storage: Ignore dangling symbolic link for filesystem pool

If there is a dangling symbolic link in filesystem pool, the pool
will fail to start or refresh, this patch is to fix it by ignoring
it with a warning log.

src/storage/storage_backend.c

@@ -977,7 +977,8 @@ virStorageBackendForType(int type) {
 /*
  * Allows caller to silently ignore files with improper mode
  *
- * Returns -1 on error, -2 if file mode is unexpected.
+ * Returns -1 on error, -2 if file mode is unexpected or the
+ * volume is a dangling symbolic link.
  */
 int
 virStorageBackendVolOpenCheckMode(const char *path, unsigned int flags)
@@ -986,6 +987,12 @@ virStorageBackendVolOpenCheckMode(const char *path, unsigned int flags)
     struct stat sb;
 
     if ((fd = open(path, O_RDONLY|O_NONBLOCK|O_NOCTTY)) < 0) {
+        if ((errno == ENOENT || errno == ELOOP) &&
+            lstat(path, &sb) == 0) {
+            VIR_WARN("ignoring dangling symlink '%s'", path);
+            return -2;
+        }
+
         virReportSystemError(errno,
                              _("cannot open volume '%s'"),
                              path);

src/storage/storage_backend_fs.c

@@ -651,7 +651,7 @@ virStorageBackendFileSystemRefresh(virConnectPtr conn ATTRIBUTE_UNUSED,
                 goto cleanup;
             else {
                 /* Silently ignore non-regular files,
-                 * eg '.' '..', 'lost+found' */
+                 * eg '.' '..', 'lost+found', dangling symbolic link */
                 virStorageVolDefFree(vol);
                 vol = NULL;
                 continue;