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fix(rcache): skip an artifact write when the object exists at the same size; retry a 429 - #1755
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| Check name | Status | Explanation | Resolution |
|---|---|---|---|
| Description check |  |
The description explains the problem and the intended behavior, but it does not include the required Summary, Testing, and Checklist sections. It also provides no test commands or output. | Add the required Summary, Testing, and Checklist sections. Include the commands run and relevant test output, state whether documentation needs updates, and include a BREAKING CHANGE footer if applicable. |
| Docstring Coverage | |
Docstring coverage is 63.16% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 19 functions across 1 files. (2 skipped: … | Write docstrings for the functions missing them to satisfy the coverage threshold. |
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Check name Status Explanation Title check The title clearly identifies the main changes: skipping redundant artifact writes and retrying GCS 429 responses. It is concise and follows the required Conventional Commit format. Linked Issues check Check skipped because no linked issues were found for this pull request. Out of Scope Changes check Check skipped because no linked issues were found for this pull request.
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Docstring coverage is 63.16% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 19 functions across 1 files. (2 skipped: 2 unsupported.)
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Write docstrings for the functions missing them to satisfy the coverage threshold.
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I checked the cache beneath the moon,
For matching bytes and checksums true.
When 429 blocked the upload lane,
I tried again with file rewound.
A matching object saved the day,
And staged its index on the way.
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Reviewed: 1 finding, none blocking at Address the comments on the diff. Clobber: none seen. |
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Actionable comments posted: 1
- 🪄 Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @crates/rcache/src/remote_writer.rs:
- Line 182: In send_buffered(), rewind each cloned tar_file to offset zero
before starting an upload attempt, mapping seek failures through Error::IO, so
retries upload the complete file.
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The same-size skip is sound: the object name is the sha256 of the compressed bytes and GCS only makes an object visible once its upload finalizes, so a same-name, same-length object is the same content. The 429 retry has a bug that writes an empty object, noted inline. Separately, commitlint is red: the commit header rcache: ... has no type, so it needs rewording to something like fix(rcache): ... (the PR title too).
| .await?; | ||
| let mut attempt = 0; | ||
| loop { | ||
| let f = tar_file.try_clone().map_err(Error::IO)?; |
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try_clone() shares the file offset with tar_file, so every retry starts where the previous attempt stopped reading. For the small objects this PR targets, send_buffered() takes the single-shot path, which reads the whole source into memory before it sends. After a 429 the offset is at EOF, so the retry reads zero bytes, computes the crc32c over zero bytes, and writes a 0-byte object under the sha256 name. That attempt succeeds, so the spec hash goes into pending and the index points at the empty object. Seek the clone back to 0 before each attempt, e.g. f.seek(SeekFrom::Start(0)).
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Right, and that was a real data bug for exactly the objects this PR targets. Fixed in 7856280: each attempt seeks its clone to the start before the write. upload_retry_after_429_sends_the_whole_file moves the shared offset to the end inside the first, failing attempt, as a single-shot read would, and asserts the retry's payload carries every byte. Without the rewind it fails with an empty payload.
| { | ||
| Ok(_) => break, | ||
| Err(e) | ||
| if e.http_status_code() == Some(TOO_MANY_REQUESTS) |
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A 429 here means another uploader just wrote this same object, so rewriting it is redundant, and with several parallel uploaders each retry is one more mutation on the hot name. Re-running the open_object size check before each retry, and skipping the write when it matches, makes the retry idempotent and lets most of them return without writing. A create-only write (set_if_generation_match(0), treating 412 as already present) would do the same in one request.
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Done in 7856280: before each retry the size check runs again, and when the object is present at the artifact's length the retry writes nothing; the spec hash still goes to the index. I kept the existing write rather than a create-only one, so behaviour against objects written before this change is the same. upload_429_then_present_does_not_write_again covers it.
| .send() | ||
| .await | ||
| && stat.object().size > 1024 * 1024 | ||
| && stat.object().size as u64 == local_len |
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Neither new path has a test. The mock already stubs open_object and write_object_buffered. One test where open_object returns a matching size and no write is expected, and one where the first write returns 429 and the second returns Ok while asserting the second payload carries the full file bytes, would cover both paths. The second test would have caught the offset bug above.
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Added in 7856280, on the existing mock: upload_skips_an_object_present_at_the_same_length (no write expected), upload_retry_after_429_sends_the_whole_file (429, then a write whose payload must hold the full file; it catches the offset bug), and upload_429_then_present_does_not_write_again.
…e size; retry a 429 Artifact objects are content-addressed, so an existing object of the same length is the same bytes; the writer only skipped objects over 1 MiB, so every small identical output (several packages produce the same empty tarball) was rewritten by every uploader, and two of them inside one second trip GCS's per-object mutation limit (429, seen from build-bot on 2026-09-28). The size check is exact now, and a 429 on the write is retried with a short backoff since the collision is another writer of the same content. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
… stops once the object is there From review. The retry cloned the artifact's file handle, and clones share one offset: a small object is read whole before it is sent, so after a 429 the retry read nothing and wrote an empty object under the content's name, which the index then pointed at. Each attempt now seeks to the start. And a 429 is another uploader writing this same content, so before each retry the size check runs again, and a present object is not written a second time. Tests: a present object is not written; a retry after a 429 sends the whole file even when the failed attempt read it to the end (it fails without the rewind, with the empty payload); and a 429 followed by a present object writes once. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Rebased onto main 11b0b44, with both commits retitled |
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Actionable comments posted: 1
- 🪄 Fix comments: Fix CodeRabbit comments on this PR
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only still-valid issues, skip the rest with a brief reason, keep changes
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Inline comments:
Review comments at @crates/rcache/src/remote_writer.rs:
- Line 161: Update the local length lookup in the remote writer to propagate
`tar_file.metadata()` failures as `Error::IO` instead of defaulting to zero, so
a failed metadata read cannot make a zero-byte remote object appear to match.
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…still looks for the object From review. A length that could not be read became 0, and 0 matched an empty object left by a partial upload under the same name. It is an error now. And the retry budget's last 429 gave up without the look that follows every other 429, so an upload whose object had just landed from another uploader failed, and the caller's unwrap took the whole upload session with it; the look runs after the last 429 too. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
| async fn object_present(&self, object: &str, len: u64) -> bool { | ||
| matches!( | ||
| self.backend | ||
| .open_object(self.base.bucket.clone(), object.to_string()) | ||
| .send() | ||
| .await, | ||
| Ok(stat) if stat.object().size as u64 == len | ||
| ) | ||
| } |
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HIGH · security · confidence 0.70 · in object_present
object_present trusts a remote object's byte length as proof that its content is the correct content-addressed bytes, and upload (line 164) skips the write on that basis. The comment's premise — 'the same name with the same length is the same bytes' — is only true when every writer of {sha256}.zst is honest; content-addressing derives the name from the content, but the storage layer guarantees nothing about a name it was handed. An attacker with write access to this shared bucket can pre-create {sha256}.zst at the exact target length with different bytes. Before this change the honest uploader rewrote any pre-existing object up to 1 MiB (the removed size > 1024 * 1024 guard, 'large enough to be real'), so a small poisoned object was repaired on the next upload. Now it is accepted as 'present' and never overwritten, and every consumer of that spec_hash fails forever in fetch_verified with HashMismatch (remote.rs:711). This turns transient, self-healing poisoning into persistent denial of service against a shared build cache, and it removes the one defense (rewrite of small objects) at exactly the small sizes the PR calls the common case (18-byte empty tarballs). The 429-retry path reaches the same size check (lines 210, 219), so it shares the flaw.
What the reviewer checked
remote_writer.rs:237 async fn object_present(&self, object: &str, len: u64) -> bool { matches!( ... open_object(...).send().await, Ok(stat) if stat.object().size as u64 == len ) } // old: stat.object().size > 1024 * 1024
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Act on a code-review finding.
File: crates/rcache/src/remote_writer.rs
Lines: 237-245
Category: security
Severity: high (reviewer confidence 0.70)
Finding:
`object_present` trusts a remote object's byte length as proof that its content is the correct content-addressed bytes, and `upload` (line 164) skips the write on that basis. The comment's premise — 'the same name with the same length is the same bytes' — is only true when every writer of `{sha256}.zst` is honest; content-addressing derives the name from the content, but the storage layer guarantees nothing about a name it was handed. An attacker with write access to this shared bucket can pre-create `{sha256}.zst` at the exact target length with different bytes. Before this change the honest uploader rewrote any pre-existing object up to 1 MiB (the removed `size > 1024 * 1024` guard, 'large enough to be real'), so a small poisoned object was repaired on the next upload. Now it is accepted as 'present' and never overwritten, and every consumer of that `spec_hash` fails forever in `fetch_verified` with HashMismatch (remote.rs:711). This turns transient, self-healing poisoning into persistent denial of service against a shared build cache, and it removes the one defense (rewrite of small objects) at exactly the small sizes the PR calls the common case (18-byte empty tarballs). The 429-retry path reaches the same size check (lines 210, 219), so it shares the flaw.
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Agreed: the name is only a claim about the bytes, and the length check let a small planted object stand where the old code wrote over it. Fixed in 7042df0. GCS keeps a CRC32C of every object it stores; the upload now computes the artifact's and object_present asks for a match as well as the length, on the first look and after every 429 (the retry path shares the check). An object with no checksum reads as absent. upload_writes_over_an_object_of_the_same_length_but_other_bytes covers it: same length, other bytes, one write.
…tifact's
`object_present` took a remote object's length as proof it held the
artifact's bytes, and `upload` skipped the write on that. The name is
content-addressed, but storage guarantees nothing about a name it is handed:
anyone who can write the bucket can put other bytes under `{sha256}.zst` at
the right length, and where the old code wrote over any small object, the
length check let a planted one stand for good, failing every consumer of
that spec hash on its hash check.
GCS computes a CRC32C over every object it stores. The upload now computes
the artifact's and asks for a match as well as the length, on the first look
and after every 429; an object with no checksum reads as absent. A new test
writes over a same-length object with other bytes, and the one 429 path that
is an error (out of retries, nothing under the name) now has a test too.
| async fn object_present(&self, object: &str, len: u64, crc32c: u32) -> bool { | ||
| matches!( | ||
| self.backend | ||
| .open_object(self.base.bucket.clone(), object.to_string()) | ||
| .send() | ||
| .await, | ||
| Ok(stat) if stat.object().size as u64 == len | ||
| && stat.object().checksums.as_ref().and_then(|c| c.crc32c) == Some(crc32c) | ||
| ) | ||
| } | ||
|
|
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MEDIUM · security · confidence 0.50 · in object_present
The skip decision trusts size plus CRC32C to prove that the remote bytes equal the local artifact. CRC32C is a linear error-detecting code, not a hash. A party with bucket write access can craft other bytes with the same length and CRC32C. Upload will then skip and keep the planted object under the sha256 name. The comments and test say this case is written over, but only a cryptographic hash can prove it. Use a stronger check, such as a sha256 stored in object metadata at write time, or always overwrite objects of this trust level.
What the reviewer checked
Ok(stat) if stat.object().size as u64 == len && ...checksums...crc32c == Some(crc32c)
Hand this to an agent
Act on a code-review finding.
File: crates/rcache/src/remote_writer.rs
Lines: 249-259
Category: security
Severity: medium (reviewer confidence 0.50)
Finding:
The skip decision trusts size plus CRC32C to prove that the remote bytes equal the local artifact. CRC32C is a linear error-detecting code, not a hash. A party with bucket write access can craft other bytes with the same length and CRC32C. Upload will then skip and keep the planted object under the sha256 name. The comments and test say this case is written over, but only a cryptographic hash can prove it. Use a stronger check, such as a sha256 stored in object metadata at write time, or always overwrite objects of this trust level.
First verify this against the code yourself. It is model output and
may be wrong. If it is wrong, or the tradeoff was deliberate, say so
and change nothing.
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I don't think a stronger check changes what this path can promise. The party in the scenario has bucket write access, and with it can replace the object after any honest write, so neither a skip nor an unconditional overwrite keeps planted bytes out of the bucket; the thing that keeps them out of a build is the consumer's sha256 check on fetch (fetch_verified, HashMismatch), which this path never weakens. What the skip check has to tell apart is the same bytes from a partial or stale upload by an honest uploader, and length plus GCS's own CRC32C does that; a sha256 in object metadata would be written by the same bucket writer the scenario distrusts, so it adds a round trip and no trust. No change; happy to revisit if the registry's threat model names bucket-write as in scope for this path.
Artifact objects are content-addressed, so an existing object of the same length is the same bytes. The writer skipped only objects over 1 MiB, so every small identical output (several packages produce the same empty tarball) was rewritten by every uploader, and two writes inside one second trip GCS's per-object mutation limit:
(build-bot, pkgs#775, 2026-09-28; the object is 18 bytes.) The size check is exact now, and a 429 on the write is retried with a short backoff, since the collision is another writer of the same content. build-servers carries a matching retry at its call site until the pin moves.
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