Backport two small cleanups from trunk-iplus tree:

    
1) No mutex and no function call if we're not using
plugins.
2) If we're above the table definition cache limit,
delete the oldest unused share, not the share on our hands.

sql/sql_base.cc

@@ -686,7 +686,6 @@ static TABLE_SHARE
 
 void release_table_share(TABLE_SHARE *share)
 {
-  bool to_be_deleted= 0;
   DBUG_ENTER("release_table_share");
   DBUG_PRINT("enter",
              ("share: 0x%lx  table: %s.%s  ref_count: %u  version: %lu",
@@ -700,7 +699,7 @@ void release_table_share(TABLE_SHARE *share)
   {
     if (share->version != refresh_version ||
         table_def_shutdown_in_progress)
-      to_be_deleted=1;
+      my_hash_delete(&table_def_cache, (uchar*) share);
     else
     {
       /* Link share last in used_table_share list */
@@ -712,15 +711,14 @@ void release_table_share(TABLE_SHARE *share)
       end_of_unused_share.prev= &share->next;
       share->next= &end_of_unused_share;
 
-      to_be_deleted= (table_def_cache.records > table_def_size);
+      if (table_def_cache.records > table_def_size)
+      {
+        /* Delete the least used share to preserve LRU order. */
+        my_hash_delete(&table_def_cache, (uchar*) oldest_unused_share);
+      }
     }
   }
 
-  if (to_be_deleted)
-  {
-    DBUG_PRINT("info", ("Deleting share"));
-    my_hash_delete(&table_def_cache, (uchar*) share);
-  }
   DBUG_VOID_RETURN;
 }
 

sql/sql_lex.cc

@@ -435,8 +435,11 @@ void lex_end(LEX *lex)
   DBUG_PRINT("enter", ("lex: 0x%lx", (long) lex));
 
   /* release used plugins */
-  plugin_unlock_list(0, (plugin_ref*)lex->plugins.buffer, 
-                     lex->plugins.elements);
+  if (lex->plugins.elements) /* No function call and no mutex if no plugins. */
+  {
+    plugin_unlock_list(0, (plugin_ref*)lex->plugins.buffer, 
+                       lex->plugins.elements);
+  }
   reset_dynamic(&lex->plugins);
 
   DBUG_VOID_RETURN;