Bug#40536: SELECT is blocked by INSERT DELAYED waiting on

upgrading lock, even with low_priority_updates

The problem is that there is no mechanism to control whether a
delayed insert takes a high or low priority lock on a table.

The solution is to modify the delayed insert thread ("handler")
to take into account the global value of low_priority_updates
when taking table locks. The value of low_priority_updates is
retrieved when the insert delayed thread is created and will
remain the same for the duration of the thread.

include/thr_lock.h

@@ -159,7 +159,8 @@ void thr_multi_unlock(THR_LOCK_DATA **data,uint count);
 void thr_abort_locks(THR_LOCK *lock, my_bool upgrade_lock);
 my_bool thr_abort_locks_for_thread(THR_LOCK *lock, my_thread_id thread);
 void thr_print_locks(void);		/* For debugging */
-my_bool thr_upgrade_write_delay_lock(THR_LOCK_DATA *data);
+my_bool thr_upgrade_write_delay_lock(THR_LOCK_DATA *data,
+                                     enum thr_lock_type new_lock_type);
 void    thr_downgrade_write_lock(THR_LOCK_DATA *data,
                                  enum thr_lock_type new_lock_type);
 my_bool thr_reschedule_write_lock(THR_LOCK_DATA *data);

mysql-test/r/delayed.result

@@ -284,4 +284,30 @@ ERROR 22007: Incorrect date value: '0000-00-00' for column 'f1' at row 1
 INSERT DELAYED INTO t2 VALUES (0,'2007-00-00');
 ERROR 22007: Incorrect date value: '2007-00-00' for column 'f1' at row 1
 DROP TABLE t1,t2;
+set @old_delayed_updates = @@global.low_priority_updates;
+set global low_priority_updates = 1;
+select @@global.low_priority_updates;
+@@global.low_priority_updates
+1
+drop table if exists t1;
+create table t1 (a int, b int);
+insert into t1 values (1,1);
+lock table t1 read;
+connection: update
+insert delayed into t1 values (2,2);;
+connection: select
+select * from t1;
+a	b
+1	1
+connection: default
+select * from t1;
+a	b
+1	1
+unlock tables;
+select * from t1;
+a	b
+1	1
+2	2
+drop table t1;
+set global low_priority_updates = @old_delayed_updates;
 End of 5.1 tests

mysql-test/t/delayed.test

@@ -285,4 +285,47 @@ INSERT DELAYED INTO t2 VALUES (0,'0000-00-00');
 INSERT DELAYED INTO t2 VALUES (0,'2007-00-00');
 DROP TABLE t1,t2;
 
+#
+# Bug#40536: SELECT is blocked by INSERT DELAYED waiting on upgrading lock,
+#            even with low_priority_updates
+#
+
+set @old_delayed_updates = @@global.low_priority_updates;
+set global low_priority_updates = 1;
+select @@global.low_priority_updates;
+
+--disable_warnings
+drop table if exists t1;
+--enable_warnings
+create table t1 (a int, b int);
+insert into t1 values (1,1);
+lock table t1 read;
+connect (update,localhost,root,,);
+connection update;
+--echo connection: update
+--send insert delayed into t1 values (2,2);
+connection default;
+let $wait_condition=
+  select count(*) = 1 from information_schema.processlist
+  where command = "Delayed insert" and state = "upgrading lock";
+--source include/wait_condition.inc
+connect (select,localhost,root,,);
+--echo connection: select
+select * from t1;
+connection default;
+--echo connection: default
+select * from t1;
+connection default;
+disconnect update;
+disconnect select;
+unlock tables;
+let $wait_condition=
+  select count(*) = 1 from information_schema.processlist
+  where command = "Delayed insert" and state = "Waiting for INSERT";
+--source include/wait_condition.inc
+select * from t1;
+drop table t1;
+
+set global low_priority_updates = @old_delayed_updates;
+
 --echo End of 5.1 tests

mysys/thr_lock.c

@@ -1359,7 +1359,8 @@ void thr_downgrade_write_lock(THR_LOCK_DATA *in_data,
 
 /* Upgrade a WRITE_DELAY lock to a WRITE_LOCK */
 
-my_bool thr_upgrade_write_delay_lock(THR_LOCK_DATA *data)
+my_bool thr_upgrade_write_delay_lock(THR_LOCK_DATA *data,
+                                     enum thr_lock_type new_lock_type)
 {
   THR_LOCK *lock=data->lock;
   DBUG_ENTER("thr_upgrade_write_delay_lock");
@@ -1372,7 +1373,7 @@ my_bool thr_upgrade_write_delay_lock(THR_LOCK_DATA *data)
   }
   check_locks(lock,"before upgrading lock",0);
   /* TODO:  Upgrade to TL_WRITE_CONCURRENT_INSERT in some cases */
-  data->type=TL_WRITE;				/* Upgrade lock */
+  data->type= new_lock_type;                    /* Upgrade lock */
 
   /* Check if someone has given us the lock */
   if (!data->cond)
@@ -1411,6 +1412,7 @@ my_bool thr_upgrade_write_delay_lock(THR_LOCK_DATA *data)
 my_bool thr_reschedule_write_lock(THR_LOCK_DATA *data)
 {
   THR_LOCK *lock=data->lock;
+  enum thr_lock_type write_lock_type;
   DBUG_ENTER("thr_reschedule_write_lock");
 
   pthread_mutex_lock(&lock->mutex);
@@ -1420,6 +1422,7 @@ my_bool thr_reschedule_write_lock(THR_LOCK_DATA *data)
     DBUG_RETURN(0);
   }
 
+  write_lock_type= data->type;
   data->type=TL_WRITE_DELAYED;
   if (lock->update_status)
     (*lock->update_status)(data->status_param);
@@ -1438,7 +1441,7 @@ my_bool thr_reschedule_write_lock(THR_LOCK_DATA *data)
   free_all_read_locks(lock,0);
 
   pthread_mutex_unlock(&lock->mutex);
-  DBUG_RETURN(thr_upgrade_write_delay_lock(data));
+  DBUG_RETURN(thr_upgrade_write_delay_lock(data, write_lock_type));
 }
 
 

sql/sql_insert.cc

@@ -1690,6 +1690,7 @@ public:
 
 class Delayed_insert :public ilink {
   uint locks_in_memory;
+  thr_lock_type delayed_lock;
 public:
   THD thd;
   TABLE *table;
@@ -1731,6 +1732,8 @@ public:
     pthread_cond_init(&cond_client,NULL);
     VOID(pthread_mutex_lock(&LOCK_thread_count));
     delayed_insert_threads++;
+    delayed_lock= global_system_variables.low_priority_updates ?
+                                          TL_WRITE_LOW_PRIORITY : TL_WRITE;
     VOID(pthread_mutex_unlock(&LOCK_thread_count));
   }
   ~Delayed_insert()
@@ -2540,7 +2543,7 @@ bool Delayed_insert::handle_inserts(void)
   table->use_all_columns();
 
   thd_proc_info(&thd, "upgrading lock");
-  if (thr_upgrade_write_delay_lock(*thd.lock->locks))
+  if (thr_upgrade_write_delay_lock(*thd.lock->locks, delayed_lock))
   {
     /*
       This can happen if thread is killed either by a shutdown