Fixed debug_sync timeout in deadlock_drop_table

The issue was that we sent two different signals to different threads
after each other. The DEBUG_SYNC functionality cannot handle this (as
the signal is stored in a global variable) and the first one can get
lost.

Fixed by using the same signal for both threads.

mysql-test/suite/stress/r/deadlock_drop_table.result

@@ -19,12 +19,11 @@ a	b	c
 1	NULL	NULL
 set debug_sync='now SIGNAL go';
 set debug_sync='now WAIT_FOR parked2';
-set debug_sync='before_wait_for_refs SIGNAL waiting WAIT_FOR go3';
+set debug_sync='before_wait_for_refs SIGNAL waiting WAIT_FOR go2';
 drop table t1;;
 connection con2;
 set debug_sync='now WAIT_FOR waiting';
 set debug_sync='now SIGNAL go2';
-set debug_sync='now SIGNAL go3';
 connection default;
 connection con1;
 connection default;

mysql-test/suite/stress/t/deadlock_drop_table.test

@@ -19,12 +19,16 @@ set debug_sync='now WAIT_FOR parked';
 select * from t1;
 set debug_sync='now SIGNAL go';
 set debug_sync='now WAIT_FOR parked2';
-set debug_sync='before_wait_for_refs SIGNAL waiting WAIT_FOR go3';
+set debug_sync='before_wait_for_refs SIGNAL waiting WAIT_FOR go2';
 --send drop table t1;
 --connection con2
 set debug_sync='now WAIT_FOR waiting';
 set debug_sync='now SIGNAL go2';
-set debug_sync='now SIGNAL go3';
+
+# Write out show processlist if the debug sync point times out
+let $wait_condition= select count(*)=0 from information_schema.processlist where state like "%debug%";
+source include/wait_condition.inc;
+
 --connection default
 --reap
 --connection con1

sql/backup.cc

@@ -258,16 +258,19 @@ static bool backup_flush(THD *thd)
 
 static bool backup_block_ddl(THD *thd)
 {
+  PSI_stage_info org_stage;
   DBUG_ENTER("backup_block_ddl");
 
   kill_delayed_threads();
   mysql_ha_cleanup_no_free(thd);
 
+  thd->backup_stage(&org_stage);
+  THD_STAGE_INFO(thd, stage_waiting_for_flush);
   /* Wait until all non trans statements has ended */
   if (thd->mdl_context.upgrade_shared_lock(backup_flush_ticket,
                                            MDL_BACKUP_WAIT_FLUSH,
                                            thd->variables.lock_wait_timeout))
-    DBUG_RETURN(1);
+    goto err;
 
   /*
     Remove not used tables from the table share.  Flush all changes to
@@ -284,6 +287,7 @@ static bool backup_block_ddl(THD *thd)
     We didn't do this lock above, as we wanted DDL's to be executed while
     we wait for non transactional tables (which may take a while).
  */
+  THD_STAGE_INFO(thd, stage_waiting_for_ddl);
   if (thd->mdl_context.upgrade_shared_lock(backup_flush_ticket,
                                            MDL_BACKUP_WAIT_DDL,
                                            thd->variables.lock_wait_timeout))
@@ -293,12 +297,16 @@ static bool backup_block_ddl(THD *thd)
       was called so that this function can be called again
     */
     backup_flush_ticket->downgrade_lock(MDL_BACKUP_FLUSH);
-    DBUG_RETURN(1);
+    goto err;
   }
 
   /* There can't be anything more that needs to be logged to ddl log */
+  THD_STAGE_INFO(thd, org_stage);
   stop_ddl_logging();
   DBUG_RETURN(0);
+err:
+  THD_STAGE_INFO(thd, org_stage);
+  DBUG_RETURN(1);
 }
 
 

sql/debug_sync.cc

@@ -995,6 +995,15 @@ static char *debug_sync_number(ulong *number_p, char *actstrptr,
     The input string needs to be ASCII NUL ('\0') terminated. We split
     nul-terminated tokens in it without copy.
 
+  @note
+    The current implementation does not support two 'now SIGNAL xxx' commands
+    in a row for multiple threads as the first one can get lost while
+    the waiting threads are sleeping on mysql_cond_timedwait().
+    One reason for this is that the signal name is stored in a global variable
+    that is overwritten.  A better way would be to store all signals in
+    an array together with a 'time' when the signal was sent. This array
+    should be checked on broadcast.
+
   @see the function comment of debug_sync_token() for more constraints
     for the string.
 */

sql/mysqld.cc

@@ -9146,6 +9146,8 @@ PSI_stage_info stage_slave_background_process_request= { 0, "Processing requests
 PSI_stage_info stage_slave_background_wait_request= { 0, "Waiting for requests", 0};
 PSI_stage_info stage_waiting_for_deadlock_kill= { 0, "Waiting for parallel replication deadlock handling to complete", 0};
 PSI_stage_info stage_starting= { 0, "starting", 0};
+PSI_stage_info stage_waiting_for_flush= { 0, "Waiting for non trans tables to be flushed", 0};
+PSI_stage_info stage_waiting_for_ddl= { 0, "Waiting for DDLs", 0};
 
 PSI_memory_key key_memory_DATE_TIME_FORMAT;
 PSI_memory_key key_memory_DDL_LOG_MEMORY_ENTRY;

sql/mysqld.h

@@ -639,7 +639,9 @@ extern PSI_stage_info stage_upgrading_lock;
 extern PSI_stage_info stage_user_lock;
 extern PSI_stage_info stage_user_sleep;
 extern PSI_stage_info stage_verifying_table;
+extern PSI_stage_info stage_waiting_for_ddl;
 extern PSI_stage_info stage_waiting_for_delay_list;
+extern PSI_stage_info stage_waiting_for_flush;
 extern PSI_stage_info stage_waiting_for_gtid_to_be_written_to_binary_log;
 extern PSI_stage_info stage_waiting_for_handler_insert;
 extern PSI_stage_info stage_waiting_for_handler_lock;