onetaskgraph_plugin_api/

source.rs

//! The two traits a plugin implements, and the secret lookup it is handed.

use std::collections::BTreeMap;

use schemars::{JsonSchema, Schema};
use secrecy::SecretString;
use serde::{Deserialize, Serialize};
use serde_json::Value;

use crate::{
    Capabilities, Comment, CommentBody, DependencyEdge, Direction, Document, DocumentQuery,
    ItemWrite, Label, MetadataKey, MetadataRecord, Metering, NativeId, NewComment, Page,
    PageRequest, Priority, Project, ProjectQuery, SourceError, SourceName, Status, StatusCategory,
    Task, TaskDetailRead, TaskQuery, TaskRef, TaskUpdate, TaskUpdateOutcome, WriteSupport,
    commentless, documentless, unwritable, unwritable_field, unwritable_metadata,
};

/// Whether a source is answering right now.
///
/// # Placement is an open contract question
///
/// This type lives here because [`TaskSource::health`] returns it and the trait
/// lives here: placing it in `onetaskgraph-core` would make this crate depend on
/// the engine and invert the one direction the crate split exists to establish.
/// The approved contract enumerates this crate's contents exhaustively and does
/// not name `Health`, so the enumeration and the trait as written cannot both
/// stand. Compiling forces the placement below; the resolution — add it to the
/// enumeration, or redesign `health` so no such type crosses the boundary —
/// belongs to the contract's owner, not to this crate. See `AGENTS.md`.
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize, JsonSchema)]
// llmlint: ignore[invalid_states_unrepresentable] SECOND PERMITTED REASON — this restates at a new site the justification already recorded at `Capabilities.max_page_size` (capability.rs) and `PageRequest.limit` (query.rs), and a third time in this type's own doc comment above and in AGENTS.md's "Open contract question — `Health`": `Health`'s shape is approved contract text that `TaskSource::health` returns, so an enum here would change the serialized form and the trait six undispatched nodes implement. That is the contract owner's call, not this crate's.
pub struct Health {
    /// Whether the source answered.
    ///
    /// A bare `bool` beside an untyped `detail` cannot say that an unreachable source
    /// must explain itself, or keep "reachable with a warning" apart from "reachable";
    /// an enum carrying the detail in its unreachable variant would.
    // llmlint: ignore[invalid_states_unrepresentable] SECOND PERMITTED REASON — this
    // restates at this field the justification already recorded at
    // `Capabilities.max_page_size` (capability.rs), `PageRequest.limit` (query.rs), this
    // type's own doc comment above, and AGENTS.md's "Open contract question — `Health`":
    // `Health`'s shape is approved contract text that `TaskSource::health` returns, so an
    // enum here would change the serialized form and the trait six undispatched nodes
    // implement. That is the contract owner's call, not this crate's.
    // llmlint: ignore[boundary_inputs_validated] making "unreachable with no reason given" unrepresentable means an enum here, which changes the serialized form and the trait six undispatched nodes implement. Deferred to the contract's owner — AGENTS.md, "Open contract question — `Health`".
    pub reachable: bool,
    /// What the source said, when it said anything useful.
    pub detail: Option<String>,
}

/// One configured source, as the engine drives it.
///
/// Dyn-compatible through `async_trait` because the engine holds
/// `Vec<Box<dyn TaskSource>>` over heterogeneous plugins.
///
/// Three rules bind every implementation, and the engine's compensation is only
/// correct while all three hold:
///
/// 1. **Apply** every predicate you declare [`Support::Native`](crate::Support::Native).
/// 2. **Ignore** every [`Support`](crate::Support)-typed predicate you declare
///    `Unsupported` — return the *wider* result set, never a narrower one.
///    Silently dropping rows for a predicate you did not declare is the one
///    failure no test above the plugin can catch.
/// 3. Never return a silently empty dependency read. Rule 2 reaches the
///    `Support`-typed *predicates* alone; a dependency read is always real, and so is a
///    document read — [`Capabilities::documents`] says whether this source has documents
///    at all, and a source that says it has none is never asked for one rather than
///    answering an empty page.
#[async_trait::async_trait]
pub trait TaskSource: Send + Sync {
    /// The plugin kind that built this source, for display and for plan output.
    fn kind(&self) -> &'static str;

    /// What this source applies itself. Read once per query by the engine.
    fn capabilities(&self) -> Capabilities;

    /// Whether the source is answering right now.
    ///
    /// # Errors
    ///
    /// Returns a [`SourceError`] when the check itself could not be made.
    async fn health(&self) -> Result<Health, SourceError>;

    /// Fetch one task by its native id, or `None` when there is no such task.
    ///
    /// # Errors
    ///
    /// Returns a [`SourceError`] when the source could not answer.
    async fn get_task(&self, id: &NativeId) -> Result<Option<Task>, SourceError>;

    /// Fetch several tasks by their native ids, each with the first page of its comments
    /// when `comments` names the page to read — one answer per id, in the order they were
    /// given.
    ///
    /// Each answer is what [`get_task`](Self::get_task) answers for that id, so a task that
    /// is not there is `Ok(None)` and a read that failed fails that id alone. Its
    /// [`TaskDetailRead::comments`] is what [`task_comments`](Self::task_comments) answers
    /// for `comments`. The engine passes `comments` only to a source declaring
    /// [`Capabilities::comments`](crate::Capabilities::comments) native.
    ///
    /// Defaulted to exactly those calls, item by item, so a source that implements nothing
    /// here answers as it always did. A source that can read many items — or an item and its
    /// comments — in fewer requests than that overrides it, which is the whole reason it
    /// exists: a caller holding twenty ids pays for one batch rather than forty reads.
    async fn get_task_details(
        &self,
        ids: &[NativeId],
        comments: Option<&PageRequest>,
    ) -> Vec<Result<Option<TaskDetailRead>, SourceError>> {
        let mut read = Vec::with_capacity(ids.len());
        for id in ids {
            read.push(match self.get_task(id).await {
                Ok(Some(task)) => {
                    let comments = match comments {
                        Some(page) => Some(self.task_comments(id, page).await),
                        None => None,
                    };
                    Ok(Some(TaskDetailRead { task, comments }))
                }
                Ok(None) => Ok(None),
                Err(error) => Err(error),
            });
        }
        read
    }

    /// Fetch one project by its native id, or `None` when there is no such project.
    ///
    /// # Errors
    ///
    /// Returns a [`SourceError`] when the source could not answer.
    async fn get_project(&self, id: &NativeId) -> Result<Option<Project>, SourceError>;

    /// One page of the tasks matching `query`.
    ///
    /// # Errors
    ///
    /// Returns a [`SourceError`] when the source could not answer.
    async fn query_tasks(
        &self,
        query: &TaskQuery,
        page: &PageRequest,
    ) -> Result<Page<Task>, SourceError>;

    /// One page of the projects matching `query`.
    ///
    /// # Errors
    ///
    /// Returns a [`SourceError`] when the source could not answer.
    async fn query_projects(
        &self,
        query: &ProjectQuery,
        page: &PageRequest,
    ) -> Result<Page<Project>, SourceError>;

    /// One page of every label this source knows.
    ///
    /// # Errors
    ///
    /// Returns a [`SourceError`] when the source could not answer.
    async fn labels(&self, page: &PageRequest) -> Result<Page<Label>, SourceError>;

    /// One page of the task dependency edges at `id`, in `direction`.
    ///
    /// # Errors
    ///
    /// Returns a [`SourceError`] when the source could not answer.
    async fn task_dependencies(
        &self,
        id: &NativeId,
        direction: Direction,
        page: &PageRequest,
    ) -> Result<Page<DependencyEdge>, SourceError>;

    /// One page of the project dependency edges at `id`, in `direction`.
    ///
    /// # Errors
    ///
    /// Returns a [`SourceError`] when the source could not answer.
    async fn project_dependencies(
        &self,
        id: &NativeId,
        direction: Direction,
        page: &PageRequest,
    ) -> Result<Page<DependencyEdge>, SourceError>;

    /// Whether this source can be written through at all.
    ///
    /// Defaulted to [`WriteSupport::Unsupported`], which is what keeps this a read
    /// interface for every source that has nothing to write into: one that cannot be
    /// written needs no edit and keeps working. Read before a write is attempted, so a
    /// copy naming such a source as its destination is refused before anything is read.
    fn writes(&self) -> WriteSupport {
        WriteSupport::Unsupported
    }

    /// Create or update one task, answering with the native id the destination holds it
    /// under.
    ///
    /// A source declaring [`WriteSupport::Supported`] owes three things here. It refuses,
    /// naming the field, anything it cannot represent rather than dropping it — including
    /// a metadata key it cannot carry, which it names. It writes every other field it was
    /// given. And it never creates when [`ItemWrite::target`] names an item it does not
    /// hold.
    ///
    /// # Errors
    ///
    /// Returns [`SourceError::Refused`] when this source has no write side, when a field
    /// or a metadata key cannot be represented, or when `target` names nothing here; and
    /// whatever else the source could not do the write for.
    async fn write_task(&self, write: &ItemWrite<Task>) -> Result<NativeId, SourceError> {
        let _ = write;
        Err(unwritable(self.kind()))
    }

    /// Create or update one project, on exactly the terms of
    /// [`write_task`](Self::write_task).
    ///
    /// # Errors
    ///
    /// As [`write_task`](Self::write_task).
    async fn write_project(&self, write: &ItemWrite<Project>) -> Result<NativeId, SourceError> {
        let _ = write;
        Err(unwritable(self.kind()))
    }

    /// Set the status of one task this source holds, and change nothing else about it,
    /// answering with the status as this source now reads it — or `None` when this source
    /// holds no such task.
    ///
    /// The category lands where this source's own mapping sends it, exactly as a
    /// [`write_task`](Self::write_task) of a task in that category would: a category this
    /// source has disabled is refused in the words a write of it is refused with. Title,
    /// content, labels, metadata, dependencies, [`Task::delivers`], [`Task::delivered_by`],
    /// project and comments are left exactly as they are.
    ///
    /// Defaulted to [`unwritable_field`], which is what keeps this an addition rather than a
    /// break: a source that cannot write a status on its own needs no edit and refuses by
    /// saying so. A source declaring [`WriteSupport::Unsupported`] is never asked.
    ///
    /// [`Task::delivers`]: crate::Task::delivers
    /// [`Task::delivered_by`]: crate::Task::delivered_by
    ///
    /// # Errors
    ///
    /// Returns [`SourceError::Refused`] when this source cannot write a status, or cannot
    /// write this one; and whatever else the source could not do the write for.
    async fn set_task_status(
        &self,
        id: &NativeId,
        category: StatusCategory,
    ) -> Result<Option<Status>, SourceError> {
        let _ = (id, category);
        Err(unwritable_field(self.kind(), "status"))
    }

    /// Set the priority of one task this source holds, and change nothing else about it,
    /// answering with the priority as this source now reads it — or `None` when this source
    /// holds no such task.
    ///
    /// [`Priority::None`] clears the priority. Title, content, status, labels, metadata,
    /// repositories, dependencies and comments are left exactly as they are. Nothing about
    /// the task's status moves, so the engine re-evaluates no delivered task after it.
    ///
    /// Defaulted to [`unwritable_field`], which is what keeps this an addition rather than a
    /// break. A source declaring [`WriteSupport::Unsupported`], or declaring
    /// [`Capabilities::priority`] unsupported, is never asked.
    ///
    /// # Errors
    ///
    /// Returns [`SourceError::Refused`] when this source cannot write a priority, or cannot
    /// write this one — a board with no option for it, naming the option; and whatever else
    /// the source could not do the write for.
    async fn set_task_priority(
        &self,
        id: &NativeId,
        priority: Priority,
    ) -> Result<Option<Priority>, SourceError> {
        let _ = (id, priority);
        Err(unwritable_field(self.kind(), "priority"))
    }

    /// Replace the content of one task this source holds with `content`, byte for byte, and
    /// change nothing else about it — or answer `None` when this source holds no such task.
    ///
    /// The content is [`Task::content`] exactly as this source reports it: what a later read
    /// answers there is `content`. Where the source keeps something else inside the same
    /// backend field — a metadata block in an issue body — that is kept as it was, and so is
    /// every other member: title, status, priority, labels, metadata, repositories,
    /// dependencies, project and comments. Nothing about the task's status moves, so the
    /// engine re-evaluates no delivered task after it.
    ///
    /// Defaulted to [`unwritable_field`] on exactly the terms of
    /// [`set_task_status`](Self::set_task_status). A source declaring
    /// [`WriteSupport::Unsupported`] is never asked.
    ///
    /// # Errors
    ///
    /// Returns [`SourceError::Refused`] when this source cannot write a task's content on its
    /// own, or cannot represent this one; and whatever else the source could not do the write
    /// for.
    async fn set_task_content(
        &self,
        id: &NativeId,
        content: &str,
    ) -> Result<Option<()>, SourceError> {
        let _ = (id, content);
        Err(unwritable_field(self.kind(), "content"))
    }

    /// Replace the [`Task::delivered_by`] of one task this source holds, and change nothing
    /// else about it — or answer `None` when this source holds no such task.
    ///
    /// Every entry is a qualified id, and the list is the whole of it: what the task held
    /// there before is replaced, not merged. It is the store's to keep in step — the engine
    /// calls this whenever it writes a task's [`Task::delivers`] — and nothing a person types
    /// reaches it directly.
    ///
    /// Defaulted to [`unwritable_field`] on exactly the terms of
    /// [`set_task_status`](Self::set_task_status).
    ///
    /// [`Task::delivers`]: crate::Task::delivers
    /// [`Task::delivered_by`]: crate::Task::delivered_by
    ///
    /// # Errors
    ///
    /// Returns [`SourceError::Refused`] when this source cannot hold the list, and whatever
    /// else it could not do the write for.
    async fn set_delivered_by(
        &self,
        id: &NativeId,
        delivered_by: &[TaskRef],
    ) -> Result<Option<()>, SourceError> {
        let _ = (id, delivered_by);
        Err(unwritable_field(self.kind(), "delivered_by"))
    }

    /// Set one key of the metadata of one task this source holds, and change nothing else
    /// about it, answering with the task as this source reads it back after the write — or
    /// `None` when this source holds no such task.
    ///
    /// The key is added when the task does not hold it and replaced when it does; every other
    /// metadata key, and every other field of the task, is left exactly as it was. A `value`
    /// the task already holds under `key` is a write that changes nothing, and a source owes
    /// it no write at all. The answer is a read, not an echo: what the engine reports as the
    /// value is what the returned task holds under `key`.
    ///
    /// Nothing about the task's status or its [`Task::delivers`] moves, so the engine
    /// re-evaluates no delivered task after this write.
    ///
    /// Defaulted to [`unwritable_metadata`] on exactly the terms of
    /// [`set_task_status`](Self::set_task_status): a source that cannot write one key on its
    /// own needs no edit and refuses by saying so. A source declaring
    /// [`WriteSupport::Unsupported`] is never asked.
    ///
    /// [`Task::delivers`]: crate::Task::delivers
    ///
    /// # Errors
    ///
    /// Returns [`SourceError::Refused`] when this source cannot write one key of a task's
    /// metadata on its own, or cannot write this one without changing something else; and
    /// whatever else the source could not do the write for.
    async fn set_task_metadata(
        &self,
        id: &NativeId,
        key: &MetadataKey,
        value: &Value,
    ) -> Result<Option<Task>, SourceError> {
        let _ = (id, key, value);
        Err(unwritable_metadata(self.kind(), MetadataRecord::Task))
    }

    /// Apply a targeted update to one task this source holds — every field `update` names, and
    /// nothing else — answering with a [`TaskUpdateOutcome`], or `None` when this source holds
    /// no such task.
    ///
    /// The outcome is everything the engine reports about the call, so it reads nothing of its
    /// own: the task as this source reads it once the update landed — from the one read it made
    /// and what it then wrote, where its backend answers a write with what it holds — the
    /// fields it actually wrote, and the `delivers` the task held before.
    ///
    /// A field already holding the requested value is sent no write, and an update in which
    /// nothing differs sends no write at all. Every field `update` leaves unnamed — labels,
    /// repositories, project, [`Task::delivered_by`] and comments always among them — is left
    /// exactly as it is. An update naming one metadata key both to set and to remove is refused
    /// before anything is written, in the words of [`TaskUpdate::consistent`].
    ///
    /// Defaulted to [`TaskUpdate::rewrite`]: read the task, apply the update, and — only when
    /// anything differs — [`write_task`](Self::write_task) it with its `target` set. That keeps
    /// this an addition rather than a break, and a source that predates it behaves correctly and
    /// is merely not minimal; one whose backend can take a field on its own owes an override
    /// that sends only what differs. A source declaring [`WriteSupport::Unsupported`] is never
    /// asked.
    ///
    /// [`Task::delivered_by`]: crate::Task::delivered_by
    ///
    /// # Errors
    ///
    /// Returns [`SourceError::Refused`] when the update contradicts itself, when a field it
    /// names cannot be represented here — a category this source has disabled included — and
    /// whatever else the source could not do the write for.
    async fn update_task(
        &self,
        id: &NativeId,
        update: &TaskUpdate,
    ) -> Result<Option<TaskUpdateOutcome>, SourceError> {
        update.rewrite(self, id).await
    }

    /// Set one key of the metadata of one project this source holds, on exactly the terms of
    /// [`set_task_metadata`](Self::set_task_metadata).
    ///
    /// # Errors
    ///
    /// As [`set_task_metadata`](Self::set_task_metadata).
    async fn set_project_metadata(
        &self,
        id: &NativeId,
        key: &MetadataKey,
        value: &Value,
    ) -> Result<Option<Project>, SourceError> {
        let _ = (id, key, value);
        Err(unwritable_metadata(self.kind(), MetadataRecord::Project))
    }

    /// Set one key of the metadata of one document this source holds, on exactly the terms of
    /// [`set_task_metadata`](Self::set_task_metadata).
    ///
    /// A source declaring [`Capabilities::documents`] unsupported is never asked, exactly as
    /// it is never asked for a document read.
    ///
    /// # Errors
    ///
    /// As [`set_task_metadata`](Self::set_task_metadata).
    async fn set_document_metadata(
        &self,
        id: &NativeId,
        key: &MetadataKey,
        value: &Value,
    ) -> Result<Option<Document>, SourceError> {
        let _ = (id, key, value);
        Err(unwritable_metadata(self.kind(), MetadataRecord::Document))
    }

    /// Remove one task this destination holds, so a copy that could not finish can put
    /// the destination back the way it found it.
    ///
    /// This is not a verb of the product: nothing a user types deletes anything, and a
    /// copy never deletes an item it did not itself create in the run that is failing.
    /// It exists because a copy is either complete or it never happened — a half-written
    /// project has to be run again, and the re-run is the mutation burst that trips a
    /// hosted destination's rate limiter. Undoing this run's own creates is what removes
    /// that retry at source.
    ///
    /// A source declaring [`WriteSupport::Supported`] owes a real implementation, for the
    /// reason it owes [`write_task`](Self::write_task) one: the engine will create items
    /// there, so it has to be able to remove the ones it created. An `id` naming nothing
    /// is **not** an error — the item is already gone, which is the state this asks for.
    ///
    /// # Errors
    ///
    /// Returns [`SourceError::Refused`] when this source has no write side, and whatever
    /// else the source could not remove the item for.
    async fn delete_task(&self, id: &NativeId) -> Result<(), SourceError> {
        let _ = id;
        Err(unwritable(self.kind()))
    }

    /// Remove one project this destination holds, on exactly the terms of
    /// [`delete_task`](Self::delete_task).
    ///
    /// # Errors
    ///
    /// As [`delete_task`](Self::delete_task).
    async fn delete_project(&self, id: &NativeId) -> Result<(), SourceError> {
        let _ = id;
        Err(unwritable(self.kind()))
    }

    /// Fetch one document by its native id, or `None` when there is no such document.
    ///
    /// Defaulted to [`documentless`], which is what keeps documents an addition rather
    /// than a break: a source with none needs no edit, keeps working, and says so in the
    /// same words every other document-free source does. A source that has documents
    /// declares [`Support::Native`](crate::Support::Native) for
    /// [`Capabilities::documents`] and owes a real implementation here, because that
    /// declaration is what makes the engine ask.
    ///
    /// # Errors
    ///
    /// Returns [`SourceError::Refused`] when this source has no documents, and whatever
    /// else the source could not answer for.
    async fn get_document(&self, id: &NativeId) -> Result<Option<Document>, SourceError> {
        let _ = id;
        Err(documentless(self.kind()))
    }

    /// One page of the documents matching `query`.
    ///
    /// Defaulted on exactly the terms of [`get_document`](Self::get_document). A source
    /// with no documents refuses rather than answering an empty page: an empty page reads
    /// as a source that has documents and holds none matching, which is the one wrong
    /// answer this method can give.
    ///
    /// # Errors
    ///
    /// As [`get_document`](Self::get_document).
    async fn query_documents(
        &self,
        query: &DocumentQuery,
        page: &PageRequest,
    ) -> Result<Page<Document>, SourceError> {
        let _ = (query, page);
        Err(documentless(self.kind()))
    }

    /// Create or update one document, on exactly the terms of
    /// [`write_task`](Self::write_task).
    ///
    /// # Errors
    ///
    /// As [`write_task`](Self::write_task).
    async fn write_document(&self, write: &ItemWrite<Document>) -> Result<NativeId, SourceError> {
        let _ = write;
        Err(unwritable(self.kind()))
    }

    /// Remove one document this destination holds, on exactly the terms of
    /// [`delete_task`](Self::delete_task).
    ///
    /// # Errors
    ///
    /// As [`delete_task`](Self::delete_task).
    async fn delete_document(&self, id: &NativeId) -> Result<(), SourceError> {
        let _ = id;
        Err(unwritable(self.kind()))
    }

    /// One page of the comments on `task`, oldest first, or `None` when this source holds
    /// no such task.
    ///
    /// Defaulted to [`commentless`], which is what keeps comments an addition rather than a
    /// break: a source with none needs no edit and keeps working. A source whose tasks have
    /// comments declares [`Support::Native`](crate::Support::Native) for
    /// [`Capabilities::comments`] and owes a real implementation of all four comment methods,
    /// because that declaration is what makes the engine ask.
    ///
    /// "No such task" is `None` rather than an error, exactly as it is for
    /// [`get_task`](Self::get_task); a task that exists and has no comments is an empty page.
    ///
    /// # Errors
    ///
    /// Returns [`SourceError::Refused`] when this source has no comments, and whatever else
    /// the source could not answer for.
    async fn task_comments(
        &self,
        task: &NativeId,
        page: &PageRequest,
    ) -> Result<Option<Page<Comment>>, SourceError> {
        let _ = (task, page);
        Err(commentless(self.kind()))
    }

    /// Add one comment to `task`, answering with the comment as the source now holds it, or
    /// `None` when this source holds no such task.
    ///
    /// The body is stored byte for byte. A source that records the author itself refuses a
    /// [`NewComment::author`] rather than dropping it, naming why; a source that cannot
    /// represent the body refuses it, naming why, rather than escaping it into something
    /// else.
    ///
    /// # Errors
    ///
    /// Returns [`SourceError::Refused`] when this source has no comments or cannot be
    /// written, when it cannot record what it was given, and whatever else it could not do
    /// the write for.
    async fn add_comment(
        &self,
        task: &NativeId,
        comment: &NewComment,
    ) -> Result<Option<Comment>, SourceError> {
        let _ = (task, comment);
        Err(commentless(self.kind()))
    }

    /// Replace the body of the comment `comment` on `task`, answering with the comment as the
    /// source now holds it, or `None` when this source holds no such task or that task has no
    /// such comment.
    ///
    /// Only the body and the time it last changed move: the id, the author and the time it
    /// was written are the comment's own.
    ///
    /// # Errors
    ///
    /// As [`add_comment`](Self::add_comment).
    async fn edit_comment(
        &self,
        task: &NativeId,
        comment: &NativeId,
        body: &CommentBody,
    ) -> Result<Option<Comment>, SourceError> {
        let _ = (task, comment, body);
        Err(commentless(self.kind()))
    }

    /// Remove the comment `comment` from `task`, answering with the id it removed, or `None`
    /// when this source holds no such task or that task has no such comment.
    ///
    /// Unlike [`delete_task`](Self::delete_task), this *is* a verb of the product — a person
    /// removes a comment they posted — so a comment that is not there is reported as `None`
    /// for the engine to refuse by name, rather than treated as already gone.
    ///
    /// # Errors
    ///
    /// As [`add_comment`](Self::add_comment).
    async fn delete_comment(
        &self,
        task: &NativeId,
        comment: &NativeId,
    ) -> Result<Option<NativeId>, SourceError> {
        let _ = (task, comment);
        Err(commentless(self.kind()))
    }

    /// Whether this source keeps template answers beside its items at all.
    ///
    /// What lets a regenerate tell an item whose answers went missing — a block deleted by
    /// hand, which the regenerate writes back — from an item of a source that never keeps any,
    /// where a missing answer is no difference. Defaulted to `false`, as
    /// [`task_template_answers`](Self::task_template_answers) is defaulted to `None`; a source
    /// that keeps answers answers `true` here.
    fn keeps_template_answers(&self) -> bool {
        false
    }

    /// The template answers the task `id` was last rendered from, as this source keeps them
    /// beside the task — or `None` when it keeps none for it, or holds no such task.
    ///
    /// Keeping answers is a source's own choice, never an obligation: a source whose items
    /// are one record in a hosted system has no room beside the item that is not the item, and
    /// answers written into its content or its metadata would duplicate what the content
    /// already says. Defaulted to `None`, which is what keeps this an addition rather than a
    /// break. A source that keeps them owes three things: they are in neither
    /// [`Task::content`] nor [`Task::metadata`], they are written only by
    /// [`write_task_rendered`](Self::write_task_rendered) and
    /// [`set_task_rendering`](Self::set_task_rendering), and they come back here exactly as
    /// they were written, JSON types intact.
    ///
    /// # Errors
    ///
    /// Returns a [`SourceError`] when the source could not answer, a record whose answers it
    /// cannot read included.
    async fn task_template_answers(
        &self,
        id: &NativeId,
    ) -> Result<Option<BTreeMap<String, Value>>, SourceError> {
        let _ = id;
        Ok(None)
    }

    /// The template answers the document `id` was last rendered from, on exactly the terms of
    /// [`task_template_answers`](Self::task_template_answers).
    ///
    /// # Errors
    ///
    /// As [`task_template_answers`](Self::task_template_answers).
    async fn document_template_answers(
        &self,
        id: &NativeId,
    ) -> Result<Option<BTreeMap<String, Value>>, SourceError> {
        let _ = id;
        Ok(None)
    }

    /// Create or update one task exactly as [`write_task`](Self::write_task) does, keeping
    /// `answers` — the answers its content was rendered from — beside it in the same write
    /// where this source keeps answers at all.
    ///
    /// The task arrives carrying its provenance under [`MetadataKey::TEMPLATE_KEY`] like any
    /// other metadata entry. Defaulted to [`write_task`](Self::write_task) alone: a source that
    /// keeps no answers writes the task and nothing beside it, which is the whole of what it
    /// owes. A source that keeps them writes the task and the answers together, so a reader
    /// never finds one without the other.
    ///
    /// # Errors
    ///
    /// As [`write_task`](Self::write_task).
    async fn write_task_rendered(
        &self,
        write: &ItemWrite<Task>,
        answers: &BTreeMap<String, Value>,
    ) -> Result<NativeId, SourceError> {
        let _ = answers;
        self.write_task(write).await
    }

    /// Create or update one document, on exactly the terms of
    /// [`write_task_rendered`](Self::write_task_rendered).
    ///
    /// # Errors
    ///
    /// As [`write_document`](Self::write_document).
    async fn write_document_rendered(
        &self,
        write: &ItemWrite<Document>,
        answers: &BTreeMap<String, Value>,
    ) -> Result<NativeId, SourceError> {
        let _ = answers;
        self.write_document(write).await
    }

    /// Replace one task's rendering — its content, byte for byte, its
    /// [`MetadataKey::TEMPLATE_KEY`] entry, set to `provenance`, and the answers it keeps
    /// beside the task where it keeps any — in one write, and change nothing else about it;
    /// or answer `None` when this source holds no such task.
    ///
    /// The content is [`Task::content`] exactly as a later read reports it, on the terms of
    /// [`set_task_content`](Self::set_task_content). Title, status, priority, labels, every
    /// other metadata entry, repositories, dependencies, [`Task::delivers`],
    /// [`Task::delivered_by`], project and comments are left exactly as they are. The write is
    /// one write: a reader sees the task as it was or as it is now, never a new content beside
    /// the old provenance or the old answers.
    ///
    /// Defaulted to [`unwritable_field`] on exactly the terms of
    /// [`set_task_status`](Self::set_task_status). A source declaring
    /// [`WriteSupport::Unsupported`] is never asked.
    ///
    /// [`Task::delivers`]: crate::Task::delivers
    /// [`Task::delivered_by`]: crate::Task::delivered_by
    ///
    /// # Errors
    ///
    /// Returns [`SourceError::Refused`] when this source cannot replace a task's rendering on
    /// its own, or cannot represent this one; and whatever else the source could not do the
    /// write for.
    async fn set_task_rendering(
        &self,
        id: &NativeId,
        content: &str,
        provenance: &Value,
        answers: &BTreeMap<String, Value>,
    ) -> Result<Option<()>, SourceError> {
        let _ = (id, content, provenance, answers);
        Err(unwritable_field(self.kind(), "rendering"))
    }

    /// Replace one document's rendering, on exactly the terms of
    /// [`set_task_rendering`](Self::set_task_rendering). A source declaring
    /// [`Capabilities::documents`] unsupported is never asked.
    ///
    /// # Errors
    ///
    /// As [`set_task_rendering`](Self::set_task_rendering).
    async fn set_document_rendering(
        &self,
        id: &NativeId,
        content: &str,
        provenance: &Value,
        answers: &BTreeMap<String, Value>,
    ) -> Result<Option<()>, SourceError> {
        let _ = (id, content, provenance, answers);
        Err(SourceError::Refused {
            message: format!(
                "the {} plugin cannot write a document's rendering on its own",
                self.kind()
            ),
        })
    }

    /// What this source has sent to its backend since it was built and what that spent, or
    /// `None` when it does not meter its own requests.
    ///
    /// Defaulted to `None`, which is what keeps metering an addition rather than a break: a
    /// source that does not count its requests needs no edit, and is reported as not
    /// metering rather than as having spent nothing. A source that answers owes a running
    /// total — see [`Metering`] — because what one command spent is read as the difference
    /// between two readings.
    ///
    /// # Errors
    ///
    /// Returns a [`SourceError`] when the reading itself could not be taken. A caller
    /// reports such a source as not metering; what a command cost is never a reason for the
    /// command to fail.
    async fn metering(&self) -> Result<Option<Metering>, SourceError> {
        Ok(None)
    }
}

/// The factory that turns one configuration block into a live [`TaskSource`].
///
/// Having the compile-time registry and the subprocess seam be the same shape is
/// the whole reason this is a trait rather than a free function.
pub trait SourcePlugin: Send + Sync + 'static {
    /// The name a configuration document's `plugin:` field names.
    fn kind(&self) -> &'static str;

    /// The JSON Schema for this plugin's own `config:` block.
    fn config_schema(&self) -> Schema;

    /// Build a live source from one configuration block.
    ///
    /// `name` is the configured source's name, for error messages only — a
    /// plugin never learns it for any other purpose.
    ///
    /// # Errors
    ///
    /// Returns [`SourceError::Config`] when `config` is not valid for this
    /// plugin, or [`SourceError::Auth`] when a named credential is absent.
    fn build(
        &self,
        name: &SourceName,
        config: &serde_json::Value,
        secrets: &dyn SecretResolver,
    ) -> Result<Box<dyn TaskSource>, SourceError>;

    /// The fields of this plugin's `config:` block that name a filesystem path, as dotted
    /// paths into that block.
    ///
    /// A relative value at one of these, **supplied by a configuration document**, is
    /// resolved against the directory holding that document before [`Self::build`] sees it;
    /// supplied through the environment or a flag it keeps resolving against the process
    /// working directory, because there is no document to rebase it on. A plugin is handed
    /// values and no origins, so this declaration is the only way it can say which of its
    /// own fields that rule reaches.
    ///
    /// Defaulted to none, which is what keeps this an addition rather than a break: a
    /// plugin whose block holds no path needs no edit, and a caller asks every plugin
    /// rather than keeping a table of which ones answer.
    // llmlint: ignore[invalid_states_unrepresentable] The identity of a configuration field
    // is a name, and no type can make a wrong one unrepresentable here: every string is a
    // syntactically valid dotted path, so a newtype would validate nothing and would only
    // move where a name that is not a field of *this* plugin is accepted. What decides that
    // is whether the name is a property of the schema `config_schema` publishes — a
    // per-plugin fact no shared type can hold — so the gate is per plugin and executable:
    // `document_relative_fields_are_fields_this_plugin_declares` in
    // `onetaskgraph-local-md/tests/plugin.rs`, which a plugin adding a declaration owes its
    // own copy of.
    fn document_relative_paths(&self) -> &'static [&'static str] {
        &[]
    }
}

/// How a plugin reads the credential its configuration names.
///
/// A configuration document never carries a credential value, only the name of
/// the environment variable holding it.
pub trait SecretResolver: Send + Sync {
    /// The value of `var`, or `None` when nothing defines it.
    ///
    /// The returned value is never logged and never appears in `Debug` output.
    fn get(&self, var: &str) -> Option<SecretString>;
}