Fix for bug #21587: FLUSH TABLES causes server crash when used with HANDLER statements

Problems (appear only under some circumstances): 
  1. we get a reference to a deleted table searching in the 
     thd->handler_tables_hash in the mysql_ha_read().

  2. DBUG_ASSERT(table->file->inited == handler::NONE); assert fails in the
     close_thread_table().

Fix: end open index scans and table scans and remove references to the 
tables from the handler tables hash. After this preparation it is safe 
to close the tables. The close can no longer fail on open index/table 
scans and the closed table will not be used again by handler functions.
                                 

sql/mysql_priv.h

@@ -941,6 +941,7 @@ bool mysql_ha_read(THD *, TABLE_LIST *,enum enum_ha_read_modes,char *,
                    List<Item> *,enum ha_rkey_function,Item *,ha_rows,ha_rows);
 int mysql_ha_flush(THD *thd, TABLE_LIST *tables, uint mode_flags,
                    bool is_locked);
+void mysql_ha_mark_tables_for_reopen(THD *thd, TABLE *table);
 /* mysql_ha_flush mode_flags bits */
 #define MYSQL_HA_CLOSE_FINAL        0x00
 #define MYSQL_HA_REOPEN_ON_USAGE    0x01

sql/sql_base.cc

@@ -519,7 +519,15 @@ void close_thread_tables(THD *thd, bool lock_in_use, bool skip_derived)
 
   DBUG_PRINT("info", ("thd->open_tables: %p", thd->open_tables));
 
- found_old_table= 0;
+  
+  /* 
+    End open index scans and table scans and remove references to the tables 
+    from the handler tables hash. After this preparation it is safe to close 
+    the tables.
+  */
+  mysql_ha_mark_tables_for_reopen(thd, thd->open_tables);
+
+  found_old_table= 0;
   while (thd->open_tables)
     found_old_table|=close_thread_table(thd, &thd->open_tables);
   thd->some_tables_deleted=0;

sql/sql_handler.cc

@@ -748,3 +748,41 @@ static int mysql_ha_flush_table(THD *thd, TABLE **table_ptr, uint mode_flags)
 
   DBUG_RETURN(0);
 }
+
+
+/*
+  Mark tables for reopen.
+
+  SYNOPSIS
+    mysql_ha_mark_tables_for_reopen()
+    thd                         Thread identifier.
+    table                       Table list to mark for reopen.
+
+  DESCRIPTION
+    For each table found in the handler hash mark it as closed
+    (ready for reopen) and end all index/table scans.
+
+  NOTE
+    The caller must lock LOCK_open.
+*/
+
+void mysql_ha_mark_tables_for_reopen(THD *thd, TABLE *table)
+{
+  DBUG_ENTER("mysql_ha_mark_tables_for_reopen");
+
+  safe_mutex_assert_owner(&LOCK_open);
+  for (; table; table= table->next)
+  {
+    TABLE_LIST *hash_tables;
+    if ((hash_tables= (TABLE_LIST*) hash_search(&thd->handler_tables_hash,
+                                                (byte*) table->alias,
+                                                strlen(table->alias) + 1)))
+    {
+      /* Mark table as ready for reopen. */
+      hash_tables->table= NULL;
+      /* End open index/table scans. */
+      table->file->ha_index_or_rnd_end();
+    }
+  }
+  DBUG_VOID_RETURN;
+}