fix(upload): retry on O_EXCL collision in single-file upload (B12)

backend/cmd/webui/handlers/libraries.go

@@ -135,23 +135,40 @@ func (h *H) Upload(c *gin.Context) {
 		err(c, http.StatusBadRequest, "bad_format", "extension must be cbz/pdf/epub/txt/md")
 		return
 	}
-	dst, e := h.uniquePath(root, name)
-	if e != nil {
-		err(c, http.StatusForbidden, "forbidden", e.Error())
+	// B12: retry on O_EXCL collision — concurrent uploads with the same name
+	// can both get the same candidate from uniquePath (stat-then-create race).
+	var dst, tmp string
+	var out *os.File
+	for attempt := 0; attempt < 5; attempt++ {
+		var e error
+		dst, e = h.uniquePath(root, name)
+		if e != nil {
+			err(c, http.StatusForbidden, "forbidden", e.Error())
+			return
+		}
+		tmp = dst + ".upload-" + strconv.FormatInt(time.Now().UnixNano(), 36)
+		out, e = os.OpenFile(tmp, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0o644)
+		if e == nil {
+			break
+		}
+		if !os.IsExist(e) {
+			err(c, http.StatusInternalServerError, "internal", "create tmp")
+			return
+		}
+		// O_EXCL collision — retry with fresh uniquePath.
+	}
+	if out == nil {
+		err(c, http.StatusConflict, "conflict", "too many concurrent uploads with same name")
 		return
 	}
 	src, e := fh.Open()
 	if e != nil {
+		out.Close()
+		os.Remove(tmp)
 		err(c, http.StatusInternalServerError, "internal", "open upload")
 		return
 	}
 	defer src.Close()
-	tmp := dst + ".upload-" + strconv.FormatInt(time.Now().UnixNano(), 36)
-	out, e := os.OpenFile(tmp, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0o644)
-	if e != nil {
-		err(c, http.StatusInternalServerError, "internal", "create tmp")
-		return
-	}
 	// B6: only MaxBytesError returns 413; other io.Copy failures (disk full,
 	// connection drop) return 500.
 	if _, e := io.Copy(out, src); e != nil {