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deckd: read a revision back, and report its parent #7

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751fa56
Read a revision's tree and files back from an explicit-commit repo
lukemelia Oct 27, 2026
eb11636
Report a revision's own parent from /commit and /outcome
lukemelia Oct 28, 2026
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76 changes: 72 additions & 4 deletions packages/deckd/README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -72,11 +72,13 @@ When Hub uses `DECKD_URL`, Node watchers publish + HMR only — they do not
| ensure / seal / restore | per-workspace **write** lock |
| list / file-at / file-list-at | per-workspace **read** lock (concurrent) |
| commit / outcome | per-repo **file** lock (cross-process) |
| tree / read / tree-hash | none; the file lock only on a miss |

## Explicit commits

`/commit`, `/outcome`, `/head` and `/tree-hash` record a revision from content
the caller staged, never from a working copy. A writer such as realm-server
the caller staged, never from a working copy. `/tree` and `/read` read a
revision back. A writer such as realm-server
can then record each write exactly once, even when deckd's response is lost.

Each call names `repo`, the absolute path of one history directory (for
Expand Down Expand Up @@ -169,12 +171,19 @@ Each commit's message ends with the caller's trailers, then
```json
{
"revision": "6f1c2d8e0a9b4c3d2e1f0a9b8c7d6e5f4a3b2c1d",
"parent": "0000000000000000000000000000000000000000",
"tree": "9a3e1b7c5d2f4e6a8b0c1d3e5f7a9b1c3d5e7f9a",
"created": true,
"replayed": false
}
```

- `parent` is `revision`'s own parent, which on a replay may be older than
the request's `parent`. It is `null` only for the root commit.
- `created` is false when the changes left HEAD's tree as it was, and
`revision` is that HEAD. On a replay (`replayed: true`) it says whether the
operation's first commit wrote `revision` or found the tree unchanged.

- `operationId`: 1–128 characters from `[A-Za-z0-9._-]`, not starting with `.`.
- `changes`: each entry has `path` plus exactly one of `contentBase64` or
`delete: true`. A path appears at most once. Paths are relative and may not
Expand All @@ -193,10 +202,16 @@ Each commit's message ends with the caller's trailers, then
```

```json
{ "state": "committed", "revision": "6f1c2d8e0a9b4c3d2e1f0a9b8c7d6e5f4a3b2c1d" }
{
"state": "committed",
"revision": "6f1c2d8e0a9b4c3d2e1f0a9b8c7d6e5f4a3b2c1d",
"parent": "0000000000000000000000000000000000000000",
"created": true
}
```

or `{ "state": "absent" }`, after which the ID is fenced. It is a POST because
`parent` and `created` mean what they do in `/commit`'s answer. Or the answer
is `{ "state": "absent" }`, after which the ID is fenced. It is a POST because
it can change state.

### `POST /head`
Expand Down Expand Up @@ -230,6 +245,59 @@ any repo, whoever committed it and whatever the message. The revision must be
in the repo's history; a commit object that was written but never published
is `revision-not-found`.

### `POST /tree`

```json
{
"repo": "/realms/acme/.history",
"revision": "6f1c2d8e0a9b4c3d2e1f0a9b8c7d6e5f4a3b2c1d"
}
```

```json
{
"entries": [
{
"path": "Author/jane.json",
"blob": "3b18e512dba79e4c8300dd08aeb37f8e728b8dad",
"executable": false
}
]
}
```

Every file in the revision's tree, in path order. `blob` is the git blob ID

@habdelra habdelra Oct 28, 2026

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[Claude Code 🤖] "In path order" doesn't describe the order /tree actually returns. A caller that merge-joins the list against a sorted disk listing will get mismatches. CommitService::tree passes along jj's MergedTree::entries(), which walks depth-first and sorts each directory by component name (jj-lib 0.46 TreeEntriesIterator; GitBackend::read_tree re-sorts git's entries by component). So a directory comes before a sibling file whose name it prefixes:

  • /tree returns Author/jane.json, then Author.gts
  • full-path byte order, git order, and JS Array.prototype.sort all put Author.gts first (. 0x2e < / 0x2f)

Realms routinely have a Foo.gts next to a Foo/ instance directory, so recovery will hit this case.

Either sort entries by the full path string before returning (entries.sort_by(|a, b| a.path.cmp(&b.path)), then "byte order of path" is a precise contract), or document the order as it is. I'd sort, since a flat byte order is what a caller can reproduce. Either way, tree_lists_every_file_with_its_git_blob_id should cover a.txt + a/b.txt: the current b.json / dir/a.gts pair has the same order under both rules, so it can't tell them apart. The doc comment on CommitService::tree makes the same claim.

Regression (new contract in this PR), non-blocking unless the realm-server consumer relies on the order.

of the file's bytes (`git hash-object`), so a caller can compare files on disk
with a revision without reading them out of history. The revision must be in
the repo's history, as for `/tree-hash`.

### `POST /read`

```json
{
"repo": "/realms/acme/.history",
"revision": "6f1c2d8e0a9b4c3d2e1f0a9b8c7d6e5f4a3b2c1d",
"paths": ["Author/jane.json", "Author/old.json"]
}
```

```json
{
"files": [
{ "path": "Author/jane.json", "contentBase64": "eyJuYW1lIjoiSmFuZSJ9" },
{ "path": "Author/old.json", "absent": true }
],
"remaining": []
}
```

- Each path's bytes at the revision, in the order asked. A path the revision
has no file at, including a directory, is `absent`.
- One answer carries at most 32 MiB of content. Paths past that point are not
read and are listed in `remaining`, in order, for the caller to ask for
again. The first path is always read, so every file can be read.
- Paths follow `/commit`'s rules.

### Errors

Every error body is `{ "code", "error" }`, plus fields named below, on every
Expand All @@ -238,7 +306,7 @@ route, including a request deckd can't parse.
| Status | `code` | When |
| ----------- | -------------------- | ----------------------------------------------------------------------- |
| 400/415/422 | `invalid-request` | Unparseable JSON, a missing field, the wrong content type, or bad input |
| 404 | `revision-not-found` | `/tree-hash` for a revision the repo doesn't have |
| 404 | `revision-not-found` | `/tree-hash`, `/tree` or `/read` for a revision the repo doesn't have |
| 409 | `parent-mismatch` | `parent` isn't HEAD; body has `head` |
| 409 | `operation-fenced` | The operation ID was reported `absent` by `/outcome` |
| 409 | `operation-mismatch` | The operation ID committed a different request; body has `revision` |
Expand Down
68 changes: 67 additions & 1 deletion packages/deckd/src/api.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -13,7 +13,7 @@ use tower_http::trace::TraceLayer;

use crate::history::commit::{
Author, Change, CommitError, CommitOutcome, CommitRequest, CommitService, CommitTrailer,
OperationOutcome, RevisionTree,
OperationOutcome, RevisionTree, TreeEntry,
};
use crate::history::{Actor, HistoryBackend, HistoryBackendError, HistoryEntry, RestorePlan};
use crate::watch::WatchPlane;
Expand Down Expand Up @@ -54,6 +54,8 @@ pub fn router(state: AppState) -> Router {
.route("/outcome", post(outcome))
.route("/head", post(head))
.route("/tree-hash", post(tree_hash))
.route("/tree", post(tree))
.route("/read", post(read))
.route("/ensure-build", post(ensure_build))
.route("/artifact-at", post(artifact_at))
.layer(TraceLayer::new_for_http())
Expand All @@ -77,6 +79,8 @@ pub fn router(state: AppState) -> Router {
.route("/outcome", post(outcome))
.route("/head", post(head))
.route("/tree-hash", post(tree_hash))
.route("/tree", post(tree))
.route("/read", post(read))
.layer(TraceLayer::new_for_http())
.with_state(state);

Expand Down Expand Up @@ -312,6 +316,34 @@ struct TreeHashResponse {
tree: String,
}

#[derive(Debug, Serialize)]
struct TreeResponse {
entries: Vec<TreeEntry>,
}

#[derive(Debug, Deserialize)]
struct ReadBody {
repo: String,
revision: String,
paths: Vec<String>,
}

#[derive(Debug, Serialize)]
#[serde(rename_all = "camelCase")]
struct ReadFile {
path: String,
#[serde(skip_serializing_if = "Option::is_none")]
content_base64: Option<String>,
#[serde(skip_serializing_if = "std::ops::Not::not")]
absent: bool,
}

#[derive(Debug, Serialize)]
struct ReadResponse {
files: Vec<ReadFile>,
remaining: Vec<String>,
}

async fn commit(
State(st): State<AppState>,
Body(body): Body<CommitBody>,
Expand Down Expand Up @@ -383,6 +415,40 @@ async fn tree_hash(
Ok(Json(TreeHashResponse { tree: found.tree }))
}

async fn tree(
State(st): State<AppState>,
Body(body): Body<TreeHashBody>,
) -> Result<Json<TreeResponse>, ApiError> {
let repo = std::path::PathBuf::from(body.repo);
let revision = body.revision;
let entries = run_commits(&st, move |c| c.tree(&repo, &revision)).await?;
Ok(Json(TreeResponse { entries }))
}

async fn read(
State(st): State<AppState>,
Body(body): Body<ReadBody>,
) -> Result<Json<ReadResponse>, ApiError> {
let repo = std::path::PathBuf::from(body.repo);
let revision = body.revision;
let paths = body.paths;
let found = run_commits(&st, move |c| c.read(&repo, &revision, &paths)).await?;
Ok(Json(ReadResponse {
files: found
.files
.into_iter()
.map(|file| ReadFile {
path: file.path,
absent: file.content.is_none(),
content_base64: file
.content
.map(|bytes| base64::engine::general_purpose::STANDARD.encode(bytes)),
})
.collect(),
remaining: found.remaining,
}))
}

/// Commit work waits on repo locks and drives jj-lib's `!Send` futures, so it
/// runs on a blocking thread.
async fn run_commits<T, F>(st: &AppState, f: F) -> Result<T, ApiError>
Expand Down
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