* fix: resolve Kafka gateway response deadlocks causing Sarama client hangs
Fix three bugs in the Kafka protocol handler that caused sequential
clients (notably Sarama) to hang during E2E tests:
1. Race condition in correlation queue ordering: the correlation ID was
added to the response ordering queue AFTER sending the request to
the processing channel. A fast processor (e.g. ApiVersions) could
finish and send its response before the ID was in the queue, causing
the response writer to miss it — permanently deadlocking the
connection. Now the ID is added BEFORE the channel send, with error
response injection on send failure.
2. Silent error response drops: when processRequestSync returned an
error, the response writer logged it but never sent anything back to
the client. The client would block forever waiting for bytes that
never arrived. Now sends a Kafka UNKNOWN_SERVER_ERROR response.
3. Produce V0/V1 missing timeout_ms parsing: the handler skipped the
4-byte timeout field, reading it as topicsCount instead. This caused
incorrect parsing of the entire produce request for V0/V1 clients.
* fix: API-versioned error responses, unsupported-version queue fix, V0V1 header alignment
1. errors.go — BuildAPIErrorResponse: emits a minimal-but-valid error
body whose layout matches the schema the client expects for each API
key and version (throttle_time position, array fields, etc.). The
old 2-byte generic body corrupted the protocol stream for APIs whose
response begins with throttle_time_ms or an array.
2. handler.go — unsupported-version path: the correlationID was never
added to correlationQueue before sending to responseChan, so the
response writer could never match it and the client hung. Now
appends the ID under correlationQueueMu before the send.
3. produce.go — handleProduceV0V1: requestBody is already post-header
(HandleConn strips client_id). The handler was erroneously parsing
acks bytes as a client_id length, misaligning all subsequent field
reads. Removed the client_id parsing; offset now starts at 0 with
acks(2) + timeout_ms(4) + topicsCount(4), matching handleProduceV2Plus.
* fix: free pooled message buffer per-iteration instead of deferring
The read loop allocated messageBuf via mem.Allocate and deferred
mem.Free. Since the defer only runs when HandleConn returns, pool
buffers accumulated for the entire connection lifetime — one per
request. Worse, the deferred frees ran in LIFO order before
wg.Wait(), so processing goroutines could read from already-freed
pool buffers.
Now: read into a pooled buffer, immediately copy to Go-managed
memory, and return the pool buffer. messageBuf is a regular slice
safe for async goroutine access with no defer accumulation.
* fix: cancel context before wg.Wait and on worker response-send timeout
Two related issues:
1. Cleanup defer ordering deadlock: defers run LIFO — the cleanup defer
(close channels, wg.Wait) ran before the cancel() defer. The
response writer is in the WaitGroup and exits only on ctx.Done() or
responseChan close, but both signals came after wg.Wait(). Deadlock
on every normal connection close (EOF, read error, queue-full).
Fix: call cancel() at the start of the cleanup defer, before
wg.Wait().
2. Worker 5s response-send timeout: when the timeout fired, the
response was silently dropped but the correlationID remained in the
ordered queue. The response writer could never advance past it,
stalling all subsequent responses permanently.
Fix: call cancel() to tear down the connection — if we cannot
deliver a response in 5s the connection is irrecoverable.
* chore: remove empty no-op ListOffsets conditional
The `if apiKey == 2 {}` block had no body — leftover debug code.
ListOffsets routing is handled by isDataPlaneAPI (returns false,
sending it to the control channel). No behavior change.
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