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.go

@@ -0,0 +1,70 @@
+package handler
+
+import (
+	"bytes"
+	"errors"
+	"io"
+	"mime"
+	"net/http"
+	"strings"
+
+	"github.com/gin-gonic/gin"
+)
+
+var errSystemProxyResponseTooLarge = errors.New("系统渠道流式响应超过配置上限")
+
+func isSystemProxyEventStream(resp *http.Response) bool {
+	mediaType, _, err := mime.ParseMediaType(resp.Header.Get("Content-Type"))
+	return err == nil && strings.EqualFold(mediaType, "text/event-stream")
+}
+
+func copySystemProxyResponseHeaders(c *gin.Context, resp *http.Response) {
+	for _, key := range []string{"Content-Type", "Cache-Control", "Content-Disposition"} {
+		if value := resp.Header.Get(key); value != "" {
+			c.Header(key, value)
+		}
+	}
+	c.Header("X-Content-Type-Options", "nosniff")
+}
+
+func streamSystemProxyResponse(c *gin.Context, resp *http.Response, responseLimit int64) ([]byte, error) {
+	copySystemProxyResponseHeaders(c, resp)
+	// 同时通知镜像内和可能存在的外层 Nginx，不得重新缓冲模型事件流。
+	c.Header("Cache-Control", "no-cache, no-store, no-transform")
+	c.Header("X-Accel-Buffering", "no")
+	c.Status(resp.StatusCode)
+	c.Writer.WriteHeaderNow()
+	c.Writer.Flush()
+
+	var captured bytes.Buffer
+	buffer := make([]byte, 32<<10)
+	remaining := responseLimit
+	for {
+		readLimit := int64(len(buffer))
+		if remaining < readLimit {
+			readLimit = remaining + 1
+		}
+		if readLimit < 1 {
+			readLimit = 1
+		}
+		read, readErr := resp.Body.Read(buffer[:int(readLimit)])
+		if read > 0 {
+			if int64(read) > remaining {
+				return captured.Bytes(), errSystemProxyResponseTooLarge
+			}
+			chunk := buffer[:read]
+			_, _ = captured.Write(chunk)
+			if _, writeErr := c.Writer.Write(chunk); writeErr != nil {
+				return captured.Bytes(), writeErr
+			}
+			c.Writer.Flush()
+			remaining -= int64(read)
+		}
+		if readErr != nil {
+			if errors.Is(readErr, io.EOF) {
+				return captured.Bytes(), nil
+			}
+			return captured.Bytes(), readErr
+		}
+	}
+}