fix(serve): check cover write errors, clean tmp only on failure path (B11)

backend/cmd/webui/handlers/content.go

@@ -2,6 +2,7 @@ package handlers
 
 import (
 	"fmt"
+	"log"
 	"net/http"
 	"os"
 	"path"
@@ -60,14 +61,18 @@ func (h *H) ServeCover(c *gin.Context) {
 				if e == nil {
 					dst := filepath.Join(dir, "cover"+ext)
 					if e := os.MkdirAll(dir, 0o755); e == nil {
-						tmp := fmt.Sprintf("%s.tmp-%d", dst, time.Now().UnixNano()) // 并发幂等:唯一 tmp + rename 原子
-						if e := os.WriteFile(tmp, img, 0o644); e == nil {
-							if e := os.Rename(tmp, dst); e == nil {
+						tmp := fmt.Sprintf("%s.tmp-%d", dst, time.Now().UnixNano())
+						// B11: check all write errors; clean tmp only on failure.
+						if we := os.WriteFile(tmp, img, 0o644); we != nil {
+							log.Printf("serve: write cover tmp: %v", we)
+							os.Remove(tmp)
+						} else if re := os.Rename(tmp, dst); re != nil {
+							log.Printf("serve: rename cover: %v", re)
+							os.Remove(tmp)
+						} else {
 							http.ServeFile(c.Writer, c.Request, dst)
 						}
 					}
-						os.Remove(tmp)
-					}
 				}
 			}
 		}