fix(sse): 系统模型 - 恢复 SSE 增量传输

集成 PR #257，保留原 PR 开发者为提交作者。

backend/internal/handler/system_proxy_stream_test.go

@@ -0,0 +1,85 @@
+package handler
+
+import (
+	"bufio"
+	"errors"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/gin-gonic/gin"
+)
+
+func TestSystemProxyEventStreamFlushesBeforeUpstreamCompletes(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	release := make(chan struct{})
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set("Content-Type", "text/event-stream; charset=utf-8")
+		_, _ = io.WriteString(w, "data: first\n\n")
+		w.(http.Flusher).Flush()
+		<-release
+		_, _ = io.WriteString(w, "data: second\n\n")
+	}))
+	defer upstream.Close()
+
+	proxy := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		resp, err := http.Get(upstream.URL)
+		if err != nil {
+			http.Error(w, err.Error(), http.StatusBadGateway)
+			return
+		}
+		defer resp.Body.Close()
+		context, _ := gin.CreateTestContext(w)
+		context.Request = r
+		_, _ = streamSystemProxyResponse(context, resp, 1<<20)
+	}))
+	defer proxy.Close()
+
+	client := &http.Client{Timeout: 3 * time.Second}
+	response, err := client.Get(proxy.URL)
+	if err != nil {
+		close(release)
+		t.Fatal(err)
+	}
+	if response.Header.Get("X-Accel-Buffering") != "no" {
+		close(release)
+		_ = response.Body.Close()
+		t.Fatalf("X-Accel-Buffering = %q", response.Header.Get("X-Accel-Buffering"))
+	}
+	line, err := bufio.NewReader(response.Body).ReadString('\n')
+	if err != nil {
+		close(release)
+		_ = response.Body.Close()
+		t.Fatal(err)
+	}
+	if line != "data: first\n" {
+		close(release)
+		_ = response.Body.Close()
+		t.Fatalf("first streamed line = %q", line)
+	}
+	close(release)
+	_ = response.Body.Close()
+}
+
+func TestSystemProxyEventStreamEnforcesResponseLimit(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	response := httptest.NewRecorder()
+	context, _ := gin.CreateTestContext(response)
+	context.Request = httptest.NewRequest(http.MethodPost, "/api/ai/system/channel/responses", nil)
+	upstream := &http.Response{
+		StatusCode: http.StatusOK,
+		Header:     http.Header{"Content-Type": []string{"text/event-stream"}},
+		Body:       io.NopCloser(strings.NewReader("data: too large\n\n")),
+	}
+
+	captured, err := streamSystemProxyResponse(context, upstream, 4)
+	if !errors.Is(err, errSystemProxyResponseTooLarge) {
+		t.Fatalf("error = %v", err)
+	}
+	if len(captured) != 0 || response.Body.Len() != 0 {
+		t.Fatalf("captured = %q, response = %q", captured, response.Body.String())
+	}
+}