Bug#43230: SELECT ... FOR UPDATE can hang with FLUSH TABLES WITH READ LOCK indefinitely

The problem is that a SELECT .. FOR UPDATE statement might open
a table and later wait for a impeding global read lock without
noticing whether it is holding a table that is being waited upon
the the flush phase of the process that took the global read
lock.

The same problem also affected the following statements:

LOCK TABLES .. WRITE
UPDATE .. SET (update and multi-table update)
TRUNCATE TABLE ..
LOAD DATA ..

The solution is to make the above statements wait for a impending
global read lock before opening the tables. If there is no
impending global read lock, the statement raises a temporary
protection against global read locks and progresses smoothly
towards completion.

Important notice: the patch does not try to address all possible
cases, only those which are common and can be fixed unintrusively
enough for 5.0.

1 files changed, 25 insertions, 1 deletions

diff --git a/sql/sql_parse.cc b/sql/sql_parse.cc
index ba887486aa1..02030ac2e19 100644
--- a/sql/sql_parse.cc
+++ b/sql/sql_parse.cc
@@ -2800,6 +2800,10 @@ mysql_execute_command(THD *thd)
     if (res)
       goto error;
 
+    if (!thd->locked_tables && lex->protect_against_global_read_lock &&
+        !(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
+      goto error;
+
     if (!(res= open_and_lock_tables(thd, all_tables)))
     {
       if (lex->describe)
@@ -3660,6 +3664,9 @@ end_with_restore_list:
     DBUG_ASSERT(first_table == all_tables && first_table != 0);
     if (update_precheck(thd, all_tables))
       break;
+    if (!thd->locked_tables &&
+        !(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
+      goto error;
     DBUG_ASSERT(select_lex->offset_limit == 0);
     unit->set_limit(select_lex);
     res= (up_result= mysql_update(thd, all_tables,
@@ -3686,6 +3693,15 @@ end_with_restore_list:
     else
       res= 0;
 
+    /*
+      Protection might have already been risen if its a fall through
+      from the SQLCOM_UPDATE case above.
+    */
+    if (!thd->locked_tables &&
+        lex->sql_command == SQLCOM_UPDATE_MULTI &&
+        !(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
+      goto error;
+
     res= mysql_multi_update_prepare(thd);
 
 #ifdef HAVE_REPLICATION
@@ -3853,7 +3869,8 @@ end_with_restore_list:
                  ER(ER_LOCK_OR_ACTIVE_TRANSACTION), MYF(0));
       goto error;
     }
-
+    if (!(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
+      goto error;
     res= mysql_truncate(thd, first_table, 0);
     break;
   case SQLCOM_DELETE:
@@ -4027,6 +4044,10 @@ end_with_restore_list:
     if (check_one_table_access(thd, privilege, all_tables))
       goto error;
 
+    if (!thd->locked_tables &&
+        !(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
+      goto error;
+
     res= mysql_load(thd, lex->exchange, first_table, lex->field_list,
                     lex->update_list, lex->value_list, lex->duplicates,
                     lex->ignore, (bool) lex->local_file);
@@ -4082,6 +4103,9 @@ end_with_restore_list:
       goto error;
     if (check_table_access(thd, LOCK_TABLES_ACL | SELECT_ACL, all_tables, 0))
       goto error;
+    if (lex->protect_against_global_read_lock &&
+        !(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
+      goto error;
     thd->in_lock_tables=1;
     thd->options|= OPTION_TABLE_LOCK;