fix(proxy): 首包前 response.failed 返回真实错误码(含 #318 + 时序修复)

[james-6-23]

说明

本 PR 包含 #318 的原始提交(cherry-pick,保留 @JrCx7scC 署名),并在其上修复了一个使原方案在 HTTP 层不生效的时序 bug。合并后关闭 #318。

Closes #318

#318 的问题

原方案在回调里中止 SSE 转发后,循环外的 c.JSON(4xx) 实际不生效:

1. ReadSSEStream 返回后紧跟的 streamWriter.Flush() 在零写入时也会调用 flusher.Flush();
2. gin 的 responseWriter.Flush() 第一行是 WriteHeaderNow() —— 200 + text/event-stream header 就此定型;
3. 后续 c.JSON(400) 只能把 JSON 追加进一个 200 的 event-stream 响应体。

用 gin v1.12.0 + httptest 复刻该调用顺序实测:STATUS=200 CT="text/event-stream",下游中转层照样按 200 计费,且直连 SSE 客户端连原来能收到的 response.failed 事件都没了。

修复(第二个 commit)

• 收尾 flush 加 wroteAnyBody 守卫:零写入时不触碰 transport flush,header 留到循环外真正响应时再提交(Responses / ChatCompletions 两处);
• 回调命中拦截时置位 abortedForHTTPError,外层错误分支改用该标记,避免客户端已断开等零写入场景对死连接误写 JSON;
• c.JSON 前覆盖 Content-Type: application/json; charset=utf-8(gin 的 JSON render 不覆盖已设置的 Content-Type);
• 新增 wire 级测试:真实 HTTP 连接上断言首包前失败下游收到 4xx JSON、已产出首 token 时维持 200 SSE 收尾。

未覆盖(与 #318 保持一致的范围)

IsOpenAIResponsesAPI() 直连分支、responses_ws.go 入站 WS、image-generation 强制 HTTP 分支仍保持旧行为,后续单独处理。

测试

go build ./...、go test ./... 全部通过。

Summary by CodeRabbit

• Bug Fixes
  • Improved streaming error handling so early upstream failures now return a proper JSON error instead of ending with a misleading successful stream response.
  • Prevented incomplete SSE responses from being sent when no data has reached the client yet, reducing incorrect success outcomes.
  • Preserved normal streaming behavior once output has already started, so completed streams still finish as expected.

[coderabbitai]

Caution

Review failed

The pull request is closed.

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 5d685a3e-bec1-41cc-a732-101d961b06ec

📥 Commits

Reviewing files that changed from the base of the PR and between 526db3c and 46b1af7.

📒 Files selected for processing (2)

• proxy/handler.go
• proxy/response_failed_billing_test.go

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📝 Walkthrough

Walkthrough

This change adds a helper to detect when an upstream response.failed event arrives before the first token and before any downstream body is written, then wires it into both the /v1/responses and /v1/chat/completions streaming handlers to abort SSE forwarding and return a real JSON error status instead of HTTP 200 + [DONE]. Tests cover the helper and end-to-end wire behavior.

Changes

Early response.failed error handling

Layer / File(s)	Summary
Shared detection helper proxy/handler.go	Adds shouldReturnHTTPErrorForResponseFailed to check event type, first-token state, written-body state, and client-disconnect state.
/v1/responses streaming abort path proxy/handler.go	Tracks abortedForHTTPError, resets pending SSE events and stops processing on early failure, conditionally flushes only when body was written, and returns JSON upstream error using the real status code.
/v1/chat/completions streaming abort path proxy/handler.go	Applies the same abortedForHTTPError tracking, event-loop short-circuit, conditional flush, and JSON error response for the chat completions streaming flow.
Unit and wire-level tests proxy/response_failed_billing_test.go	Adds table-driven tests for the helper and an httptest-based streamEpilogue simulation verifying 400 JSON errors before first token and unaffected 200 SSE responses after first token.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Sequence Diagram(s)

sequenceDiagram
  participant Client
  participant Handler as Streaming Handler
  participant Upstream

  Client->>Handler: Start streaming request
  Handler->>Upstream: Forward request
  alt response.failed before first token, no body written
    Upstream-->>Handler: response.failed event
    Handler->>Handler: shouldReturnHTTPErrorForResponseFailed() -> true
    Handler->>Handler: pending.Reset(), abortedForHTTPError=true
    Handler-->>Client: HTTP JSON error (real status code, message)
  else first token already produced
    Upstream-->>Handler: first token event
    Handler-->>Client: SSE chunk
    Upstream-->>Handler: response.failed event
    Handler->>Handler: shouldReturnHTTPErrorForResponseFailed() -> false
    Handler-->>Client: SSE continues, ends 200 + [DONE]
  end

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Possibly related PRs

• james-6-23/codex2api#252: Both PRs modify the streaming SSE forwarding/error handling in proxy/handler.go around early upstream response.failed events before the first token, including deferred/buffered SSE writing.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/first-token-response-failed-http-error

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