fix(llmclient): fail over when a stream opens empty or with an in-band error - #1015
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📝 WalkthroughWalkthroughThe client now validates streaming responses before returning them. Empty streams and first-event SSE errors become provider errors that can trigger failover. Healthy streams replay buffered bytes unchanged. Documentation describes the resulting status and header behavior. ChangesStreaming failover validation
Priority: ⬇️ Low Estimated code review effort: 3 (Moderate) | ~25 minutes Change: Bug fix Sequence Diagram(s)sequenceDiagram
participant Client
participant Provider
participant StreamInterceptor
participant RequestEndHook
Client->>Provider: request streaming response
Provider-->>Client: return response stream
Client->>StreamInterceptor: validate stream startup
StreamInterceptor-->>Client: return replayable stream or provider error
Client->>RequestEndHook: record startup result
Client-->>Client: expose headers only after first event
Merge Risk: 🟡 Moderate · up to The new streaming startup validation can miss a genuine empty-stream failure when a provider sends more than 64KB of keep-alive padding before stalling, meaning failover would not trigger for that pattern even though the feature is meant to catch empty streams; this should be resolved or explicitly accepted before merge. A separate minor test-lint inconsistency should also be fixed but does not block functionality. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
Full details: Docstring CoverageExplanation Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 6 functions across 3 files. (1 skipped: 1 unsupported.) ✨ Finishing Touches 💡 1📝 Generate docstrings 💡
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Actionable comments posted: 2
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Inline comments:
In `@internal/llmclient/stream_start_test.go`:
- Line 165: Replace the assert.Error call for lastInfo.Error with require.Error,
and ensure the corresponding testify/require import is available in the test.
In `@internal/llmclient/stream_start.go`:
- Line 76: Update readFirstSSEData in internal/llmclient/stream_start.go to
return a startup failure when maxStreamStartBytes is exhausted without receiving
a data event, so interceptStreamStart does not accept an empty stream. Update
the over-limit keep-alive test in internal/llmclient/stream_start_test.go at
lines 125-127 to expect that startup failure.
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docs/features/failover.mdxinternal/llmclient/client_do.gointernal/llmclient/stream_start.gointernal/llmclient/stream_start_test.go
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Repeated event:, id:, or retry: lines are retained after the configured stream-start buffer limit because only blank and comment lines are discarded. An upstream that keeps the connection open without sending data can therefore make the gateway keep accumulating the preamble instead of returning a stream. This is a non-blocking reliability concern, but concurrent stalled streams can consume unnecessary memory.
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Fixed in 72f7d6c. While no data event has arrived yet, a blank line clears the held-back bytes (a data-less event ends there), keep-alive comments past the limit are still dropped, and an event:/id:/retry: preamble that pushes past 64 KiB now ends the inspection and streams through instead of accumulating. Held-back bytes stay bounded at the limit plus one line, and preamble fields are never dropped from what replays. TestClient_DoStream_StreamStartReplaysHealthyStreams covers an oversized preamble replaying intact.
2 participants
A streaming request whose provider answered
200and then sent nothing, or opened the stream with an in-band error, reached the client as a silent EOF or a single error chunk. Failover never ran, because the stream had already been accepted.DoStreamnow waits for the first SSE data event before it accepts the stream:502 provider returned an empty stream.{"error": ...}payload fails with the status the payload carries (for example an in-band429), or502when it has none.Both failures are recorded by the circuit breaker and metrics, and they fail over under the default
429,5xxpolicy. The trade-off: the response headers now wait for the provider's first event instead of being sent right away.Verified against a fake upstream behind a two-target failover virtual model: empty
200and in-band429now fail over to the backup, and503before headers and all targets failing behave as before.Summary by CodeRabbit
Bug Fixes
Documentation