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: Add test case result for Bug#43230 mysql-test/t/lock_multi.test: Add test case for Bug#43230 sql/sql_lex.cc: Initialize flag. sql/sql_lex.h: Add a flag to the lexer. sql/sql_parse.cc: Wait for the global read lock is a write lock is going to be taken. The wait is done before opening tables. sql/sql_yacc.yy: Protect against the GRL if its a SELECT .. FOR UPDATE or LOCK TABLES .. WRITE statement.
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@ -133,3 +133,58 @@ ALTER TABLE t1 ADD COLUMN a INT;
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# 2.2.1. normal mode
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# 2.2.2. PS mode
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DROP TABLE t1;
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create table t1 (a int);
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create table t2 like t1;
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# con1
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lock tables t1 write;
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# con2
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flush tables with read lock;
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# con5
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# global read lock is taken
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# con3
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select * from t2 for update;
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# waiting for release of read lock
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# con4
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# would hang and later cause a deadlock
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flush tables t2;
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# clean up
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unlock tables;
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unlock tables;
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a
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drop table t1,t2;
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#
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# Lightweight version:
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# Ensure that the wait for a GRL is done before opening tables.
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#
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create table t1 (a int);
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create table t2 like t1;
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#
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# UPDATE
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#
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# default
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flush tables with read lock;
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# con1
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update t2 set a = 1;
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# default
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# statement is waiting for release of read lock
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# con2
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flush table t2;
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# default
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unlock tables;
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# con1
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#
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# LOCK TABLES .. WRITE
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#
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# default
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flush tables with read lock;
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# con1
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lock tables t2 write;
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# default
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# statement is waiting for release of read lock
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# con2
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flush table t2;
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# default
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unlock tables;
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# con1
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unlock tables;
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drop table t1,t2;
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@ -683,6 +683,134 @@ DROP TABLE t1;
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--disconnect locker
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--disconnect writer
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#
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# Bug#43230: SELECT ... FOR UPDATE can hang with FLUSH TABLES WITH READ LOCK indefinitely
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#
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connect (con1,localhost,root,,);
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connect (con2,localhost,root,,);
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connect (con3,localhost,root,,);
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connect (con4,localhost,root,,);
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connect (con5,localhost,root,,);
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create table t1 (a int);
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create table t2 like t1;
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connection con1;
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--echo # con1
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lock tables t1 write;
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connection con2;
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--echo # con2
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send flush tables with read lock;
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connection con5;
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--echo # con5
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let $show_statement= SHOW PROCESSLIST;
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let $field= State;
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let $condition= = 'Flushing tables';
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--source include/wait_show_condition.inc
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--echo # global read lock is taken
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connection con3;
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--echo # con3
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send select * from t2 for update;
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connection con5;
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let $show_statement= SHOW PROCESSLIST;
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let $field= State;
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let $condition= = 'Waiting for release of readlock';
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--source include/wait_show_condition.inc
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--echo # waiting for release of read lock
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connection con4;
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--echo # con4
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--echo # would hang and later cause a deadlock
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flush tables t2;
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connection con1;
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--echo # clean up
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unlock tables;
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connection con2;
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--reap
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unlock tables;
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connection con3;
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--reap
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connection default;
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disconnect con5;
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disconnect con4;
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disconnect con3;
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disconnect con2;
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disconnect con1;
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drop table t1,t2;
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--echo #
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--echo # Lightweight version:
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--echo # Ensure that the wait for a GRL is done before opening tables.
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--echo #
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connect (con1,localhost,root,,);
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connect (con2,localhost,root,,);
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create table t1 (a int);
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create table t2 like t1;
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--echo #
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--echo # UPDATE
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--echo #
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connection default;
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--echo # default
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flush tables with read lock;
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connection con1;
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--echo # con1
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send update t2 set a = 1;
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connection default;
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--echo # default
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let $show_statement= SHOW PROCESSLIST;
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let $field= State;
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let $condition= = 'Waiting for release of readlock';
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--source include/wait_show_condition.inc
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--echo # statement is waiting for release of read lock
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connection con2;
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--echo # con2
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flush table t2;
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connection default;
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--echo # default
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unlock tables;
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connection con1;
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--echo # con1
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--reap
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--echo #
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--echo # LOCK TABLES .. WRITE
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--echo #
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connection default;
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--echo # default
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flush tables with read lock;
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connection con1;
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--echo # con1
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send lock tables t2 write;
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connection default;
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--echo # default
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let $show_statement= SHOW PROCESSLIST;
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let $field= State;
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let $condition= = 'Waiting for release of readlock';
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--source include/wait_show_condition.inc
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--echo # statement is waiting for release of read lock
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connection con2;
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--echo # con2
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flush table t2;
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connection default;
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--echo # default
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unlock tables;
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connection con1;
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--echo # con1
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--reap
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unlock tables;
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connection default;
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disconnect con2;
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disconnect con1;
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drop table t1,t2;
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# End of 5.0 tests
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# Wait till all disconnects are completed
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@ -204,6 +204,7 @@ void lex_start(THD *thd)
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lex->nest_level=0 ;
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lex->allow_sum_func= 0;
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lex->in_sum_func= NULL;
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lex->protect_against_global_read_lock= FALSE;
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DBUG_VOID_RETURN;
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}
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@ -1176,6 +1176,22 @@ typedef struct st_lex : public Query_tables_list
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bool escape_used;
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/*
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Special case for SELECT .. FOR UPDATE and LOCK TABLES .. WRITE.
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Protect from a impending GRL as otherwise the thread might deadlock
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if it starts waiting for the GRL in mysql_lock_tables.
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The protection is needed because there is a race between setting
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the global read lock and waiting for all open tables to be closed.
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The problem is a circular wait where a thread holding "old" open
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tables will wait for the global read lock to be released while the
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thread holding the global read lock will wait for all "old" open
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tables to be closed -- the flush part of flush tables with read
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lock.
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*/
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bool protect_against_global_read_lock;
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st_lex();
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virtual ~st_lex()
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@ -2800,6 +2800,10 @@ mysql_execute_command(THD *thd)
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if (res)
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goto error;
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if (!thd->locked_tables && lex->protect_against_global_read_lock &&
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!(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
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goto error;
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if (!(res= open_and_lock_tables(thd, all_tables)))
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{
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if (lex->describe)
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@ -3660,6 +3664,9 @@ end_with_restore_list:
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DBUG_ASSERT(first_table == all_tables && first_table != 0);
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if (update_precheck(thd, all_tables))
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break;
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if (!thd->locked_tables &&
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!(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
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goto error;
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DBUG_ASSERT(select_lex->offset_limit == 0);
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unit->set_limit(select_lex);
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res= (up_result= mysql_update(thd, all_tables,
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@ -3686,6 +3693,15 @@ end_with_restore_list:
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else
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res= 0;
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/*
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Protection might have already been risen if its a fall through
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from the SQLCOM_UPDATE case above.
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*/
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if (!thd->locked_tables &&
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lex->sql_command == SQLCOM_UPDATE_MULTI &&
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!(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
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goto error;
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res= mysql_multi_update_prepare(thd);
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#ifdef HAVE_REPLICATION
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@ -3853,7 +3869,8 @@ end_with_restore_list:
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ER(ER_LOCK_OR_ACTIVE_TRANSACTION), MYF(0));
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goto error;
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}
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if (!(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
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goto error;
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res= mysql_truncate(thd, first_table, 0);
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break;
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case SQLCOM_DELETE:
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@ -4027,6 +4044,10 @@ end_with_restore_list:
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if (check_one_table_access(thd, privilege, all_tables))
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goto error;
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if (!thd->locked_tables &&
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!(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
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goto error;
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res= mysql_load(thd, lex->exchange, first_table, lex->field_list,
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lex->update_list, lex->value_list, lex->duplicates,
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lex->ignore, (bool) lex->local_file);
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@ -4082,6 +4103,9 @@ end_with_restore_list:
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goto error;
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if (check_table_access(thd, LOCK_TABLES_ACL | SELECT_ACL, all_tables, 0))
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goto error;
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if (lex->protect_against_global_read_lock &&
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!(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
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goto error;
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thd->in_lock_tables=1;
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thd->options|= OPTION_TABLE_LOCK;
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LEX *lex=Lex;
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lex->current_select->set_lock_for_tables(TL_WRITE);
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lex->safe_to_cache_query=0;
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lex->protect_against_global_read_lock= TRUE;
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}
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{
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@ -10058,8 +10059,12 @@ table_lock_list:
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table_lock:
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table_ident opt_table_alias lock_option
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{
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if (!Select->add_table_to_list(YYTHD, $1, $2, 0, (thr_lock_type) $3))
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thr_lock_type lock_type= (thr_lock_type) $3;
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if (!Select->add_table_to_list(YYTHD, $1, $2, 0, lock_type))
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MYSQL_YYABORT;
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/* If table is to be write locked, protect from a impending GRL. */
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if (lock_type >= TL_WRITE_ALLOW_WRITE)
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Lex->protect_against_global_read_lock= TRUE;
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}
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;
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