perf(service): 优化 model 替换函数，用 gjson/sjson 替代全量 JSON 序列化

SSE 热路径中 replaceModelInSSELine 和 replaceModelInResponseBody 原来
使用 json.Unmarshal/Marshal 对每个事件做全量反序列化再序列化，现改为
gjson.Get/sjson.Set 精确字段操作，消除 O(n) 中间 map 分配，保持 JSON
字段顺序不变。涉及 OpenAIGatewayService 和 GatewayService 两个服务。

新增 23 个单元测试覆盖：顶层/嵌套 model 替换、不匹配跳过、空行/[DONE]/
非法 JSON 等边界情况。

Fixes: P1-08

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

backend/internal/service/gateway_service.go

@@ -4577,24 +4577,16 @@ func (s *GatewayService) handleNonStreamingResponse(ctx context.Context, resp *h
 }
 
 // replaceModelInResponseBody 替换响应体中的model字段
+// 使用 gjson/sjson 精确替换，避免全量 JSON 反序列化
 func (s *GatewayService) replaceModelInResponseBody(body []byte, fromModel, toModel string) []byte {
-	var resp map[string]any
-	if err := json.Unmarshal(body, &resp); err != nil {
-		return body
+	if m := gjson.GetBytes(body, "model"); m.Exists() && m.Str == fromModel {
+		newBody, err := sjson.SetBytes(body, "model", toModel)
+		if err != nil {
+			return body
+		}
+		return newBody
 	}
-
-	model, ok := resp["model"].(string)
-	if !ok || model != fromModel {
-		return body
-	}
-
-	resp["model"] = toModel
-	newBody, err := json.Marshal(resp)
-	if err != nil {
-		return body
-	}
-
-	return newBody
+	return body
 }
 
 func (s *GatewayService) replaceToolNamesInResponseBody(body []byte, toolNameMap map[string]string) []byte {

backend/internal/service/openai_gateway_service.go

@@ -24,6 +24,8 @@ import (
 	"github.com/Wei-Shaw/sub2api/internal/util/responseheaders"
 	"github.com/Wei-Shaw/sub2api/internal/util/urlvalidator"
 	"github.com/gin-gonic/gin"
+	"github.com/tidwall/gjson"
+	"github.com/tidwall/sjson"
 )
 
 const (
@@ -1430,31 +1432,22 @@ func (s *OpenAIGatewayService) replaceModelInSSELine(line, fromModel, toModel st
 		return line
 	}
 
-	var event map[string]any
-	if err := json.Unmarshal([]byte(data), &event); err != nil {
-		return line
-	}
-
-	// Replace model in response
-	if m, ok := event["model"].(string); ok && m == fromModel {
-		event["model"] = toModel
-		newData, err := json.Marshal(event)
+	// 使用 gjson 精确检查 model 字段，避免全量 JSON 反序列化
+	if m := gjson.Get(data, "model"); m.Exists() && m.Str == fromModel {
+		newData, err := sjson.Set(data, "model", toModel)
 		if err != nil {
 			return line
 		}
-		return "data: " + string(newData)
+		return "data: " + newData
 	}
 
-	// Check nested response
-	if response, ok := event["response"].(map[string]any); ok {
-		if m, ok := response["model"].(string); ok && m == fromModel {
-			response["model"] = toModel
-			newData, err := json.Marshal(event)
-			if err != nil {
-				return line
-			}
-			return "data: " + string(newData)
+	// 检查嵌套的 response.model 字段
+	if m := gjson.Get(data, "response.model"); m.Exists() && m.Str == fromModel {
+		newData, err := sjson.Set(data, "response.model", toModel)
+		if err != nil {
+			return line
 		}
+		return "data: " + newData
 	}
 
 	return line
@@ -1674,23 +1667,15 @@ func (s *OpenAIGatewayService) validateUpstreamBaseURL(raw string) (string, erro
 }
 
 func (s *OpenAIGatewayService) replaceModelInResponseBody(body []byte, fromModel, toModel string) []byte {
-	var resp map[string]any
-	if err := json.Unmarshal(body, &resp); err != nil {
-		return body
+	// 使用 gjson/sjson 精确替换 model 字段，避免全量 JSON 反序列化
+	if m := gjson.GetBytes(body, "model"); m.Exists() && m.Str == fromModel {
+		newBody, err := sjson.SetBytes(body, "model", toModel)
+		if err != nil {
+			return body
+		}
+		return newBody
 	}
-
-	model, ok := resp["model"].(string)
-	if !ok || model != fromModel {
-		return body
-	}
-
-	resp["model"] = toModel
-	newBody, err := json.Marshal(resp)
-	if err != nil {
-		return body
-	}
-
-	return newBody
+	return body
 }
 
 // OpenAIRecordUsageInput input for recording usage

backend/internal/service/openai_gateway_service_test.go

@@ -1187,3 +1187,226 @@ func TestOpenAIValidateUpstreamBaseURLEnabledEnforcesAllowlist(t *testing.T) {
 		t.Fatalf("expected non-allowlisted host to fail")
 	}
 }
+
+// ==================== P1-08 修复：model 替换性能优化测试 ====================
+
+func TestReplaceModelInSSELine(t *testing.T) {
+	svc := &OpenAIGatewayService{}
+
+	tests := []struct {
+		name     string
+		line     string
+		from     string
+		to       string
+		expected string
+	}{
+		{
+			name:     "顶层 model 字段替换",
+			line:     `data: {"id":"chatcmpl-123","model":"gpt-4o","choices":[]}`,
+			from:     "gpt-4o",
+			to:       "my-custom-model",
+			expected: `data: {"id":"chatcmpl-123","model":"my-custom-model","choices":[]}`,
+		},
+		{
+			name:     "嵌套 response.model 替换",
+			line:     `data: {"type":"response","response":{"id":"resp-1","model":"gpt-4o","output":[]}}`,
+			from:     "gpt-4o",
+			to:       "my-model",
+			expected: `data: {"type":"response","response":{"id":"resp-1","model":"my-model","output":[]}}`,
+		},
+		{
+			name:     "model 不匹配时不替换",
+			line:     `data: {"id":"chatcmpl-123","model":"gpt-3.5-turbo","choices":[]}`,
+			from:     "gpt-4o",
+			to:       "my-model",
+			expected: `data: {"id":"chatcmpl-123","model":"gpt-3.5-turbo","choices":[]}`,
+		},
+		{
+			name:     "无 model 字段时不替换",
+			line:     `data: {"id":"chatcmpl-123","choices":[]}`,
+			from:     "gpt-4o",
+			to:       "my-model",
+			expected: `data: {"id":"chatcmpl-123","choices":[]}`,
+		},
+		{
+			name:     "空 data 行",
+			line:     `data: `,
+			from:     "gpt-4o",
+			to:       "my-model",
+			expected: `data: `,
+		},
+		{
+			name:     "[DONE] 行",
+			line:     `data: [DONE]`,
+			from:     "gpt-4o",
+			to:       "my-model",
+			expected: `data: [DONE]`,
+		},
+		{
+			name:     "非 data: 前缀行",
+			line:     `event: message`,
+			from:     "gpt-4o",
+			to:       "my-model",
+			expected: `event: message`,
+		},
+		{
+			name:     "非法 JSON 不替换",
+			line:     `data: {invalid json}`,
+			from:     "gpt-4o",
+			to:       "my-model",
+			expected: `data: {invalid json}`,
+		},
+		{
+			name:     "无空格 data: 格式",
+			line:     `data:{"id":"x","model":"gpt-4o"}`,
+			from:     "gpt-4o",
+			to:       "my-model",
+			expected: `data: {"id":"x","model":"my-model"}`,
+		},
+		{
+			name:     "model 名含特殊字符",
+			line:     `data: {"model":"org/model-v2.1-beta"}`,
+			from:     "org/model-v2.1-beta",
+			to:       "custom/alias",
+			expected: `data: {"model":"custom/alias"}`,
+		},
+		{
+			name:     "空行",
+			line:     "",
+			from:     "gpt-4o",
+			to:       "my-model",
+			expected: "",
+		},
+		{
+			name:     "保持其他字段不变",
+			line:     `data: {"id":"abc","object":"chat.completion.chunk","model":"gpt-4o","created":1234567890,"choices":[{"index":0,"delta":{"content":"hi"}}]}`,
+			from:     "gpt-4o",
+			to:       "alias",
+			expected: `data: {"id":"abc","object":"chat.completion.chunk","model":"alias","created":1234567890,"choices":[{"index":0,"delta":{"content":"hi"}}]}`,
+		},
+		{
+			name:     "顶层优先于嵌套：同时存在两个 model",
+			line:     `data: {"model":"gpt-4o","response":{"model":"gpt-4o"}}`,
+			from:     "gpt-4o",
+			to:       "replaced",
+			expected: `data: {"model":"replaced","response":{"model":"gpt-4o"}}`,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := svc.replaceModelInSSELine(tt.line, tt.from, tt.to)
+			require.Equal(t, tt.expected, got)
+		})
+	}
+}
+
+func TestReplaceModelInSSEBody(t *testing.T) {
+	svc := &OpenAIGatewayService{}
+
+	tests := []struct {
+		name     string
+		body     string
+		from     string
+		to       string
+		expected string
+	}{
+		{
+			name:     "多行 SSE body 替换",
+			body:     "data: {\"model\":\"gpt-4o\",\"choices\":[]}\n\ndata: {\"model\":\"gpt-4o\",\"choices\":[{\"delta\":{\"content\":\"hi\"}}]}\n\ndata: [DONE]\n",
+			from:     "gpt-4o",
+			to:       "alias",
+			expected: "data: {\"model\":\"alias\",\"choices\":[]}\n\ndata: {\"model\":\"alias\",\"choices\":[{\"delta\":{\"content\":\"hi\"}}]}\n\ndata: [DONE]\n",
+		},
+		{
+			name:     "无需替换的 body",
+			body:     "data: {\"model\":\"gpt-3.5-turbo\"}\n\ndata: [DONE]\n",
+			from:     "gpt-4o",
+			to:       "alias",
+			expected: "data: {\"model\":\"gpt-3.5-turbo\"}\n\ndata: [DONE]\n",
+		},
+		{
+			name:     "混合 event 和 data 行",
+			body:     "event: message\ndata: {\"model\":\"gpt-4o\"}\n\n",
+			from:     "gpt-4o",
+			to:       "alias",
+			expected: "event: message\ndata: {\"model\":\"alias\"}\n\n",
+		},
+		{
+			name:     "空 body",
+			body:     "",
+			from:     "gpt-4o",
+			to:       "alias",
+			expected: "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := svc.replaceModelInSSEBody(tt.body, tt.from, tt.to)
+			require.Equal(t, tt.expected, got)
+		})
+	}
+}
+
+func TestReplaceModelInResponseBody(t *testing.T) {
+	svc := &OpenAIGatewayService{}
+
+	tests := []struct {
+		name     string
+		body     string
+		from     string
+		to       string
+		expected string
+	}{
+		{
+			name:     "替换顶层 model",
+			body:     `{"id":"chatcmpl-123","model":"gpt-4o","choices":[]}`,
+			from:     "gpt-4o",
+			to:       "alias",
+			expected: `{"id":"chatcmpl-123","model":"alias","choices":[]}`,
+		},
+		{
+			name:     "model 不匹配不替换",
+			body:     `{"id":"chatcmpl-123","model":"gpt-3.5-turbo","choices":[]}`,
+			from:     "gpt-4o",
+			to:       "alias",
+			expected: `{"id":"chatcmpl-123","model":"gpt-3.5-turbo","choices":[]}`,
+		},
+		{
+			name:     "无 model 字段不替换",
+			body:     `{"id":"chatcmpl-123","choices":[]}`,
+			from:     "gpt-4o",
+			to:       "alias",
+			expected: `{"id":"chatcmpl-123","choices":[]}`,
+		},
+		{
+			name:     "非法 JSON 返回原值",
+			body:     `not json`,
+			from:     "gpt-4o",
+			to:       "alias",
+			expected: `not json`,
+		},
+		{
+			name:     "空 body 返回原值",
+			body:     ``,
+			from:     "gpt-4o",
+			to:       "alias",
+			expected: ``,
+		},
+		{
+			name:     "保持嵌套结构不变",
+			body:     `{"model":"gpt-4o","usage":{"prompt_tokens":10,"completion_tokens":20},"choices":[{"message":{"role":"assistant","content":"hello"}}]}`,
+			from:     "gpt-4o",
+			to:       "alias",
+			expected: `{"model":"alias","usage":{"prompt_tokens":10,"completion_tokens":20},"choices":[{"message":{"role":"assistant","content":"hello"}}]}`,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := svc.replaceModelInResponseBody([]byte(tt.body), tt.from, tt.to)
+			require.Equal(t, tt.expected, string(got))
+		})
+	}
+}