fix(version): refuse a snapshot read below the retained history - #617
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`SuperVersions::get_version_for_snapshot` panicked with "should always find
a SuperVersion" when a read asked for a snapshot at or below the oldest
retained version's seqno. Compaction maintenance (with the caller's GC
watermark) and `clear` prune the history past such snapshots, so every
read API could take the tree down on a valid argument.
- Return `Error::SnapshotBelowRetention { requested, oldest_retained }`
instead; point reads (get, get_pinned, multi_get, size_of, contains_key,
len, is_empty, approximate_range_*) surface it directly, iterators
(iter, range, prefix, range_seekable, batch_range_scan) yield it as their
first and only item, on both the standard and the KV-separated tree
- Add `AbstractTree::oldest_retained_seqno()` so a caller can validate a
snapshot before reading: servable iff `0` or strictly above it
- Keep snapshot `0` served from the oldest retained version (nothing is
visible at `0` from any version, so the choice is immaterial)
- `Tree::create_{iter,range,prefix,seekable_range_bounds}` now return
`Result`, raising the error before any I/O
- Document the retention boundary and the storage cost of a retention
window (proportional to write + compaction volume) in docs/INVARIANTS.md
Closes #616
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📝 WalkthroughWalkthroughThe change adds ChangesSnapshot retention handling
Estimated code review effort: 4 (Complex) | ~60 minutes Merge Risk: 🟡 Moderate · up to Repair operators could configure a floor lower than the source tree’s actual boundary and expose invalid historical reads. The guidance should be corrected before merge; the remaining test-contract issue is bounded by existing disk-backed integration coverage. Sequence Diagram(s)sequenceDiagram
participant Caller
participant Tree
participant SuperVersionHistory
participant Manifest
Caller->>Tree: read at snapshot seqno
Tree->>SuperVersionHistory: resolve retained version
SuperVersionHistory-->>Tree: version or SnapshotBelowRetention
Tree-->>Caller: value, iterator error item, or structured error
Tree->>Manifest: persist retention floor during destructive install
Manifest-->>Tree: restore retention floor after reopen or repair
🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
Full details: Docstring CoverageExplanation Docstring coverage is 73.94% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 165 functions across 26 files. (3 skipped: 3 unsupported.) ✨ Finishing Touches 💡 1📝 Generate docstrings 💡
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Inline comments:
In `@src/abstract_tree.rs`:
- Line 1003: Update the doctest for oldest_retained_seqno to avoid requiring
tempfile in no_std builds: gate the complete example with cfg(feature = "std")
while keeping the API available for no_std, or replace tempfile with a
no_std-compatible setup.
In `@src/blob_tree/mod.rs`:
- Line 1431: Update BlobTree::multi_get to acquire and validate the snapshot via
snapshot_for_read before the empty-batch early return. Preserve Ok(Vec::new())
for an empty batch only after validation, ensuring seqno values at or below
retention return Error::SnapshotBelowRetention consistently with
Tree::multi_get.
In `@src/version/super_version/tests.rs`:
- Around line 294-295: Update the three retention
tests—get_version_for_snapshot_at_zero_serves_oldest_retained,
get_version_for_snapshot_below_retention_returns_error, and
oldest_retained_seqno_tracks_the_history_front—to create temporary directories
with tempfile::tempdir(), write their test data on disk, and use StdFs instead
of MemFs so removal exercises std::fs::remove_file.
In `@tests/snapshot_below_retention.rs`:
- Line 135: The snapshot-below-retention fixtures in both tree-mode tests need
an oldest retained sequence number of at least 2. Update the version/sequence
setup around SequenceNumberCounter::default and the below value so the tested
snapshot is a deterministic nonzero sequence strictly less than
oldest_retained_seqno(), while preserving the SnapshotBelowRetention assertions.
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docs/INVARIANTS.mdsrc/abstract_tree.rssrc/blob_tree/mod.rssrc/error.rssrc/iter_guard.rssrc/tree/columnar_scan.rssrc/tree/mod.rssrc/version/super_version.rssrc/version/super_version/tests.rstests/snapshot_below_retention.rstests/tree_prefix_bloom.rs
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The boundary a GC compaction or `clear` established lived only in the in-memory version history: `SuperVersions::new` seeded the recovered version at seqno 0, so after a restart every historical snapshot was served from the surviving version, silently answering with data the snapshot never saw (the exact outcome `SnapshotBelowRetention` exists to refuse). - `Version::retention_floor`: the highest snapshot seqno the version can no longer serve, monotone across versions; persisted as the manifest `retention_floor` section and an appended edit-log field (both optional, so older manifests recover as floor 0) - `RetentionEffect` names what an install does to older snapshots and is passed to `upgrade_version`: a GC compaction (`GcBelow(w)`) raises the floor to `w - 1`, a `clear` / table drop (`DropsData`) to its own install seqno, additive installs (`Keep`) leave it alone; the floor rides in the same version edit as the data loss it records - a reopened history is seeded at the floor, version seqnos are clamped non-decreasing (a counter reset below the floor cannot slip a version under it) and `get_version_for_snapshot` checks the front explicitly - `Config::repair_retention_floor` (default 0): a rebuilt manifest cannot derive the floor (a GC compaction zeroes the settled rows' seqnos) and must not guess it (the external-WAL reconciliation reads intermediate snapshots back), so the deployment that ran the compactions supplies it - `BlobTree::multi_get` validates the snapshot before the empty-batch return, matching `Tree::multi_get` - the retention fixture pins `first + 1 < oldest_retained_seqno()` so the strict-below probes stay strict - docs: INVARIANTS (durable boundary), manifest-recovery (retention floor), external-wal (repair floor) Regression tests cover reopen after GC compaction / clear / drop_range / FIFO eviction / leveled merge, additive installs, manifest rotation, a reopen chain, a reset counter, checkpoints and repair, on both tree types; edit / diff / recovery / history unit tests cover the codec and the boundary check. Part of #616
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Inline comments:
In `@docs/external-wal.md`:
- Around line 247-248: Update the repair-floor guidance in docs/external-wal.md
lines 247-248 to use the maximum floor established by every retention-advancing
operation, including clear and table drops, with 0 when no pruning occurred;
update docs/manifest-recovery.md lines 334-335 to apply the same rule instead of
only the highest compaction watermark.
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docs/INVARIANTS.mddocs/external-wal.mddocs/manifest-recovery.mdsrc/abstract_tree.rssrc/blob_tree/ingest.rssrc/blob_tree/mod.rssrc/compaction/flavour.rssrc/compaction/worker.rssrc/config/mod.rssrc/error.rssrc/repair.rssrc/scrub/ecc_tests.rssrc/tree/ingest.rssrc/tree/mod.rssrc/version/diff.rssrc/version/diff/tests.rssrc/version/edit.rssrc/version/edit/tests.rssrc/version/mod.rssrc/version/recovery.rssrc/version/recovery/tests.rssrc/version/super_version.rssrc/version/super_version/tests.rstests/common/mod.rstests/repair.rstests/snapshot_below_retention.rs
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Three gaps in how an install raises the persisted retention floor, plus the accessor a deployment needs to record it: - A GC watermark above the counter (`SeqNo::MAX` to collect all history) pushed the floor to `MAX_SEQNO - 1`, so after a reopen every snapshot the counter could still hand out was refused. The floor is now capped at the compaction's own install seqno: a snapshot taken after the install sees the installed, complete data - A compaction whose user filter removed or rewrote rows recorded only the watermark-derived floor, so at watermark 0 a reopen served the filtered version for a snapshot the live tree still answered from the retained pre-compaction version. The filter adapter now counts its own non-`Keep` verdicts (`TransformCounters`), the producer marks the output, and `install_merge` raises the floor to the install seqno - A `drop_range` with no fully contained table installed an empty drop and raised the floor for nothing; `drop_tables` returns `CompactionResult::nothing()` for an empty id set and the drop-range strategy answers `DoNothing` like FIFO does - `AbstractTree::retention_floor()`: the persisted boundary (what a reopen enforces), distinct from the live `oldest_retained_seqno()`; this is the value to record for `Config::repair_retention_floor`, since it already folds in clears, drops and filtering compactions, not only GC watermarks. Docs (config, external-wal, manifest-recovery, INVARIANTS) now say so Regression tests: a `SeqNo::MAX` watermark reopen, a filter rewrite at watermark 0 across a reopen, an empty drop_range across a reopen, and the persisted-vs-live boundary after a drop. Part of #616
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Summary
SuperVersions::get_version_for_snapshotpanicked withshould always find a SuperVersionwhen a read asked for a snapshot at or below the oldest retained version's seqno. Compaction maintenance (with the caller's GC watermark) andclearprune the version history past such snapshots, so every read API (get,multi_get, range / prefix iterators, seekable and batch scans, columnar scan, range estimates) could take the tree down on a valid argument.The read is now refused with a typed error instead of served from a newer version (which would silently return data the snapshot never saw), and the boundary is durable:
Error::SnapshotBelowRetention { requested, oldest_retained }: point reads return it directly; iterators yield it as their first and only item (also viapeek_keyon the seekable iterator, whose seeks become no-ops). Standard and KV-separated trees alike, including an emptymulti_getbatch.AbstractTree::oldest_retained_seqno(): the read boundary, so a caller can validate a long-lived snapshot before reading. A snapshot is servable iff it is0or strictly above that seqno.0stays served from the oldest retained version: nothing is visible at0from any version, so the choice is immaterial and probing an empty tree keeps working after pruning.Version::retention_floor(persisted as a manifest section plus an appended edit-log field, both optional so older manifests recover as0) records the highest snapshot an install made unservable: a GC compaction with watermarkwsets it tow - 1, capped at the compaction's own install seqno (so aSeqNo::MAXwatermark cannot refuse every later snapshot); aclear/drop_range/ FIFO eviction / a compaction whose user filter removed or rewrote rows sets it to its own install seqno; additive installs (flush, ingest, trivial move) and an empty drop leave it alone. The install passes aRetentionEffecttoupgrade_version, so the floor rides in the same version edit as the data loss it records. A reopened history is seeded at the floor, version seqnos are clamped non-decreasing, and the front is checked explicitly, so a counter reset below the floor cannot slip a version under it.AbstractTree::retention_floor(): the persisted boundary (what a reopen enforces), distinct from the liveoldest_retained_seqno(), and the value a deployment records for a later repair.Config::repair_retention_floor(default0): a rebuilt manifest cannot derive the floor (a GC compaction zeroes the settled rows' seqnos) and must not guess it (the external-WAL reconciliation reads intermediate snapshots back after a repair), so the deployment supplies theretention_floor()it last recorded.Tree::create_{iter,range,prefix,seekable_range_bounds}(doc-hidden) returnResultand raise the error before any I/O; the happy path gains no per-item branch, and the lock-freesnapshot_for_readfast path is unchanged.INVARIANTS.md(retention boundary, its durability, and the storage cost of a retention window),manifest-recovery.md(retention floor after a repair),external-wal.md(repair floor), rustdoc on the new API.Testing
tests/snapshot_below_retention.rs(31 tests): every read surface on both tree types, theclearpath, snapshot0, the exact boundary (oldestfails,oldest + 1succeeds), the unpruned case; the boundary after a reopen following a GC compaction,clear,drop_range, FIFO eviction, a leveled merge and a filtering compaction at watermark 0, aSeqNo::MAXwatermark capped at the install, an emptydrop_rangeleaving it alone, additive installs leaving it at0, manifest rotation (floor read from the snapshot section), a reopen chain with a rising floor, a reset counter, the persisted-vs-live boundary, checkpoints, and repair with and without the configured floor.diffemits the floor only when changed, recovery applies it, the history seeds at the floor and checks the front before searching.cargo fmt --check,cargo clippy --workspace --all-targets(default and--all-features) with-D warnings,cargo doc --no-deps(default and--all-features) 0 warnings, no-std check 0 errors,cargo nextest run --workspace --all-features3288/3288,cargo test --doc --all-features80/80.Closes #616