test(repro): minimal Workers reproducer for the streamed-PUT failures

[alukach]

Standalone reproducer under tests/repro/streamed-put-io-context/. Not wired into CI — it is a manual tool and an artifact to hand upstream.

What it isolates

No S3, no auth, no registry. One variable: whether the inbound body stream is touched before or after an await.

• /before — attach the stream in the request's own I/O context. Mirrors ProxyGateway::op_needs_buffered_body, the guard that covers the multipart control ops and batch delete.
• /after — await first (standing in for the registry lookup and STS exchange), then attach. Mirrors WorkerBackend::forward for PutObject/UploadPart, which the classifier explicitly excludes.

forward() copies WorkerBackend::forward faithfully, including the FixedLengthStream wrapper and its dropped pipe_to promise.

Result

1 failure in 32 on /after, 0 in 32 on /before, at concurrency 8 × 16 MiB:

[after] forward failed: Error: Network connection lost.

Intermittent — a later 96/96 run was clean. A green run is inconclusive.

Scope

Network connection lost maps to BackendError → 503, the minority prod mode (2 of 23 in one sample). It does not reproduce the majority 520.

Prod evidence now shows the 520 is relayed from the backend fetch: GatewayResponse::Forward passes the upstream status through verbatim, and the worker's own 5xx log (from #201) records status=520 with handle_request returning normally. tessera's backend is plain S3 in us-west-2, which does not emit 520 — so the runtime is synthesizing it for a subrequest the worker issued. That remains open.

The accompanying TypeError: Can't read from request stream after responding with an exception fires ~1 ms after the response commits — the orphaned pipe_to still reading the inbound body. A consequence rather than the cause, but it is what resets the connection so the client never sees the relayed status.

🤖 Generated with Claude Code

[claude]

Claude finished @alukach's task in 1m 6s —— View job

---

Reviewing PR #203

• Gather context (PR body, diff, changed files)
• Read changed files
• Analyze for correctness/security/best-practice issues
• Post final review

---

✅ No blocking issues — safe to merge.

This is a self-contained, non-production reproducer (isolated under tests/repro/streamed-put-io-context/, not wired into CI, build artifacts gitignored), and it's well-constructed for its purpose: the /before vs /after split faithfully isolates the one variable (timing of body-stream attachment relative to an await) and forward() mirrors WorkerBackend::forward closely enough (including the dropped pipe_to promise) to be a credible upstream artifact.

A couple of very minor, non-blocking observations:

• tests/repro/streamed-put-io-context/drive.py:36 — the except Exception as e: branch doesn't close conn, so a failed/reset connection's socket isn't explicitly released. Harmless for a short-lived manual driver script, but worth a finally: conn.close() if this gets reused for longer soak runs.
• tests/repro/streamed-put-io-context/src/lib.rs:61 — text(599, &msg) uses a non-standard HTTP status (599) purely as an internal sentinel for "the worker crate surfaced an Err"; it's commented as intentional and is within the Fetch spec's valid response-status range, so it's fine, just worth knowing it's not a real status code if anyone greps logs for it later.
• .gitignore excludes Cargo.lock for what is effectively a standalone binary-ish crate — normally you'd commit the lockfile for reproducibility, but given this is explicitly a throwaway/manual tool (per the Cargo.toml comment "deliberately not a member of the parent crate's build"), that's a reasonable call, not a defect.

Nothing here rises to a correctness bug or security issue — the harness only talks to 127.0.0.1, has no auth/secrets, and the intentional "bug-mirroring" behaviors (dropped promise, ignored slow_call error on /before) are clearly documented as deliberate.

[github-actions]

🚀 Latest commit deployed to https://source-data-proxy-pr-203.source-coop.workers.dev

• Date: 2026-07-28T19:44:59Z
• Commit: d40fa84