the [`ProjectRoot`] in the
/// [`ProjectWorkspace`].
fn insert_plugin_data(world: &mut World) {
    if world.contains_resource::<PluginData>() {
        return;
    }
    let home = RigHome::of(world);
    let session = world
        .get_resource::<SessionStore>()
        .map(|store| -> Arc<dyn Files> { store.0.clone() });
    let root = world
        .get_resource::<ProjectRoot>()
        .cloned()
        .unwrap_or_default();
    let workspace = world.get_resource::<ProjectWorkspace>();
    let host = workspace.and_then(|workspace| workspace.0.host());
    let data = PluginData::new(&home, session, &root.0, host);
    world.insert_resource(data);
}

/// Opens the session's store, the one in the world or else the one the
/// run's front or [`StoreOpener`] opens for [`SessionPaths`], and reads
/// the session it holds ([`open_session`]), before anything is written to
/// it. A sessionless run never marks its session saved, so a crash leaves
/// no session for `/resume` to list.
fn open_store(world: &mut World) -> Result<(), SessionError> {
    let store = match world.remove_resource::<SessionStore>() {
        Some(SessionStore(store)) => store,
        None => {
            cli::choose(world);
            let front = world.get_resource::<Cli>().and_then(Cli::chosen);
            let keeps = !front.is_some_and(|front| front.sessionless);
            let stores = front
                .and_then(|front| front.session_store.as_ref())
                .or_else(|| world.get_resource::<StoreOpener>());
            let (Some(paths), Some(stores)) = (world.get_resource::<SessionPaths>(), stores) else {
                return Ok(());
            };
            let store = stores.open(paths)?;
            if keeps && !paths.is_saved() {
                world.insert_resource(Unsaved);
            }
            store
        }
    };
    open_session(world, store)
}

/// Keeps a refused run out of its session's directory: what it logged
/// goes to the launcher's run directory, where the launcher points, or
/// nowhere; it keeps no scratch files; and the session is not marked
/// started.
pub(crate) fn leave_session(world: &mut World) {
    let elsewhere = world.get_resource::<LauncherRun>().map(|run| run.0.log());
    move_text_log(world, elsewhere);
    world.insert_resource(Scratch(None));
    world.remove_resource::<Unmarked>();
}

/// Marks the session saved after its log first wrote.
fn mark_saved(log: Option<Res<SessionLog>>, paths: Res<SessionPaths>, mut commands: Commands) {
    if !log.is_some_and(|log| log.has_written()) {
        return;
    }
    commands.remove_resource::<Unsaved>();
    if let Err(failure) = paths.mark_saved() {
        warn!("could not mark the session saved: {failure}");
    }
}

/// Marks the session started at the end of its first app's first frame.
fn mark_started(paths: Res<SessionPaths>, mut commands: Commands) {
    commands.remove_resource::<Unmarked>();
    if let Err(failure) = fs::write(paths.started(), b"") {
        error!("could not mark the session started: {failure}");
    }
}

/// Removes the session's directory when the run leaves it for good with
/// nothing saved there, such as a run that switches to another session at
/// startup, or when a sessionless front's run ends, however, once
/// everything was written there; never a saved session a front keeps,
/// never on a reload, which runs the session again, never on a switch the
/// launcher is told of (its switch file is in the directory, and the
/// launcher removes an unsaved session once it read it), and never a
/// resumed session's, which a sessionless front refuses. A run that
/// failed first moves its text log into the launcher's run directory,
/// where the launcher's failure message points.
fn discard_unsaved(
    mut exits: MessageReader<AppExit>,
    (cli, start, reason): (
        Option<Res<Cli>>,
        Option<Res<AppStart>>,
        Option<Res<ExitReason>>,
    ),
    paths: Res<SessionPaths>,
    (run, scratch): (Option<Res<LauncherRun>>, Option<Res<Scratch>>),
) {
    let exits: Vec<&AppExit> = exits.read().collect();
    let reload = match reason.as_deref() {
        Some(reason) => *reason == ExitReason::Reload,
        None => exits
            .iter()
            .any(|exit| ExitReason::of(exit) == ExitReason::Reload),
    };
    let discard = if cli.is_some_and(|cli| cli.sessionless()) {
        start.is_none_or(|start| start.0 != StartReason::Resumed)
    } else {
        !reload && !paths.is_saved() && !paths.switch().exists()
    };
    if !discard {
        return;
    }
    if !exits.iter().all(|exit| exit.is_success())
        && let Some(run) = run
        && let Some(log) = scratch.and_then(|scratch| scratch.log())
    {
        fs::rename(log, run.0.log()).ok();
    }
    fs::remove_dir_all(paths.path()).ok();
}

/// Changes the saved session in `dir`, which no rig runs, as
/// [`rewrite_stopped`] rewrites one: read, given
/// to `edit`, and saved, its listing brought up to it
/// ([`StoreOpener::relist`]). It holds the session's launcher lock
/// ([`SessionDir::hold`]) from the check to the last write, so a rig
/// starting meanwhile finds it running elsewhere. The running session,
/// `running`, is refused: edit its agents' `Conversation` instead.
///
/// # Errors
///
/// When `dir` is the running session or another rig runs it, when its
/// store does not open or its session cannot be read (nothing written),
/// when `edit` fails (nothing written), or when a write fails.
pub fn edit_stopped(
    stores: &StoreOpener,
    running: &SessionDir,
    dir: &SessionDir,
    edit: impl FnOnce(&mut Session) -> Result<(), String>,
) -> Result<(), String> {
    let _held = hold_stopped(running, dir, "edit its agents' conversations instead")?;
    let store = stores.open(dir).map_err(|failure| failure.to_string())?;
    let mut session = Session::read(&*store).map_err(|failure| failure.to_string())?;
    edit(&mut session)?;
    session
        .save(&*store)
        .map_err(|failure| failure.to_string())?;
    relist(stores, &*store, &session);
    Ok(())
}

/// Brings the listing of `session`, saved to `store`, up to it; a listing
/// not written is logged, as a running rig's is, since the session is.
fn relist(stores: &StoreOpener, store: &dyn Store, session: &Session) {
    if let Err(failure) = stores.relist(store, session) {
        warn!("could not write a stopped session's listing: {failure}");
    }
}

/// The launcher lock of the session in `dir`, held for an edit of it:
/// refused for the running session (`running`, which is edited as `this`
/// says), a session that is not there and one another rig runs.
fn hold_stopped(running: &SessionDir, dir: &SessionDir, this: &str) -> Result<File, String> {
    if dir.path() == running.path() {
        return Err(format!("it is this session: {this}"));
    }
    if !dir.path().is_dir() {
        return Err("there is no such session".to_owned());
    }
    dir.hold()
        .map_err(|failure| failure.to_string())?
        .ok_or_else(|| "another rig runs it".to_owned())
}

/// Rewrites the saved session in `dir`, which no rig runs, through
/// `with`, as done by `by` (rig-memory's `rewrite`), holding its launcher
/// lock throughout, as [`edit_stopped`] does: its store opened by
/// `stores`, then the files kept in its directory: its name
/// ([`SessionDir::name`]), the text log and the spilled tool output, and
/// its listing brought up to it ([`StoreOpener::relist`]). The running
/// session, `running`, is refused: only a stopped session is rewritten.
/// For a plugin redacting another saved session:
///
/// ```no_run
/// use rig_harness::harness_protocol::SessionId;
/// use rig_harness::prelude::*;
///
/// fn redact(
///     other: SessionId,
///     (home, stores, paths): (Res<RigHome>, Res<StoreOpener>, Res<SessionPaths>),
/// ) -> Result<(), String> {
///     let redact = Strings(|text: &mut String| {
///         let hit = text.contains("sk-123");
///         if hit {
///             *text = text.replace("sk-123", "[REDACTED]");
///         }
///         hit
///     });
///     let dir = home.0.session(&other);
///     rewrite_stopped(&stores, &paths, &dir, Author::plugin("redact"), &redact).map(drop)
/// }
/// ```
///
/// # Errors
///
/// When `dir` is the running session or another rig runs it, when its
/// store does not open or its session cannot be read (nothing written), or
/// when a write fails.
pub fn rewrite_stopped(
    stores: &StoreOpener,
    running: &SessionDir,
    dir: &SessionDir,
    by: Author,
    with: &dyn Rewrite,
) -> Result<Rewritten, String> {
    let _held = hold_stopped(running, dir, "only a stopped session is rewritten")?;
    let store = stores.open(dir).map_err(|failure| failure.to_string())?;
    let rewritten = rewrite(&*store, by, with).map_err(|failure| failure.to_string())?;
    let scratch = DirFiles::new(dir.path());
    map_spill(&scratch, with)
        .and_then(|()| map_name(dir, with))
        .and_then(|_| rewrite_file(&scratch, LOG, with))
        .map_err(|failure| format!("rewriting its scratch files failed: {failure}"))?;
    match Session::read(&*store) {
        Ok(session) => relist(stores, &*store, &session),
        Err(failure) => warn!("could not read a rewritten session to list it: {failure}"),
    }
    Ok(rewritten)
}

/// The text log's name to a [`Rewrite`].
const LOG: &str = "agent.log";

/// Maps the session's name in `dir` ([`SessionDir::name`]) through
/// `with`, as the text file `name`; whether it changed. A name mapped to
/// blank is removed.
fn map_name(dir: &SessionDir, with: &dyn Rewrite) -> io::Result<bool> {
    let Some(name) = dir.name() else {
        return Ok(false);
    };
    let mut bytes = name.into_bytes();
    if !with.file("name", &mut bytes) {
        return Ok(false);
    }
    dir.set_name(Some(&String::from_utf8_lossy(&bytes)))?;
    Ok(true)
}

/// Maps every file of the spilled tool output in `scratch`, a session's
/// scratch directory, through `with`, each named `spill/<file>`.
fn map_spill(scratch: &DirFiles, with: &dyn Rewrite) -> io::Result<()> {
    for name in scratch.files()? {
        if name.starts_with("spill/") {
            rewrite_file(scratch, &name, with)?;
        }
    }
    Ok(())
}

/// Removes a saved session: not this one, and not one another rig runs.
/// The session is marked deleting first ([`SessionDir::mark_deleting`]),
/// which hides it from listings, then everything its store holds is
/// destroyed, wherever [`StoreOpener`] keeps it, then its directory is
/// removed. A delete a crash cut short is finished at the next start.
/// [`SessionDeleted`] follows, or [`SessionDeleteFailed`] and an error
/// notice say why not.
#[derive(Event, Clone, Debug)]
pub struct DeleteSession(pub SessionId);

/// A session removed with [`DeleteSession`], or by the next start after
/// a crash cut its delete short.
#[derive(Message, Clone, Debug)]
pub struct SessionDeleted(pub SessionId);

/// A session [`DeleteSession`], or the next start finishing a delete a
/// crash cut short, could not remove, and why, such as `another rig runs
/// it`.
#[derive(Message, Clone, Debug)]
pub struct SessionDeleteFailed {
    /// The session.
    pub id: SessionId,
    /// Why it is still there.
    pub why: String,
}

fn delete_session(
    delete: On<DeleteSession>,
    (paths, stores): (Res<SessionPaths>, Res<StoreOpener>),
    home: Option<Res<RigHome>>,
    mut deleted: MessageWriter<SessionDeleted>,
    mut failed: MessageWriter<SessionDeleteFailed>,
    mut notices: MessageWriter<Notice>,
) {
    let home = home.map_or_else(Home::from_env, |home| home.0.clone());
    let session = home.session(&delete.0);
    let refusal = if session.path() == paths.path() {
        Some("it is this session".to_owned())
    } else if !session.path().is_dir() {
        Some("there is no such session".to_owned())
    } else if session.runs_elsewhere() {
        Some("another rig runs it".to_owned())
    } else {
        session
            .mark_deleting()
            .map_err(SessionError::from)
            .and_then(|()| finish_delete(&session, &stores))
            .err()
            .map(|failure| failure.to_string())
    };
    match refusal {
        None => {
            deleted.write(SessionDeleted(delete.0.clone()));
        }
        Some(why) => {
            notices.write(Notice::error(
                None,
                format!("Could not delete session {}: {why}.", delete.0),
            ));
            let id = delete.0.clone();
            failed.write(SessionDeleteFailed { id, why });
        }
    }
}

/// Removes the session in `session`, marked deleting: everything its store
/// holds, then its directory. Running it again after a crash finishes it.
/// A session in the layout before `rig-session/2` has nothing outside it.
fn finish_delete(session: &SessionDir, stores: &StoreOpener) -> Result<(), SessionError> {
    match (stores.open)(session) {
        Ok(store) => store.destroy()?,
        Err(SessionError::OldLayout) => {}
        Err(failure) => return Err(failure),
    }
    Ok(fs::remove_dir_all(session.path())?)
}

/// Finishes the deletes a crash cut short: every session of the app's
/// home marked deleting that no other rig runs.
fn finish_deletes(
    home: Option<Res<RigHome>>,
    (paths, stores): (Res<SessionPaths>, Res<StoreOpener>),
    mut deleted: MessageWriter<SessionDeleted>,
    mut failed: MessageWriter<SessionDeleteFailed>,
    mut notices: MessageWriter<Notice>,
) {
    let Some(home) = home else {
        return;
    };
    let Ok(entries) = fs::read_dir(home.0.sessions()) else {
        return;
    };
    for entry in entries.flatten() {
        let session = SessionDir::at(entry.path());
        if !session.is_deleting() || session.path() == paths.path() || session.runs_elsewhere() {
            continue;
        }
        match (finish_delete(&session, &stores), session.id()) {
            (Ok(()), Some(id)) => {
                deleted.write(SessionDeleted(id));
            }
            (Ok(()), None) => {}
            (Err(failure), id) => {
                notices.write(Notice::warn(
                    None,
                    format!(
                        "Could not finish deleting the session in {}: {failure}.",
                        session.path().display()
                    ),
                ));
                if let Some(id) = id {
                    let why = failure.to_string();
                    failed.write(SessionDeleteFailed { id, why });
                }
            }
        }
    }
}

/// The `LogPlugin` formatter: plain text appended to the session's text
/// log, so nothing is written to stderr. The log comes before the
/// session's other plugins, so it takes the session from the environment
/// first, and logs into the session's directory until every plugin is
/// built, when the log moves where [`Scratch`] says.
pub(crate) fn log_layer(app: &mut App) -> Option<BoxedFmtLayer> {
    let at = session_paths(app.world_mut()).log();
    let earlier = fs::metadata(&at).ok().map(|earlier| earlier.len());
    let file = OpenOptions::new().create(true).append(true).open(&at).ok();
    let log = TextLog {
        at: file.is_some().then_some(at),
        earlier,
        file: Arc::new(Mutex::new(file)),
    };
    let writer = log.file.clone();
    app.insert_resource(log);
    let layer = fmt::Layer::default().with_ansi(false);
    Some(Box::new(
        layer.with_writer(move || LogWriter(writer.clone())),
    ))
}

/// The text log's file, and where it is.
#[derive(Resource)]
struct TextLog {
    at: Option<PathBuf>,
    /// The length of the file earlier runs left at `at`, which this run
    /// appends to; `None` when this run made it.
    earlier: Option<u64>,
    file: Arc<Mutex<Option<File>>>,
}

/// Writes a line of the text log, or drops it without a file.
struct LogWriter(Arc<Mutex<Option<File>>>);

impl io::Write for LogWriter {
    fn write(&mut self, bytes: &[u8]) -> io::Result<usize> {
        let mut file = self.0.lock().unwrap_or_else(PoisonError::into_inner);
        match file.as_mut() {
            Some(file) => file.write(bytes),
            None => Ok(bytes.len()),
        }
    }

    fn flush(&mut self) -> io::Result<()> {
        let mut file = self.0.lock().unwrap_or_else(PoisonError::into_inner);
        file.as_mut().map_or(Ok(()), io::Write::flush)
    }
}

/// Moves what this run wrote to the text log so far to `wanted`, or
/// removes it for no place, and logs there from now on. What earlier runs
/// logged stays where it was.
fn move_text_log(world: &mut World, wanted: Option<PathBuf>) {
    let Some(mut log) = world.get_resource_mut::<TextLog>() else {
        return;
    };
    if log.at == wanted {
        return;
    }
    let handle = log.file.clone();
    let mut file = handle.lock().unwrap_or_else(PoisonError::into_inner);
    let moved = match (&log.at, log.earlier, &wanted) {
        (Some(at), None, Some(to)) => move_file(at, to),
        (Some(at), None, None) => fs::remove_file(at),
        (Some(at), Some(earlier), to) => take_tail(at, earlier, to.as_deref()),
        (None, _, _) => Ok(()),
    };
    if let Err(failure) = moved {
        // Said in the old log, which stays where it was.
        drop(file);
        error!("could not move the text log: {failure}");
        return;
    }
    *file = wanted
        .as_deref()
        .and_then(|to| OpenOptions::new().create(true).append(true).open(to).ok());
    drop(file);
    log.at = wanted;
    log.earlier = None;
}

/// Cuts the file `from` back to its first `length` bytes, appending what
/// followed to `to` when there is one.
fn take_tail(from: &Path, length: u64, to: Option<&Path>) -> io::Result<()> {
    if let Some(to) = to {
        let mut tail = File::open(from)?;
        tail.seek(SeekFrom::Start(length))?;
        if let Some(directory) = to.parent() {
            fs::create_dir_all(directory)?;
        }
        let mut moved = OpenOptions::new().create(true).append(true).open(to)?;
        io::copy(&mut tail, &mut moved)?;
    }
    OpenOptions::new().write(true).open(from)?.set_len(length)
}

/// Moves the file `from` to `to`, making its directory, across file
/// systems too.
fn move_file(from: &Path, to: &Path) -> io::Result<()> {
    if let Some(directory) = to.parent() {
        fs::create_dir_all(directory)?;
    }
    fs::rename(from, to).or_else(|_| {
        fs::copy(from, to)?;
        fs::remove_file(from)
    })
}

/// A warning or error this process logged, Bevy's own included.
#[derive(Reflect, Clone, Debug, Default)]
pub struct Logged {
    /// `Warn` or `Error`.
    pub level: NoticeLevel,
    /// The module that logged it, such as `bevy_app::hierarchy`.
    pub target: String,
    /// Its message, then its other fields.
    pub message: String,
    /// The `AgentId` its `agent` field names.
    pub agent: Option<String>,
    /// The plugin (its `Name`) whose module logged it, where that can be told.
    pub plugin: Option<String>,
}

/// The warnings and errors logged that nothing took yet. At most 256
/// wait; the rest are only in the text log.
#[derive(Resource)]
pub struct LoggedQueue(pub Receiver<Logged>);

/// The `LogPlugin` layer passing warnings and errors on to [`LoggedQueue`],
/// which exists before any plugin `plugins.toml` lists is added, and
/// timing systems into [`SystemTimings`].
pub fn log_events(app: &mut App) -> Option<BoxedLayer> {
    let (sender, receiver) = crossbeam_channel::bounded(256);
    let timings = SystemTimings::default();
    app.insert_resource(LoggedQueue(receiver))
        .insert_resource(timings.clone());
    Some(Box::new(PassOn(sender).and_then(TimeSystems(timings))))
}

struct PassOn(Sender<Logged>);

impl Layer<Registry> for PassOn {
    fn on_event(&self, event: &Event<'_>, _: Context<'_, Registry>) {
        let level = *event.metadata().level();
        if level <= Level::WARN {
            let target = event.metadata().target().to_owned();
            let level = if level == Level::ERROR {
                NoticeLevel::Error
            } else {
                NoticeLevel::Warn
            };
            let mut logged = Logged {
                level,
                target,
                ..Logged::default()
            };
            event.record(&mut logged);
            self.0.try_send(logged).ok();
        }
    }
}

/// A `log` record, such as Bevy's own warnings, names its module in
/// `log.target`.
impl Visit for Logged {
    fn record_str(&mut self, field: &Field, value: &str) {
        match field.name() {
            "log.target" => self.target = value.to_owned(),
            _ => self.record_debug(field, &format_args!("{value}")),
        }
    }

    fn record_debug(&mut self, field: &Field, value: &dyn Debug) {
        match field.name() {
            "agent" => self.agent = Some(format!("{value:?}")),
            "message" => self.message.insert_str(0, &format!("{value:?}")),
            name if name.starts_with("log.") => {}
            name => write!(self.message, " {name}={value:?}").unwrap_or_default(),
        }
    }
}

#[cfg(test)]
mod tests;