perf(stelae): a directory states its blob map instead of rebuilding it

[scarmuega]

Plan: plans/dolos-stelae-restore-operability.md (Trellis domain) — both defects it names: (1) a half-restored node reads as restored, (2) a directory restore decompresses every blob twice.

The defect

An inscription names layers by diffId — identity, over uncompressed bytes — and deliberately never by the compressed digest a blob is addressed by. A registry hands that map over off its manifest. A directory had no manifest, so SteleDir::blob_index rebuilt the map by hashing and decompressing every blob in the stele, in full, before the restore went on to read them all again.

A directory restore therefore decompressed the whole stele twice.

The change

SteleWriter::seal now writes the map beside inscription.json, as blobs.json:

{"layers":[{"diffId":"sha256:…","blob":"sha256:…"}, …]}

blob_index reads it and opens no blob. A directory that carries one is a degenerate registry, which is what having a manifest means.

Four properties hold it together:

• Transport, never identity. Nothing signs blobs.json, nothing hashes it into the stele's digest, and both halves of every entry are recoverable from the blobs. So it is additive: a stele without one still restores, by the scan that was the only way before.
• Absence is the older format; corruption is a fault. A missing file falls back to the scan. A file that is there and does not parse is an error, on the same principle RestoreProgress and PublishRecord already apply — reading a corrupt file as an absent one would restore correctly while silently paying the pass the file exists to remove.
• It exists only when it is complete. A reader that finds one stops looking at blobs, so a map missing an entry would read as a stele missing a layer. seal writes the file only if this handle wrote every layer the inscription lists, and removes any stale one otherwise. Postcondition: after a seal, blobs.json describes that inscription or does not exist.
• Written through a staging file and renamed, so the name never exists over half a document.

What moved, verification-wise

The scan was also a full verification pass, and that is not lost — it is narrowed to the layers somebody actually reads. A layer's diffId, uncompressed size and record count are proven by LayerReader::finish against the descriptor the signed inscription carries, on both the buffered and the streaming path, exactly as before. What a directory stops paying for is verifying blobs nobody asked for — which is the position a registry has always been in.

One error shape is preserved on purpose: a map can now place a layer whose file is gone, where before a missing file was simply a missing entry. SteleDir::open_blob maps that NotFound back to Error::LayerNotFound, so an operator is told which layer is missing rather than No such file or directory.

The saving, measured

cargo test --release -p stelae --test toy_profile the_scan_the_sidecar_removes -- --nocapture, on a stele of 8 layers / 8.39 MB (incompressible bodies, so the measurement is the pass and not the compressor):

8 layers, 8394544 compressed / 8394280 uncompressed bytes
blob_index by scan:    46.205ms
blob_index by sidecar: 36.125µs

≈1,280×, and the shape is what matters more than the ratio: the scan is linear in the size of the stele at ≈182 MB/s on this machine — ≈5.5 s per GB — and the sidecar is linear in the number of layers, at tens of microseconds for a stele of this one's shape. That pass ran before the restore fetched its first record, and a mainnet-sized stele is where it is worth having removed.

(1) A half-restored node reads as restored

ADR-004's restore pipeline set the state cursor at the end of step 5 — the state tip — and only then rebuilt the live-UTxO index dimensions (utxo::{address,payment,stake,policy,asset}) at step 6. has_existing_data() reads the cursor and nothing else, so an interruption between the two left a node that bootstrap --skip-if-data silently skipped and that answered address and asset queries with fewer rows rather than an error.

The ADR's owner ruled the step order rather than the progress file: set_cursor moves after the rebuild, as the last write of the restore. The state cursor becomes the completion marker for the whole restore instead of for the state tip alone; the progress file keeps its shape, and has_existing_data() keeps reading exactly one thing.

Nothing else has to move with it. rebuild_utxo_indexes takes the chain point as an argument, from plan.position.point, and never reads it back off the state store, so nothing between the old step 5 and step 6 consumes the cursor. And it costs nothing on resume: the tip is never checkpointed and the rebuild is unconditional, so a resumed restore already redid precisely the work that now follows the cursor.

The consequence, stated in the ADR and the module doc rather than left to be discovered: an interruption anywhere in a restore now leaves a node has_existing_data() reports as empty, which is what it is. --continue repairs it cheaply, because the epoch layers stay checkpointed; without it the stele is restored again from the top over keyed writes — a rewrite, not a duplication, and the behaviour every interruption before the tip already had.

restore.rs's module doc sections "The order is the specification" and "Why set_cursor is last, and what that does and does not buy" are superseded: the question is closed, and the docs now say what the order is rather than that an owner owes an answer. The progress file is cleared after the cursor, since the cursor is now what says the restore finished.

How it is pinned

a_restore_interrupted_in_the_live_utxo_rebuild_leaves_no_cursor (crates/snapshot/tests/restore.rs) fails the restore inside step 6 and asserts the store set has UTxOs and no cursor. The failure is injected through a new TestFault::IndexApplyError in dolos-testing, which fails only IndexWriter::apply — the restore's single call to it, in the rebuild — where the existing IndexStoreError fails start_writer and never gets that far. Reverting the set_cursor move fails the test on its cursor assertion, so it pins the order and not merely the outcome.

Verification

• cargo test --workspace --all-targets
• cargo test --workspace --all-targets --all-features --exclude dolos-minibf --exclude dolos-minikupo --exclude dolos-trp
• cargo clippy --all-targets --all-features -- -D warnings
• cargo +nightly fmt --all -- --check
• cargo deny check advisories
• cargo tree -p stelae -e normal --all-features — matches nothing ^dolos(-|$); the sidecar lives in the protocol crate and adds no dependency.

All re-run over the full branch after part (1) landed.

New test in crates/snapshot/tests/restore.rs:

• a_restore_interrupted_in_the_live_utxo_rebuild_leaves_no_cursor — part (1), described above.

New tests in crates/stelae/tests/toy_profile.rs:

• a_sealed_directory_states_its_blob_map — the sidecar's map is the map the scan rebuilds.
• the_sidecar_is_read_instead_of_the_blobs — every blob emptied in place; the sidecar path returns the same map (so it opened none), and the scan refuses the same directory.
• a_corrupt_sidecar_is_not_an_absent_one.
• an_incomplete_map_is_not_written.
• the_scan_the_sidecar_removes — the measurement above.

Two existing tests changed with the behaviour they pin: tampering_is_caught_on_read now shows where a corrupt blob is caught under each map, and a_malformed_record_is_reported_not_counted asks for the scan explicitly, because the blob it plants by hand is named in no inscription.

One red check, not from this branch

CI's cargo fmt job is red, and none of its diff is this PR's. Today's nightly rewraps comments at a narrower width than the last one, so cargo +nightly fmt --all -- --check reports 86 files across the repo — xtask/, tests/memory.rs, crates/cardano/** and the rest — none of which this branch touches; the one hunk it reports in crates/snapshot/src/restore.rs is at line 1588, in a test comment that predates this work. The same job is red on the other PRs opened against this repo today. Locally, on nightly 1.10.0-nightly (969b803cbe 2026-08-09), --check is clean over this branch.

Left alone deliberately: reformatting the repo to today's nightly is its own change, and not this plan's.

Summary by CodeRabbit

• New Features

  • Directory-based snapshots now include a blobs.json sidecar that maps layers to blob files, enabling faster reads and restores.
  • Snapshot sealing automatically creates or removes the sidecar as needed.

• Bug Fixes

  • Restore completion is now recorded only after live UTxO indexes are rebuilt, preventing incomplete restores from appearing finished.
  • Older snapshots without the sidecar remain supported through automatic reconstruction.
  • Corrupt or incomplete sidecar data is detected and reported appropriately.

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📒 Files selected for processing (2)

• crates/stelae/src/dir.rs
• crates/stelae/tests/toy_profile.rs

📝 Walkthrough

Walkthrough

Changes

The directory transport now writes and reads an unsigned blobs.json sidecar for layer-to-blob mappings, with blob scanning as a fallback. Restore now rebuilds live-UTxO indexes before writing the cursor. Fault-injection tests verify that failed rebuilds leave no cursor.

Snapshot integrity

Layer / File(s)	Summary
Directory blob-index sidecar adrs/004_stelae_snapshots.md, crates/stelae/src/dir.rs, crates/stelae/src/oci.rs, crates/stelae/src/transport.rs, crates/stelae/tests/toy_profile.rs	SteleDir records layer mappings, writes blobs.json during sealing, reads it during blob_index, and scans blobs only when the sidecar is absent. Tests cover corruption, incomplete maps, fallback behavior, and scan comparison.
Restore completion marker crates/snapshot/src/restore.rs, crates/snapshot/tests/restore.rs, crates/testing/src/faults.rs	Restore rebuilds live-UTxO indexes before committing the cursor. Fault injection targets IndexWriter::apply and verifies that interrupted restores leave no cursor.

Estimated code review effort: 4 (Complex) | ~45 minutes

Merge Risk: 🟡 Moderate · up to cff6b

This PR changes snapshot publication and restore completion behavior. A failed or concurrent reseal could leave the inscription and blob map inconsistent, while a mismatched snapshot layer may write partial records before rejection; interrupted index rebuilding may also expose an incomplete index as current. The PR should receive explicit owner review and follow-up on publication atomicity and rollback behavior before merging.

Sequence Diagram(s)

sequenceDiagram
  participant LayerSink
  participant SteleDir
  participant blobs_json
  LayerSink->>SteleDir: record layer mapping
  SteleDir->>blobs_json: write mapping during seal
  SteleDir->>blobs_json: read mapping during blob_index
  SteleDir->>SteleDir: scan blobs if sidecar is absent

Loading

sequenceDiagram
  participant Restore
  participant StateStore
  participant IndexWriter
  Restore->>StateStore: commit restored state tip
  Restore->>IndexWriter: rebuild live-UTxO indexes
  IndexWriter-->>Restore: apply index records
  Restore->>StateStore: commit cursor as final write

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Suggested reviewers: akashbhalla-svg

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 77.27% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 44 functions across 7 files. (1 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: Stelae directories persist their blob map instead of rebuilding it, improving lookup performance. It does not mention the separate restore cu…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Title check

Explanation

The title clearly and concisely describes the main change: Stelae directories persist their blob map instead of rebuilding it, improving lookup performance. It does not mention the separate restore cursor ordering change, but the title does not need to cover every change.

Full details: Docstring Coverage

Explanation

Docstring coverage is 77.27% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 44 functions across 7 files. (1 skipped: 1 unsupported.)

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Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

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Inline comments:
In `@crates/stelae/src/dir.rs`:
- Line 656: Update seal and write_blob_index to serialize sealing per directory,
invalidate the existing sidecar before replacing the inscription, and publish
inscription.json and blobs.json through one recoverable transaction. Replace the
shared .blobs.json.staging path with a unique per-operation staging path, and
route all storage writes through the established transactional writers so
concurrent or failed seals cannot publish mismatched files.

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📒 Files selected for processing (8)

• adrs/004_stelae_snapshots.md
• crates/snapshot/src/restore.rs
• crates/snapshot/tests/restore.rs
• crates/stelae/src/dir.rs
• crates/stelae/src/oci.rs
• crates/stelae/src/transport.rs
• crates/stelae/tests/toy_profile.rs
• crates/testing/src/faults.rs

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[scarmuega]

Superseded by the post-repo-split re-implementation: the crates/stelae half landed in txpipe/stelae#1 (released as v0.2.0), the profile half is #1297, which pins that release. Same content, reviewed history preserved on both replacements.