MDEV-27850: MTR tests can hang due to DEBUG_SYNC race condition

DEBUG_SYNC signals can get lost in certain tests due to later
DEBUG_SYNC commands overwriting them. This patch addresses
these issues in three tests: main.query_cache_debug,
main.partition_debug_sync, and
rpl.rpl_dump_request_retry_warning.

Additionally, main.partition_debug_sync needed changes to the
result file (the others did not). The synchronization happened
between two commands, one based on ALTER, the other on DROP.
A new thread/connection was needed to synchronize the DEBUG_SYNC
actions between these commands, thereby changing the result file.
Additional comments were added for clarification.

Reviewed By:
============
Andrei Elkin <andrei.elkin@mariadb.com>

mysql-test/r/partition_debug_sync.result

@@ -53,9 +53,14 @@ SET DEBUG_SYNC= 'alter_table_before_rename_result_table WAIT_FOR delete_done';
 ALTER TABLE t2 REMOVE PARTITIONING;
 connection default;
 SET DEBUG_SYNC= 'now WAIT_FOR removing_partitions';
-SET DEBUG_SYNC= 'rm_table_no_locks_before_delete_table SIGNAL waiting_for_alter';
+SET DEBUG_SYNC= 'rm_table_no_locks_before_delete_table SIGNAL waiting_for_alter WAIT_FOR mtr_continue';
 SET DEBUG_SYNC= 'rm_table_no_locks_before_binlog SIGNAL delete_done';
 DROP TABLE IF EXISTS t2;
+# Create a new connection thread to orchestrate the ALTER and DROP
+# threads' DEBUG_SYNC points to avoid lost signals
+connect con2,localhost,root,,;
+SET DEBUG_SYNC= 'now SIGNAL mtr_continue';
+connection default;
 connection con1;
 ERROR 42S02: Table 'test.t2' doesn't exist
 SET DEBUG_SYNC= 'RESET';

mysql-test/suite/rpl/t/rpl_dump_request_retry_warning.test

@@ -43,6 +43,12 @@ START SLAVE;
 SET DEBUG_SYNC= 'now WAIT_FOR parked';
 SET @@GLOBAL.debug_dbug = @saved_dbug;
 SET DEBUG_SYNC= 'now SIGNAL continue';
+
+# Ensure the last DEBUG_SYNC signal was received by the target thread before
+# reset; otherwise, the reset can drop the last signal before it gets
+# acknowledged
+let $wait_condition= select count(*)=0 from information_schema.processlist where state like "%debug%";
+source include/wait_condition.inc;
 SET DEBUG_SYNC= 'RESET';
 
 --source include/wait_for_slave_io_to_start.inc

mysql-test/t/partition_debug_sync.test

@@ -66,12 +66,27 @@ SET DEBUG_SYNC= 'alter_table_before_rename_result_table WAIT_FOR delete_done';
 --send ALTER TABLE t2 REMOVE PARTITIONING
 connection default;
 SET DEBUG_SYNC= 'now WAIT_FOR removing_partitions';
-SET DEBUG_SYNC= 'rm_table_no_locks_before_delete_table SIGNAL waiting_for_alter';
+
+SET DEBUG_SYNC= 'rm_table_no_locks_before_delete_table SIGNAL waiting_for_alter WAIT_FOR mtr_continue';
 SET DEBUG_SYNC= 'rm_table_no_locks_before_binlog SIGNAL delete_done';
-DROP TABLE IF EXISTS t2;
+--send DROP TABLE IF EXISTS t2
+
+--echo # Create a new connection thread to orchestrate the ALTER and DROP
+--echo # threads' DEBUG_SYNC points to avoid lost signals
+connect(con2,localhost,root,,);
+let $wait_condition= select count(*)=0 from information_schema.processlist where state like "debug sync point: alter_table_before_open_tables";
+source include/wait_condition.inc;
+SET DEBUG_SYNC= 'now SIGNAL mtr_continue';
+
+# Return execution for DROP TABLE
+connection default;
+--reap
+
+# Return execution for ALTER TABLE
 connection con1;
 --error ER_NO_SUCH_TABLE
 --reap
+
 SET DEBUG_SYNC= 'RESET';
 disconnect con1;
 --source include/wait_until_disconnected.inc

mysql-test/t/query_cache_debug.test

@@ -197,8 +197,17 @@ SET DEBUG_SYNC="now WAIT_FOR parked1_2";
 --echo ** and finally release the mutex. The threads will continue to wait
 --echo ** until a broadcast signal reaches them causing both threads to 
 --echo ** come alive and check the condition.
+
+# Before sending signals back-to-back, we have to ensure the previous signal
+# was received
+let $wait_condition= select count(*)=3 from information_schema.processlist where state like "%debug%";
+source include/wait_condition.inc;
 SET DEBUG_SYNC="now SIGNAL go2";
+let $wait_condition= select count(*)=2 from information_schema.processlist where state like "%debug%";
+source include/wait_condition.inc;
 SET DEBUG_SYNC="now SIGNAL go3";
+let $wait_condition= select count(*)=1 from information_schema.processlist where state like "%debug%";
+source include/wait_condition.inc;
 
 --echo **
 --echo ** Finally signal the DELETE statement on THD1 one last time.
@@ -208,6 +217,8 @@ SET DEBUG_SYNC="now SIGNAL go3";
 --echo ** invalidations and a broadcast signal will be sent to the thread 
 --echo ** group holding result set writers.
 SET DEBUG_SYNC="now SIGNAL go1_2";
+let $wait_condition= select count(*)=0 from information_schema.processlist where state like "%debug%";
+source include/wait_condition.inc;
 
 --echo **
 --echo *************************************************************************