perf(service): SSE Scanner buffer 改用 sync.Pool 复用，减少高并发 GC 压力

将流式响应中 bufio.Scanner 的 64KB buffer 从每次 make 分配改为
sync.Pool 复用，统一切片表达式为 [:0]、变量命名为 scanBuf，
并补充对应的单元测试。

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/service/antigravity_gateway_service.go

@@ -1587,8 +1587,9 @@ func (s *AntigravityGatewayService) streamUpstreamResponse(c *gin.Context, resp
 	var firstTokenRecorded bool
 
 	scanner := bufio.NewScanner(resp.Body)
-	buf := make([]byte, 0, 64*1024)
-	scanner.Buffer(buf, 1024*1024)
+	scanBuf := getSSEScannerBuf64K()
+	defer putSSEScannerBuf64K(scanBuf)
+	scanner.Buffer(scanBuf[:0], 1024*1024)
 
 	for scanner.Scan() {
 		line := scanner.Bytes()
@@ -2120,7 +2121,8 @@ func (s *AntigravityGatewayService) handleGeminiStreamingResponse(c *gin.Context
 	if s.settingService.cfg != nil && s.settingService.cfg.Gateway.MaxLineSize > 0 {
 		maxLineSize = s.settingService.cfg.Gateway.MaxLineSize
 	}
-	scanner.Buffer(make([]byte, 64*1024), maxLineSize)
+	scanBuf := getSSEScannerBuf64K()
+	scanner.Buffer(scanBuf[:0], maxLineSize)
 	usage := &ClaudeUsage{}
 	var firstTokenMs *int
 
@@ -2141,7 +2143,8 @@ func (s *AntigravityGatewayService) handleGeminiStreamingResponse(c *gin.Context
 	}
 	var lastReadAt int64
 	atomic.StoreInt64(&lastReadAt, time.Now().UnixNano())
-	go func() {
+	go func(scanBuf *sseScannerBuf64K) {
+		defer putSSEScannerBuf64K(scanBuf)
 		defer close(events)
 		for scanner.Scan() {
 			atomic.StoreInt64(&lastReadAt, time.Now().UnixNano())
@@ -2152,7 +2155,7 @@ func (s *AntigravityGatewayService) handleGeminiStreamingResponse(c *gin.Context
 		if err := scanner.Err(); err != nil {
 			_ = sendEvent(scanEvent{err: err})
 		}
-	}()
+	}(scanBuf)
 	defer close(done)
 
 	// 上游数据间隔超时保护（防止上游挂起长期占用连接）
@@ -2277,7 +2280,8 @@ func (s *AntigravityGatewayService) handleGeminiStreamToNonStreaming(c *gin.Cont
 	if s.settingService.cfg != nil && s.settingService.cfg.Gateway.MaxLineSize > 0 {
 		maxLineSize = s.settingService.cfg.Gateway.MaxLineSize
 	}
-	scanner.Buffer(make([]byte, 64*1024), maxLineSize)
+	scanBuf := getSSEScannerBuf64K()
+	scanner.Buffer(scanBuf[:0], maxLineSize)
 
 	usage := &ClaudeUsage{}
 	var firstTokenMs *int
@@ -2305,7 +2309,8 @@ func (s *AntigravityGatewayService) handleGeminiStreamToNonStreaming(c *gin.Cont
 
 	var lastReadAt int64
 	atomic.StoreInt64(&lastReadAt, time.Now().UnixNano())
-	go func() {
+	go func(scanBuf *sseScannerBuf64K) {
+		defer putSSEScannerBuf64K(scanBuf)
 		defer close(events)
 		for scanner.Scan() {
 			atomic.StoreInt64(&lastReadAt, time.Now().UnixNano())
@@ -2316,7 +2321,7 @@ func (s *AntigravityGatewayService) handleGeminiStreamToNonStreaming(c *gin.Cont
 		if err := scanner.Err(); err != nil {
 			_ = sendEvent(scanEvent{err: err})
 		}
-	}()
+	}(scanBuf)
 	defer close(done)
 
 	// 上游数据间隔超时保护（防止上游挂起长期占用连接）
@@ -2728,7 +2733,8 @@ func (s *AntigravityGatewayService) handleClaudeStreamToNonStreaming(c *gin.Cont
 	if s.settingService.cfg != nil && s.settingService.cfg.Gateway.MaxLineSize > 0 {
 		maxLineSize = s.settingService.cfg.Gateway.MaxLineSize
 	}
-	scanner.Buffer(make([]byte, 64*1024), maxLineSize)
+	scanBuf := getSSEScannerBuf64K()
+	scanner.Buffer(scanBuf[:0], maxLineSize)
 
 	var firstTokenMs *int
 	var last map[string]any
@@ -2754,7 +2760,8 @@ func (s *AntigravityGatewayService) handleClaudeStreamToNonStreaming(c *gin.Cont
 
 	var lastReadAt int64
 	atomic.StoreInt64(&lastReadAt, time.Now().UnixNano())
-	go func() {
+	go func(scanBuf *sseScannerBuf64K) {
+		defer putSSEScannerBuf64K(scanBuf)
 		defer close(events)
 		for scanner.Scan() {
 			atomic.StoreInt64(&lastReadAt, time.Now().UnixNano())
@@ -2765,7 +2772,7 @@ func (s *AntigravityGatewayService) handleClaudeStreamToNonStreaming(c *gin.Cont
 		if err := scanner.Err(); err != nil {
 			_ = sendEvent(scanEvent{err: err})
 		}
-	}()
+	}(scanBuf)
 	defer close(done)
 
 	// 上游数据间隔超时保护（防止上游挂起长期占用连接）
@@ -2908,7 +2915,8 @@ func (s *AntigravityGatewayService) handleClaudeStreamingResponse(c *gin.Context
 	if s.settingService.cfg != nil && s.settingService.cfg.Gateway.MaxLineSize > 0 {
 		maxLineSize = s.settingService.cfg.Gateway.MaxLineSize
 	}
-	scanner.Buffer(make([]byte, 64*1024), maxLineSize)
+	scanBuf := getSSEScannerBuf64K()
+	scanner.Buffer(scanBuf[:0], maxLineSize)
 
 	// 辅助函数：转换 antigravity.ClaudeUsage 到 service.ClaudeUsage
 	convertUsage := func(agUsage *antigravity.ClaudeUsage) *ClaudeUsage {
@@ -2940,7 +2948,8 @@ func (s *AntigravityGatewayService) handleClaudeStreamingResponse(c *gin.Context
 	}
 	var lastReadAt int64
 	atomic.StoreInt64(&lastReadAt, time.Now().UnixNano())
-	go func() {
+	go func(scanBuf *sseScannerBuf64K) {
+		defer putSSEScannerBuf64K(scanBuf)
 		defer close(events)
 		for scanner.Scan() {
 			atomic.StoreInt64(&lastReadAt, time.Now().UnixNano())
@@ -2951,7 +2960,7 @@ func (s *AntigravityGatewayService) handleClaudeStreamingResponse(c *gin.Context
 		if err := scanner.Err(); err != nil {
 			_ = sendEvent(scanEvent{err: err})
 		}
-	}()
+	}(scanBuf)
 	defer close(done)
 
 	streamInterval := time.Duration(0)

backend/internal/service/antigravity_gateway_service_test.go

@@ -7,7 +7,9 @@ import (
 	"io"
 	"net/http"
 	"net/http/httptest"
+	"strings"
 	"testing"
+	"time"
 
 	"github.com/Wei-Shaw/sub2api/internal/pkg/antigravity"
 	"github.com/gin-gonic/gin"
@@ -190,3 +192,37 @@ func TestAntigravityMaxRetriesForModel_AfterSwitchFallback(t *testing.T) {
 	got := antigravityMaxRetriesForModel("gemini-2.5-flash", true)
 	require.Equal(t, 5, got)
 }
+
+func TestAntigravityStreamUpstreamResponse_UsageAndFirstToken(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	writer := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(writer)
+	c.Request = httptest.NewRequest(http.MethodPost, "/v1/messages", nil)
+
+	pr, pw := io.Pipe()
+	resp := &http.Response{StatusCode: http.StatusOK, Header: http.Header{}, Body: pr}
+
+	go func() {
+		defer func() { _ = pw.Close() }()
+		_, _ = pw.Write([]byte("data: {\"usage\":{\"input_tokens\":1,\"output_tokens\":2,\"cache_read_input_tokens\":3,\"cache_creation_input_tokens\":4}}\n"))
+		_, _ = pw.Write([]byte("data: {\"usage\":{\"output_tokens\":5}}\n"))
+	}()
+
+	svc := &AntigravityGatewayService{}
+	start := time.Now().Add(-10 * time.Millisecond)
+	usage, firstTokenMs := svc.streamUpstreamResponse(c, resp, start)
+	_ = pr.Close()
+
+	require.NotNil(t, usage)
+	require.Equal(t, 1, usage.InputTokens)
+	// 第二次事件覆盖 output_tokens
+	require.Equal(t, 5, usage.OutputTokens)
+	require.Equal(t, 3, usage.CacheReadInputTokens)
+	require.Equal(t, 4, usage.CacheCreationInputTokens)
+
+	if firstTokenMs == nil {
+		t.Fatalf("expected firstTokenMs to be set")
+	}
+	// 确保有透传输出
+	require.True(t, strings.Contains(writer.Body.String(), "data:"))
+}

backend/internal/service/gateway_service.go

@@ -4031,7 +4031,8 @@ func (s *GatewayService) handleStreamingResponse(ctx context.Context, resp *http
 	if s.cfg != nil && s.cfg.Gateway.MaxLineSize > 0 {
 		maxLineSize = s.cfg.Gateway.MaxLineSize
 	}
-	scanner.Buffer(make([]byte, 64*1024), maxLineSize)
+	scanBuf := getSSEScannerBuf64K()
+	scanner.Buffer(scanBuf[:0], maxLineSize)
 
 	type scanEvent struct {
 		line string
@@ -4050,7 +4051,8 @@ func (s *GatewayService) handleStreamingResponse(ctx context.Context, resp *http
 	}
 	var lastReadAt int64
 	atomic.StoreInt64(&lastReadAt, time.Now().UnixNano())
-	go func() {
+	go func(scanBuf *sseScannerBuf64K) {
+		defer putSSEScannerBuf64K(scanBuf)
 		defer close(events)
 		for scanner.Scan() {
 			atomic.StoreInt64(&lastReadAt, time.Now().UnixNano())
@@ -4061,7 +4063,7 @@ func (s *GatewayService) handleStreamingResponse(ctx context.Context, resp *http
 		if err := scanner.Err(); err != nil {
 			_ = sendEvent(scanEvent{err: err})
 		}
-	}()
+	}(scanBuf)
 	defer close(done)
 
 	streamInterval := time.Duration(0)

backend/internal/service/gateway_service_streaming_test.go

@@ -0,0 +1,52 @@
+package service
+
+import (
+	"context"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+
+	"github.com/Wei-Shaw/sub2api/internal/config"
+	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/require"
+)
+
+func TestGatewayService_StreamingReusesScannerBufferAndStillParsesUsage(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	cfg := &config.Config{
+		Gateway: config.GatewayConfig{
+			StreamDataIntervalTimeout: 0,
+			MaxLineSize:               defaultMaxLineSize,
+		},
+	}
+
+	svc := &GatewayService{
+		cfg:              cfg,
+		rateLimitService: &RateLimitService{},
+	}
+
+	rec := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(rec)
+	c.Request = httptest.NewRequest(http.MethodPost, "/v1/messages", nil)
+
+	pr, pw := io.Pipe()
+	resp := &http.Response{StatusCode: http.StatusOK, Header: http.Header{}, Body: pr}
+
+	go func() {
+		defer func() { _ = pw.Close() }()
+		// Minimal SSE event to trigger parseSSEUsage
+		_, _ = pw.Write([]byte("data: {\"type\":\"message_start\",\"message\":{\"usage\":{\"input_tokens\":3}}}\n\n"))
+		_, _ = pw.Write([]byte("data: {\"type\":\"message_delta\",\"usage\":{\"output_tokens\":7}}\n\n"))
+		_, _ = pw.Write([]byte("data: [DONE]\n\n"))
+	}()
+
+	result, err := svc.handleStreamingResponse(context.Background(), resp, c, &Account{ID: 1}, time.Now(), "model", "model", nil, false)
+	_ = pr.Close()
+	require.NoError(t, err)
+	require.NotNil(t, result)
+	require.NotNil(t, result.usage)
+	require.Equal(t, 3, result.usage.InputTokens)
+	require.Equal(t, 7, result.usage.OutputTokens)
+}

backend/internal/service/openai_gateway_service.go

@@ -1209,7 +1209,8 @@ func (s *OpenAIGatewayService) handleStreamingResponse(ctx context.Context, resp
 	if s.cfg != nil && s.cfg.Gateway.MaxLineSize > 0 {
 		maxLineSize = s.cfg.Gateway.MaxLineSize
 	}
-	scanner.Buffer(make([]byte, 64*1024), maxLineSize)
+	scanBuf := getSSEScannerBuf64K()
+	scanner.Buffer(scanBuf[:0], maxLineSize)
 
 	type scanEvent struct {
 		line string
@@ -1228,7 +1229,8 @@ func (s *OpenAIGatewayService) handleStreamingResponse(ctx context.Context, resp
 	}
 	var lastReadAt int64
 	atomic.StoreInt64(&lastReadAt, time.Now().UnixNano())
-	go func() {
+	go func(scanBuf *sseScannerBuf64K) {
+		defer putSSEScannerBuf64K(scanBuf)
 		defer close(events)
 		for scanner.Scan() {
 			atomic.StoreInt64(&lastReadAt, time.Now().UnixNano())
@@ -1239,7 +1241,7 @@ func (s *OpenAIGatewayService) handleStreamingResponse(ctx context.Context, resp
 		if err := scanner.Err(); err != nil {
 			_ = sendEvent(scanEvent{err: err})
 		}
-	}()
+	}(scanBuf)
 	defer close(done)
 
 	streamInterval := time.Duration(0)

backend/internal/service/openai_gateway_service_test.go

@@ -14,6 +14,7 @@ import (
 
 	"github.com/Wei-Shaw/sub2api/internal/config"
 	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/require"
 )
 
 type stubOpenAIAccountRepo struct {
@@ -1066,6 +1067,43 @@ func TestOpenAIStreamingHeadersOverride(t *testing.T) {
 	}
 }
 
+func TestOpenAIStreamingReuseScannerBufferAndStillWorks(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)
+
+	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.completed\",\"response\":{\"usage\":{\"input_tokens\":1,\"output_tokens\":2,\"input_tokens_details\":{\"cached_tokens\":3}}}}\n\n"))
+	}()
+
+	result, err := svc.handleStreamingResponse(c.Request.Context(), resp, c, &Account{ID: 1}, time.Now(), "model", "model")
+	_ = pr.Close()
+	require.NoError(t, err)
+	require.NotNil(t, result)
+	require.NotNil(t, result.usage)
+	require.Equal(t, 1, result.usage.InputTokens)
+	require.Equal(t, 2, result.usage.OutputTokens)
+	require.Equal(t, 3, result.usage.CacheReadInputTokens)
+}
+
 func TestOpenAIInvalidBaseURLWhenAllowlistDisabled(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 	cfg := &config.Config{

backend/internal/service/sse_scanner_buffer_pool.go

@@ -0,0 +1,24 @@
+package service
+
+import "sync"
+
+const sseScannerBuf64KSize = 64 * 1024
+
+type sseScannerBuf64K [sseScannerBuf64KSize]byte
+
+var sseScannerBuf64KPool = sync.Pool{
+	New: func() any {
+		return new(sseScannerBuf64K)
+	},
+}
+
+func getSSEScannerBuf64K() *sseScannerBuf64K {
+	return sseScannerBuf64KPool.Get().(*sseScannerBuf64K)
+}
+
+func putSSEScannerBuf64K(buf *sseScannerBuf64K) {
+	if buf == nil {
+		return
+	}
+	sseScannerBuf64KPool.Put(buf)
+}

backend/internal/service/sse_scanner_buffer_pool_test.go

@@ -0,0 +1,19 @@
+package service
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestSSEScannerBuf64KPool_GetPutDoesNotPanic(t *testing.T) {
+	buf := getSSEScannerBuf64K()
+	require.NotNil(t, buf)
+	require.Equal(t, sseScannerBuf64KSize, len(buf[:]))
+
+	buf[0] = 1
+	putSSEScannerBuf64K(buf)
+
+	// 允许传入 nil，确保不会 panic
+	putSSEScannerBuf64K(nil)
+}