GCF-1058 MTR test galera.MW-86 fails on repeated runs

Wait for the sync point sync.wsrep_apply_cb to be reached before
executing the test and clearing the debug flag sync.wsrep_apply_cb.

The race scenario:

Intended behavior:
node2: set sync.wsrep_apply_cb in order to start waiting in the background INSERT
node1: INSERT start
node2 (background): INSERT start
node1: INSERT end
node2: send signal to background INSERT: "stop waiting and continue executing"
node2: clear sync.wsrep_apply_cb as no longer needed
node2 (background): consume the signal
node2 (background): INSERT end
node2: DROP TABLE
node2: check no pending signals are left - ok

What happens occasionally (unexpected):
node2: set sync.wsrep_apply_cb in order to start waiting in the background INSERT
node1: INSERT start
node2 (background): INSERT start
node1: INSERT end
// The background INSERT still has _not_ reached the place where it starts
// waiting for the signal:
// DBUG_EXECUTE_IF("sync.wsrep_apply_cb", "now wait_for...");
node2: send signal to background INSERT: "stop waiting and continue executing"
node2: clear sync.wsrep_apply_cb as no longer needed
// The background INSERT reaches DBUG_EXECUTE_IF("sync.wsrep_apply_cb", ...)
// but sync.wsrep_apply_cb has already been cleared and the "wait" code is not
// executed. The signal remains unconsumed.
node2 (background): INSERT end
node2: DROP TABLE
node2: check no pending signals are left - failure, signal.wsrep_apply_cb is
pending (not consumed)

Author: vasild

mysql-test/suite/galera/r/MW-86-wait1.result

@@ -5,6 +5,7 @@ SET SESSION wsrep_sync_wait = 1;
 SET GLOBAL debug = "+d,sync.wsrep_apply_cb";
 CREATE TABLE t_wait1 (f1 INTEGER) ENGINE=InnoDB;
 INSERT INTO t_wait1 VALUES (1);
+SET SESSION debug_sync = "now WAIT_FOR sync.wsrep_apply_cb_reached";
 SHOW BINARY LOGS;
 SHOW BINLOG EVENTS;
 SHOW COLUMNS FROM t1;

mysql-test/suite/galera/r/MW-86-wait8.result

@@ -6,6 +6,7 @@ SET GLOBAL debug = "+d,sync.wsrep_apply_cb";
 CREATE TABLE t_wait8 (f1 INTEGER) ENGINE=InnoDB;
 INSERT INTO t_wait8 VALUES (1);
 SET GLOBAL wsrep_provider_options = "repl.causal_read_timeout=PT0.1S";
+SET SESSION debug_sync = "now WAIT_FOR sync.wsrep_apply_cb_reached";
 SHOW BINARY LOGS;
 SHOW BINLOG EVENTS;
 SHOW COLUMNS FROM t1;

mysql-test/suite/galera/t/MW-86-wait1.test

@@ -22,6 +22,8 @@ INSERT INTO t_wait1 VALUES (1);
 
 --connection node_2
 
+SET SESSION debug_sync = "now WAIT_FOR sync.wsrep_apply_cb_reached";
+
 --disable_result_log
 
 SHOW BINARY LOGS;

mysql-test/suite/galera/t/MW-86-wait8.test

@@ -23,6 +23,8 @@ INSERT INTO t_wait8 VALUES (1);
 --let $wsrep_provider_options_orig = `SELECT @@wsrep_provider_options`
 SET GLOBAL wsrep_provider_options = "repl.causal_read_timeout=PT0.1S";
 
+SET SESSION debug_sync = "now WAIT_FOR sync.wsrep_apply_cb_reached";
+
 --disable_result_log
 
 --error ER_LOCK_WAIT_TIMEOUT

sql/wsrep_applier.cc

@@ -207,7 +207,8 @@ wsrep_cb_status_t wsrep_apply_cb(void* const             ctx,
                  {
                    const char act[]=
                      "now "
-                     "wait_for signal.wsrep_apply_cb";
+                     "SIGNAL sync.wsrep_apply_cb_reached "
+                     "WAIT_FOR signal.wsrep_apply_cb";
                    DBUG_ASSERT(!debug_sync_set_action(thd,
                                                       STRING_WITH_LEN(act)));
                  };);