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fix(gateway): 防止流式 failover 拼接腐化导致客户端收到双 message_start

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当上游在 SSE 流中途返回 event:error 时，handleStreamingResponse 已将
部分 SSE 事件写入客户端，但原先的 failover 逻辑仍会切换到下一个账号
并写入完整流，导致客户端收到两个 message_start 进而产生 400 错误。

修复方案：在每次 Forward 调用前记录 c.Writer.Size()，若 Forward 返回
UpstreamFailoverError 后 writer 字节数增加，说明 SSE 内容已不可撤销地
发送给客户端，此时直接调用 handleFailoverExhausted 发送 SSE error 事件
终止流，而非继续 failover。

Ping-only 场景不受影响：slot 等待期的 ping 字节在 Forward 前后相等，
正常 failover 流程照常进行。

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
This commit is contained in:
Elysia
2026-03-14 22:49:23 +08:00
parent 6da5fa01b9
commit 0e23732631
1 changed files with 14 additions and 0 deletions
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14
backend/internal/handler/gateway_handler.go
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@@ -391,6 +391,8 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
if fs.SwitchCount > 0 {
requestCtx = service.WithAccountSwitchCount(requestCtx, fs.SwitchCount, h.metadataBridgeEnabled())
}
// 记录 Forward 前已写入字节数，Forward 后若增加则说明 SSE 内容已发，禁止 failover
writerSizeBeforeForward := c.Writer.Size()
if account.Platform == service.PlatformAntigravity {
result, err = h.antigravityGatewayService.ForwardGemini(requestCtx, c, account, reqModel, "generateContent", reqStream, body, hasBoundSession)
} else {
@@ -402,6 +404,11 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
if err != nil {
var failoverErr *service.UpstreamFailoverError
if errors.As(err, &failoverErr) {
// 流式内容已写入客户端，无法撤销，禁止 failover 以防止流拼接腐化
if c.Writer.Size() != writerSizeBeforeForward {
h.handleFailoverExhausted(c, failoverErr, account.Platform, true)
return
}
action := fs.HandleFailoverError(c.Request.Context(), h.gatewayService, account.ID, account.Platform, failoverErr)
switch action {
case FailoverContinue:
@@ -637,6 +644,8 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
if fs.SwitchCount > 0 {
requestCtx = service.WithAccountSwitchCount(requestCtx, fs.SwitchCount, h.metadataBridgeEnabled())
}
// 记录 Forward 前已写入字节数，Forward 后若增加则说明 SSE 内容已发，禁止 failover
writerSizeBeforeForward := c.Writer.Size()
if account.Platform == service.PlatformAntigravity && account.Type != service.AccountTypeAPIKey {
result, err = h.antigravityGatewayService.Forward(requestCtx, c, account, body, hasBoundSession)
} else {
@@ -706,6 +715,11 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
}
var failoverErr *service.UpstreamFailoverError
if errors.As(err, &failoverErr) {
// 流式内容已写入客户端，无法撤销，禁止 failover 以防止流拼接腐化
if c.Writer.Size() != writerSizeBeforeForward {
h.handleFailoverExhausted(c, failoverErr, account.Platform, true)
return
}
action := fs.HandleFailoverError(c.Request.Context(), h.gatewayService, account.ID, account.Platform, failoverErr)
switch action {
case FailoverContinue: