fix(gateway): drain upstream after client disconnect

backend/internal/service/openai_gateway_service.go

@@ -1046,8 +1046,9 @@ func (s *OpenAIGatewayService) handleStreamingResponse(ctx context.Context, resp
 
 	// 仅发送一次错误事件，避免多次写入导致协议混乱（写失败时尽力通知客户端）
 	errorEventSent := false
+	clientDisconnected := false // 客户端断开后继续 drain 上游以收集 usage
 	sendErrorEvent := func(reason string) {
-		if errorEventSent {
+		if errorEventSent || clientDisconnected {
 			return
 		}
 		errorEventSent = true
@@ -1070,6 +1071,11 @@ func (s *OpenAIGatewayService) handleStreamingResponse(ctx context.Context, resp
 					log.Printf("Context canceled during streaming, returning collected usage")
 					return &openaiStreamingResult{usage: usage, firstTokenMs: firstTokenMs}, nil
 				}
+				// 客户端已断开时，上游出错仅影响体验，不影响计费；返回已收集 usage
+				if clientDisconnected {
+					log.Printf("Upstream read error after client disconnect: %v, returning collected usage", ev.err)
+					return &openaiStreamingResult{usage: usage, firstTokenMs: firstTokenMs}, nil
+				}
 				if errors.Is(ev.err, bufio.ErrTooLong) {
 					log.Printf("SSE line too long: account=%d max_size=%d error=%v", account.ID, maxLineSize, ev.err)
 					sendErrorEvent("response_too_large")
@@ -1091,12 +1097,15 @@ func (s *OpenAIGatewayService) handleStreamingResponse(ctx context.Context, resp
 					line = s.replaceModelInSSELine(line, mappedModel, originalModel)
 				}
 
-				// Forward line
-				if _, err := fmt.Fprintf(w, "%s\n", line); err != nil {
-					sendErrorEvent("write_failed")
-					return &openaiStreamingResult{usage: usage, firstTokenMs: firstTokenMs}, err
+				// 写入客户端（客户端断开后继续 drain 上游）
+				if !clientDisconnected {
+					if _, err := fmt.Fprintf(w, "%s\n", line); err != nil {
+						clientDisconnected = true
+						log.Printf("Client disconnected during streaming, continuing to drain upstream for billing")
+					} else {
+						flusher.Flush()
+					}
 				}
-				flusher.Flush()
 
 				// Record first token time
 				if firstTokenMs == nil && data != "" && data != "[DONE]" {
@@ -1106,11 +1115,14 @@ func (s *OpenAIGatewayService) handleStreamingResponse(ctx context.Context, resp
 				s.parseSSEUsage(data, usage)
 			} else {
 				// Forward non-data lines as-is
-				if _, err := fmt.Fprintf(w, "%s\n", line); err != nil {
-					sendErrorEvent("write_failed")
-					return &openaiStreamingResult{usage: usage, firstTokenMs: firstTokenMs}, err
+				if !clientDisconnected {
+					if _, err := fmt.Fprintf(w, "%s\n", line); err != nil {
+						clientDisconnected = true
+						log.Printf("Client disconnected during streaming, continuing to drain upstream for billing")
+					} else {
+						flusher.Flush()
+					}
 				}
-				flusher.Flush()
 			}
 
 		case <-intervalCh:
@@ -1118,6 +1130,10 @@ func (s *OpenAIGatewayService) handleStreamingResponse(ctx context.Context, resp
 			if time.Since(lastRead) < streamInterval {
 				continue
 			}
+			if clientDisconnected {
+				log.Printf("Upstream timeout after client disconnect, returning collected usage")
+				return &openaiStreamingResult{usage: usage, firstTokenMs: firstTokenMs}, nil
+			}
 			log.Printf("Stream data interval timeout: account=%d model=%s interval=%s", account.ID, originalModel, streamInterval)
 			// 处理流超时，可能标记账户为临时不可调度或错误状态
 			if s.rateLimitService != nil {
@@ -1127,11 +1143,16 @@ func (s *OpenAIGatewayService) handleStreamingResponse(ctx context.Context, resp
 			return &openaiStreamingResult{usage: usage, firstTokenMs: firstTokenMs}, fmt.Errorf("stream data interval timeout")
 
 		case <-keepaliveCh:
+			if clientDisconnected {
+				continue
+			}
 			if time.Since(lastDataAt) < keepaliveInterval {
 				continue
 			}
 			if _, err := fmt.Fprint(w, ":\n\n"); err != nil {
-				return &openaiStreamingResult{usage: usage, firstTokenMs: firstTokenMs}, err
+				clientDisconnected = true
+				log.Printf("Client disconnected during streaming, continuing to drain upstream for billing")
+				continue
 			}
 			flusher.Flush()
 		}

backend/internal/service/openai_gateway_service_test.go

@@ -38,6 +38,20 @@ type cancelReadCloser struct{}
 func (c cancelReadCloser) Read(p []byte) (int, error) { return 0, context.Canceled }
 func (c cancelReadCloser) Close() error               { return nil }
 
+type failingGinWriter struct {
+	gin.ResponseWriter
+	failAfter int
+	writes    int
+}
+
+func (w *failingGinWriter) Write(p []byte) (int, error) {
+	if w.writes >= w.failAfter {
+		return 0, errors.New("write failed")
+	}
+	w.writes++
+	return w.ResponseWriter.Write(p)
+}
+
 func (c stubConcurrencyCache) AcquireAccountSlot(ctx context.Context, accountID int64, maxConcurrency int, requestID string) (bool, error) {
 	return true, nil
 }
@@ -211,6 +225,51 @@ func TestOpenAIStreamingContextCanceledDoesNotInjectErrorEvent(t *testing.T) {
 	}
 }
 
+func TestOpenAIStreamingClientDisconnectDrainsUpstreamUsage(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	cfg := &config.Config{
+		Gateway: config.GatewayConfig{
+			StreamDataIntervalTimeout: 0,
+			StreamKeepaliveInterval:   0,
+			MaxLineSize:               defaultMaxLineSize,
+		},
+	}
+	svc := &OpenAIGatewayService{cfg: cfg}
+
+	rec := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(rec)
+	c.Request = httptest.NewRequest(http.MethodPost, "/", nil)
+	c.Writer = &failingGinWriter{ResponseWriter: c.Writer, failAfter: 0}
+
+	pr, pw := io.Pipe()
+	resp := &http.Response{
+		StatusCode: http.StatusOK,
+		Body:       pr,
+		Header:     http.Header{},
+	}
+
+	go func() {
+		defer func() { _ = pw.Close() }()
+		_, _ = pw.Write([]byte("data: {\"type\":\"response.in_progress\",\"response\":{}}\n\n"))
+		_, _ = pw.Write([]byte("data: {\"type\":\"response.completed\",\"response\":{\"usage\":{\"input_tokens\":3,\"output_tokens\":5,\"input_tokens_details\":{\"cached_tokens\":1}}}}\n\n"))
+	}()
+
+	result, err := svc.handleStreamingResponse(c.Request.Context(), resp, c, &Account{ID: 1}, time.Now(), "model", "model")
+	_ = pr.Close()
+	if err != nil {
+		t.Fatalf("expected nil error, got %v", err)
+	}
+	if result == nil || result.usage == nil {
+		t.Fatalf("expected usage result")
+	}
+	if result.usage.InputTokens != 3 || result.usage.OutputTokens != 5 || result.usage.CacheReadInputTokens != 1 {
+		t.Fatalf("unexpected usage: %+v", *result.usage)
+	}
+	if strings.Contains(rec.Body.String(), "event: error") || strings.Contains(rec.Body.String(), "write_failed") {
+		t.Fatalf("expected no injected SSE error event, got %q", rec.Body.String())
+	}
+}
+
 func TestOpenAIStreamingTooLong(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 	cfg := &config.Config{