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fix(store): two ExperimentalMongoStore update fixes (truncated stream, version-CAS) - #55

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@b7r6 b7r6 commented Oct 7, 2026 •

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What and why

Lane A stability fix from the findings catalog (see nativelink-findings-2026-10-05.csv
on the meta/findings-catalog-2026-10-05 branch), rebased onto OSS main
@bf4b3aa855. This fork PR exists to validate the change on CI that is
representative of upstream PR lanes (public read-only cache, same endpoints)
before any upstream submission.

How was this verified?

The branch carries its own regression tests (fails-without where the fix
class allows it — see the branch's test commits). This PR's purpose is the
full representative CI run; green here rehearses the exact checks an
upstream PR would face.

Risk

Fork-internal validation PR; nothing lands anywhere by merging this. The
underlying fix's risk assessment happens at upstream-submission time.

AI assistance

Agent-assisted pipeline (Claude) for rebase and PR mechanics under my
direction; the fix itself carries verification in its tests.

@b7r6
b7r6 force-pushed the b7r6/mongo-update-truncated-on-stream-error branch 3 times, most recently from 0320a75 to a1b9aaf Compare October 8, 2026 15:12
@b7r6
b7r6 force-pushed the b7r6/mongo-update-truncated-on-stream-error branch 2 times, most recently from 94a1704 to ec6867d Compare October 15, 2026 11:19
b7r6 and others added 4 commits October 15, 2026 13:02
…ried blob (TraceMachina#2840)

`sizes_for_keys` snapshotted `lru.len()` once for the whole batch and
reused that stale length in `should_evict` for every key in the loop.

When the map sits one entry over `max_count` — e.g. an action-input
lease (or a TTL-pending entry) pins it at `max_count + 1` — the first
unleased queried key is popped, bringing the real length back to
`max_count`. But iterations 2..N still saw the stale `lru_len ==
max_count + 1`, so `is_over_count` stayed true and every remaining
queried, unleased entry was popped too: remove-callbacks queued,
`FileEntryImpl` unref deleting the backing files.

The batch then reports `None` for all N digests, the client treats them
as absent and re-uploads, and a plain existence check (`has()` →
FilesystemStore → `sizes_for_keys`) has destroyed every resident blob it
was only asked about.

Fix: keep a mutable working count and decrement it as entries are
reaped, so the over-count predicate is re-evaluated against the current
length each iteration. Eviction now stops the moment the map is back
at/under the limit — a size/existence query evicts at most the genuine
overage and never cascades.

Adds a white-box regression test that pins the map one over `max_count`
with a leased entry (bypassing the insert-time eviction pass the way a
held lease does) and asserts a batch `sizes_for_keys` over the resident
keys leaves all but the single-entry overage present. It fails on the
stale-len code (all N reported missing) and passes with the fix.

Provenance: adversarial review fleet, highest-severity finding.
* Update Rust crate rustls to v0.23.46 [SECURITY]
* Fix MODULE.bazel.lock
…date

update read the upload stream with while let Ok(chunk) = reader.recv().await,
which treats a stream Err identically to the empty-chunk EOF. An upload that
fails partway (client disconnect, or an upstream store erroring mid-copy) was
upserted as whatever bytes arrived and returned Ok(partial_len) -- a truncated
blob stored under the full digest key, served as success on every later read.

Propagate the recv error instead, and (belt-and-suspenders) reject a completed
read whose size disagrees with UploadSizeInfo::ExactSize.

Regression update_rejects_truncated_stream_instead_of_committing_it drops the
writer mid-stream without send_eof and asserts update returns Err and the digest
is absent; fails without the fix (commits the truncation as Ok).

Co-authored-by: Claude Opus 4.8 <[email protected]>
…ne), not Err

ExperimentalMongoStore::update_data used find_one_and_update(...).upsert(true). On a version-CAS conflict (a concurrent writer already bumped the version) the {_id, version} filter matches nothing, so upsert tries to INSERT a doc with the existing _id and MongoDB returns a DuplicateKeyError (E11000), surfaced as Err(Internal). The version-CAS contract (and the Redis backend, which returns Ok(None)) require a conflict to be retryable; instead concurrent updates on a Mongo-backed record spuriously failed. Fix: .upsert(current_version == 0) so only a genuine first-create upserts; a non-matching filter on an existing record now returns Ok(None) (retryable). Adds a deterministic fails-without regression and corrects an existing (ignored) scheduler test that had pinned the buggy Err behavior.

Co-Authored-By: Claude Opus 4.8 <[email protected]>
@b7r6
b7r6 force-pushed the b7r6/mongo-update-truncated-on-stream-error branch from ec6867d to c36fa2e Compare October 15, 2026 13:51
@b7r6 b7r6 changed the title fix(store): propagate mid-stream errors in ExperimentalMongoStore::update fix(store): two ExperimentalMongoStore update fixes (truncated stream, version-CAS) Oct 15, 2026
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