feat(backend): 提交后端审计修复与配套测试改动

backend/internal/service/upstream_response_limit.go

@@ -0,0 +1,38 @@
+package service
+
+import (
+	"errors"
+	"fmt"
+	"io"
+
+	"github.com/Wei-Shaw/sub2api/internal/config"
+)
+
+var ErrUpstreamResponseBodyTooLarge = errors.New("upstream response body too large")
+
+const defaultUpstreamResponseReadMaxBytes int64 = 8 * 1024 * 1024
+
+func resolveUpstreamResponseReadLimit(cfg *config.Config) int64 {
+	if cfg != nil && cfg.Gateway.UpstreamResponseReadMaxBytes > 0 {
+		return cfg.Gateway.UpstreamResponseReadMaxBytes
+	}
+	return defaultUpstreamResponseReadMaxBytes
+}
+
+func readUpstreamResponseBodyLimited(reader io.Reader, maxBytes int64) ([]byte, error) {
+	if reader == nil {
+		return nil, errors.New("response body is nil")
+	}
+	if maxBytes <= 0 {
+		maxBytes = defaultUpstreamResponseReadMaxBytes
+	}
+
+	body, err := io.ReadAll(io.LimitReader(reader, maxBytes+1))
+	if err != nil {
+		return nil, err
+	}
+	if int64(len(body)) > maxBytes {
+		return nil, fmt.Errorf("%w: limit=%d", ErrUpstreamResponseBodyTooLarge, maxBytes)
+	}
+	return body, nil
+}