Patch from upcoming subversion 1.9.1:

http://svn.apache.org/viewvc?view=revision&revision=1696695

--- ../subversion/libsvn_fs_x/fs_id.c	2015/08/20 01:01:06	1696694
+++ ./subversion/libsvn_fs_x/fs_id.c	2015/08/20 01:10:22	1696695
@@ -85,6 +85,13 @@
      is not.*/
   const char *fs_path;
 
+  /* If FS is NULL, this points to svn_fs_open() as passed to the library. */
+  svn_error_t *(*svn_fs_open_)(svn_fs_t **,
+      const char *,
+      apr_hash_t *,
+      apr_pool_t *,
+      apr_pool_t *);
+
   /* Pool that this context struct got allocated in. */
   apr_pool_t *owner;
 
@@ -118,11 +125,14 @@
 fs_cleanup(void *baton)
 {
   svn_fs_x__id_context_t *context = baton;
+  svn_fs_x__data_t *ffd = context->fs->fsap_data;
 
   /* Remember the FS_PATH to potentially reopen and mark the FS as n/a. */
   context->fs_path = apr_pstrdup(context->owner, context->fs->path);
+  context->svn_fs_open_ = ffd->svn_fs_open_;
   context->fs = NULL;
 
+
   /* No need for further notifications because from now on, everything is
      allocated in OWNER. */
   apr_pool_cleanup_kill(context->owner, context, owner_cleanup);
@@ -137,8 +147,12 @@
 {
   if (!context->fs)
     {
-      svn_error_t *err = svn_fs_open2(&context->fs, context->fs_path, NULL,
-                                      context->owner, context->owner);
+      svn_error_t *err;
+
+      SVN_ERR_ASSERT_NO_RETURN(context->svn_fs_open_);
+
+      err = context->svn_fs_open_(&context->fs, context->fs_path, NULL,
+                                  context->owner, context->owner);
       if (err)
         {
           svn_error_clear(err);