增加测试

backend/internal/handler/gateway_handler.go

@@ -406,7 +406,7 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 				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)
+						h.handleFailoverExhausted(c, failoverErr, service.PlatformGemini, true)
 						return
 					}
 					action := fs.HandleFailoverError(c.Request.Context(), h.gatewayService, account.ID, account.Platform, failoverErr)

backend/internal/handler/gateway_handler_stream_failover_test.go

@@ -0,0 +1,122 @@
+package handler
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/Wei-Shaw/sub2api/internal/service"
+	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// partialMessageStartSSE 模拟 handleStreamingResponse 已写入的首批 SSE 事件。
+const partialMessageStartSSE = "event: message_start\ndata: {\"type\":\"message_start\",\"message\":{\"id\":\"msg_01\",\"type\":\"message\",\"role\":\"assistant\",\"content\":[],\"model\":\"claude-sonnet-4-5\",\"stop_reason\":null,\"stop_sequence\":null,\"usage\":{\"input_tokens\":10,\"output_tokens\":1}}}\n\n" +
+	"event: content_block_start\ndata: {\"type\":\"content_block_start\",\"index\":0,\"content_block\":{\"type\":\"text\",\"text\":\"\"}}\n\n"
+
+// TestStreamWrittenGuard_MessagesPath_AbortFailoverOnSSEContentWritten 验证：
+// 当 Forward 在返回 UpstreamFailoverError 前已向客户端写入 SSE 内容时，
+// 故障转移保护逻辑必须终止循环并发送 SSE 错误事件，而不是进行下一次 Forward。
+// 具体验证：
+//  1. c.Writer.Size() 检测条件正确触发（字节数已增加）
+//  2. handleFailoverExhausted 以 streamStarted=true 调用后，响应体以 SSE 错误事件结尾
+//  3. 响应体中只出现一个 message_start，不存在第二个（防止流拼接腐化）
+func TestStreamWrittenGuard_MessagesPath_AbortFailoverOnSSEContentWritten(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Request = httptest.NewRequest(http.MethodPost, "/v1/messages", nil)
+
+	// 步骤 1：记录 Forward 前的 writer size（模拟 writerSizeBeforeForward := c.Writer.Size()）
+	sizeBeforeForward := c.Writer.Size()
+	require.Equal(t, -1, sizeBeforeForward, "gin writer 初始 Size 应为 -1（未写入任何字节）")
+
+	// 步骤 2：模拟 Forward 已向客户端写入部分 SSE 内容（message_start + content_block_start）
+	_, err := c.Writer.Write([]byte(partialMessageStartSSE))
+	require.NoError(t, err)
+
+	// 步骤 3：验证守卫条件成立（c.Writer.Size() != sizeBeforeForward）
+	require.NotEqual(t, sizeBeforeForward, c.Writer.Size(),
+		"写入 SSE 内容后 writer size 必须增加，守卫条件应为 true")
+
+	// 步骤 4：模拟 UpstreamFailoverError（上游在流中途返回 403）
+	failoverErr := &service.UpstreamFailoverError{
+		StatusCode:   http.StatusForbidden,
+		ResponseBody: []byte(`{"error":{"type":"permission_error","message":"forbidden"}}`),
+	}
+
+	// 步骤 5：守卫触发 → 调用 handleFailoverExhausted，streamStarted=true
+	h := &GatewayHandler{}
+	h.handleFailoverExhausted(c, failoverErr, service.PlatformAnthropic, true)
+
+	body := w.Body.String()
+
+	// 断言 A：响应体中包含最初写入的 message_start SSE 事件行
+	require.Contains(t, body, "event: message_start", "响应体应包含已写入的 message_start SSE 事件")
+
+	// 断言 B：响应体以 SSE 错误事件结尾（data: {"type":"error",...}\n\n）
+	require.True(t, strings.HasSuffix(strings.TrimRight(body, "\n"), "}"),
+		"响应体应以 JSON 对象结尾（SSE error event 的 data 字段）")
+	require.Contains(t, body, `"type":"error"`, "响应体末尾必须包含 SSE 错误事件")
+
+	// 断言 C：SSE event 行 "event: message_start" 只出现一次（防止双 message_start 拼接腐化）
+	firstIdx := strings.Index(body, "event: message_start")
+	lastIdx := strings.LastIndex(body, "event: message_start")
+	assert.Equal(t, firstIdx, lastIdx,
+		"响应体中 'event: message_start' 必须只出现一次，不得因 failover 拼接导致两次")
+}
+
+// TestStreamWrittenGuard_GeminiPath_AbortFailoverOnSSEContentWritten 与上述测试相同，
+// 验证 Gemini 路径使用 service.PlatformGemini（而非 account.Platform）时行为一致。
+func TestStreamWrittenGuard_GeminiPath_AbortFailoverOnSSEContentWritten(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Request = httptest.NewRequest(http.MethodPost, "/v1beta/models/gemini-2.0-flash:streamGenerateContent", nil)
+
+	sizeBeforeForward := c.Writer.Size()
+
+	_, err := c.Writer.Write([]byte(partialMessageStartSSE))
+	require.NoError(t, err)
+
+	require.NotEqual(t, sizeBeforeForward, c.Writer.Size())
+
+	failoverErr := &service.UpstreamFailoverError{
+		StatusCode: http.StatusForbidden,
+	}
+
+	h := &GatewayHandler{}
+	h.handleFailoverExhausted(c, failoverErr, service.PlatformGemini, true)
+
+	body := w.Body.String()
+
+	require.Contains(t, body, "event: message_start")
+	require.Contains(t, body, `"type":"error"`)
+
+	firstIdx := strings.Index(body, "event: message_start")
+	lastIdx := strings.LastIndex(body, "event: message_start")
+	assert.Equal(t, firstIdx, lastIdx, "Gemini 路径不得出现双 message_start")
+}
+
+// TestStreamWrittenGuard_NoByteWritten_GuardNotTriggered 验证反向场景：
+// 当 Forward 返回 UpstreamFailoverError 时若未向客户端写入任何 SSE 内容，
+// 守卫条件（c.Writer.Size() != sizeBeforeForward）为 false，不应中止 failover。
+func TestStreamWrittenGuard_NoByteWritten_GuardNotTriggered(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Request = httptest.NewRequest(http.MethodPost, "/v1/messages", nil)
+
+	// 模拟 writerSizeBeforeForward：初始为 -1
+	sizeBeforeForward := c.Writer.Size()
+
+	// Forward 未写入任何字节直接返回错误（例如 401 发生在连接建立前）
+	// c.Writer.Size() 仍为 -1
+
+	// 守卫条件：sizeBeforeForward == c.Writer.Size() → 不触发
+	guardTriggered := c.Writer.Size() != sizeBeforeForward
+	require.False(t, guardTriggered,
+		"未写入任何字节时，守卫条件必须为 false，应允许正常 failover 继续")
+}