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.

mysql-test/r/lock_multi.result

@@ -133,3 +133,58 @@ ALTER TABLE t1 ADD COLUMN a INT;
 # 2.2.1. normal mode
 # 2.2.2. PS mode
 DROP TABLE t1;
+create table t1 (a int);
+create table t2 like t1;
+# con1
+lock tables t1 write;
+# con2
+flush tables with read lock;
+# con5
+# global read lock is taken
+# con3
+select * from t2 for update;
+# waiting for release of read lock
+# con4
+# would hang and later cause a deadlock
+flush tables t2;
+# clean up
+unlock tables;
+unlock tables;
+a
+drop table t1,t2;
+#
+# Lightweight version:
+# Ensure that the wait for a GRL is done before opening tables.
+#
+create table t1 (a int);
+create table t2 like t1;
+#
+# UPDATE
+#
+# default
+flush tables with read lock;
+# con1
+update t2 set a = 1;
+# default
+# statement is waiting for release of read lock
+# con2
+flush table t2;
+# default
+unlock tables;
+# con1
+#
+# LOCK TABLES .. WRITE
+#
+# default
+flush tables with read lock;
+# con1
+lock tables t2 write;
+# default
+# statement is waiting for release of read lock
+# con2
+flush table t2;
+# default
+unlock tables;
+# con1
+unlock tables;
+drop table t1,t2;

mysql-test/t/lock_multi.test

@@ -683,6 +683,134 @@ DROP TABLE t1;
 --disconnect locker
 --disconnect writer
 
+#
+# Bug#43230: SELECT ... FOR UPDATE can hang with FLUSH TABLES WITH READ LOCK indefinitely
+#
+
+connect (con1,localhost,root,,);
+connect (con2,localhost,root,,);
+connect (con3,localhost,root,,);
+connect (con4,localhost,root,,);
+connect (con5,localhost,root,,);
+
+create table t1 (a int);
+create table t2 like t1;
+
+connection con1;
+--echo # con1
+lock tables t1 write;
+connection con2;
+--echo # con2
+send flush tables with read lock;
+connection con5;
+--echo # con5
+let $show_statement= SHOW PROCESSLIST;
+let $field= State;
+let $condition= = 'Flushing tables';
+--source include/wait_show_condition.inc
+--echo # global read lock is taken
+connection con3;
+--echo # con3
+send select * from t2 for update;
+connection con5;
+let $show_statement= SHOW PROCESSLIST;
+let $field= State;
+let $condition= = 'Waiting for release of readlock';
+--source include/wait_show_condition.inc
+--echo # waiting for release of read lock
+connection con4;
+--echo # con4
+--echo # would hang and later cause a deadlock
+flush tables t2;
+connection con1;
+--echo # clean up
+unlock tables;
+connection con2;
+--reap
+unlock tables;
+connection con3;
+--reap
+connection default;
+disconnect con5;
+disconnect con4;
+disconnect con3;
+disconnect con2;
+disconnect con1;
+
+drop table t1,t2;
+
+--echo #
+--echo # Lightweight version:
+--echo # Ensure that the wait for a GRL is done before opening tables.
+--echo #
+
+connect (con1,localhost,root,,);
+connect (con2,localhost,root,,);
+
+create table t1 (a int);
+create table t2 like t1;
+
+--echo #
+--echo # UPDATE
+--echo #
+
+connection default;
+--echo # default
+flush tables with read lock;
+connection con1;
+--echo # con1
+send update t2 set a = 1;
+connection default;
+--echo # default
+let $show_statement= SHOW PROCESSLIST;
+let $field= State;
+let $condition= = 'Waiting for release of readlock';
+--source include/wait_show_condition.inc
+--echo # statement is waiting for release of read lock
+connection con2;
+--echo # con2
+flush table t2;
+connection default;
+--echo # default
+unlock tables;
+connection con1;
+--echo # con1
+--reap
+
+--echo #
+--echo # LOCK TABLES .. WRITE
+--echo #
+
+connection default;
+--echo # default
+flush tables with read lock;
+connection con1;
+--echo # con1
+send lock tables t2 write;
+connection default;
+--echo # default
+let $show_statement= SHOW PROCESSLIST;
+let $field= State;
+let $condition= = 'Waiting for release of readlock';
+--source include/wait_show_condition.inc
+--echo # statement is waiting for release of read lock
+connection con2;
+--echo # con2
+flush table t2;
+connection default;
+--echo # default
+unlock tables;
+connection con1;
+--echo # con1
+--reap
+unlock tables;
+
+connection default;
+disconnect con2;
+disconnect con1;
+
+drop table t1,t2;
+
 # End of 5.0 tests
 
 # Wait till all disconnects are completed

sql/sql_lex.cc

@@ -204,6 +204,7 @@ void lex_start(THD *thd)
   lex->nest_level=0 ;
   lex->allow_sum_func= 0;
   lex->in_sum_func= NULL;
+  lex->protect_against_global_read_lock= FALSE;
   DBUG_VOID_RETURN;
 }
 

sql/sql_lex.h

@@ -1176,6 +1176,22 @@ typedef struct st_lex : public Query_tables_list
 
   bool escape_used;
 
+  /*
+    Special case for SELECT .. FOR UPDATE and LOCK TABLES .. WRITE.
+
+    Protect from a impending GRL as otherwise the thread might deadlock
+    if it starts waiting for the GRL in mysql_lock_tables.
+
+    The protection is needed because there is a race between setting
+    the global read lock and waiting for all open tables to be closed.
+    The problem is a circular wait where a thread holding "old" open
+    tables will wait for the global read lock to be released while the
+    thread holding the global read lock will wait for all "old" open
+    tables to be closed -- the flush part of flush tables with read
+    lock.
+  */
+  bool protect_against_global_read_lock;
+
   st_lex();
 
   virtual ~st_lex()

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 @@ mysql_execute_command(THD *thd)
     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 @@ mysql_execute_command(THD *thd)
     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 @@ mysql_execute_command(THD *thd)
                  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 @@ mysql_execute_command(THD *thd)
     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 @@ mysql_execute_command(THD *thd)
       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;
 

sql/sql_yacc.yy

@@ -4522,6 +4522,7 @@ select_lock_type:
 	    LEX *lex=Lex;
 	    lex->current_select->set_lock_for_tables(TL_WRITE);
 	    lex->safe_to_cache_query=0;
+            lex->protect_against_global_read_lock= TRUE;
 	  }
 	| LOCK_SYM IN_SYM SHARE_SYM MODE_SYM
 	  {
@@ -10058,8 +10059,12 @@ table_lock_list:
 table_lock:
 	table_ident opt_table_alias lock_option
 	{
-	  if (!Select->add_table_to_list(YYTHD, $1, $2, 0, (thr_lock_type) $3))
+          thr_lock_type lock_type= (thr_lock_type) $3;
+	  if (!Select->add_table_to_list(YYTHD, $1, $2, 0, lock_type))
 	   MYSQL_YYABORT;
+          /* If table is to be write locked, protect from a impending GRL. */
+          if (lock_type >= TL_WRITE_ALLOW_WRITE)
+            Lex->protect_against_global_read_lock= TRUE;
 	}
         ;