debug_sync: Implement NO_CLEAR_EVENT syntax

When waiting on a signal, NO_CLEAR_EVENT allows one to not clear the
signal, effectively allowing other threads to wait for the same signal.

mysql-test/main/debug_sync.result

@@ -299,4 +299,20 @@ disconnect con1;
 disconnect con2;
 connection default;
 DROP TABLE t1;
+#
+# Test NO_CLEAR_EVENT flag. The signal should still be visible after
+# the wait has completed succesfully.
+#
+SET DEBUG_SYNC= 'now SIGNAL s1';
+SHOW VARIABLES LIKE 'DEBUG_SYNC';
+Variable_name	Value
+debug_sync	ON - current signals: 's1'
+SET DEBUG_SYNC= 'now WAIT_FOR s1 NO_CLEAR_EVENT';
+SHOW VARIABLES LIKE 'DEBUG_SYNC';
+Variable_name	Value
+debug_sync	ON - current signals: 's1'
+SET DEBUG_SYNC= 'now WAIT_FOR s1';
+SHOW VARIABLES LIKE 'DEBUG_SYNC';
+Variable_name	Value
+debug_sync	ON - current signals: ''
 SET DEBUG_SYNC= 'RESET';

mysql-test/main/debug_sync.test

@@ -428,6 +428,17 @@ disconnect con2;
 connection default;
 DROP TABLE t1;
 
+--echo #
+--echo # Test NO_CLEAR_EVENT flag. The signal should still be visible after
+--echo # the wait has completed succesfully.
+--echo #
+SET DEBUG_SYNC= 'now SIGNAL s1';
+SHOW VARIABLES LIKE 'DEBUG_SYNC';
+SET DEBUG_SYNC= 'now WAIT_FOR s1 NO_CLEAR_EVENT';
+SHOW VARIABLES LIKE 'DEBUG_SYNC';
+SET DEBUG_SYNC= 'now WAIT_FOR s1';
+SHOW VARIABLES LIKE 'DEBUG_SYNC';
+
 #
 # Cleanup after test case.
 # Otherwise signal would contain 'flushed' here,

sql/debug_sync.cc

@@ -48,6 +48,8 @@ struct st_debug_sync_action
   String        wait_for;               /* signal to wait for */
   String        sync_point;             /* sync point name */
   bool          need_sort;              /* if new action, array needs sort */
+  bool          clear_event;            /* do not clear signal when waited
+                                           for if false. */
 };
 
 /* Debug sync control. Referenced by THD. */
@@ -1253,6 +1255,7 @@ static bool debug_sync_eval_action(THD *thd, char *action_str, char *action_end)
     /* Set default for EXECUTE and TIMEOUT options. */
     action->execute= 1;
     action->timeout= opt_debug_sync_timeout;
+    action->clear_event= true;
 
     /* Get next token. If none follows, set action. */
     if (!(ptr= debug_sync_token(&token, &token_length, ptr, action_end)))
@@ -1303,6 +1306,15 @@ static bool debug_sync_eval_action(THD *thd, char *action_str, char *action_end)
       goto set_action;
   }
 
+  /*
+    Try NO_CLEAR_EVENT.
+  */
+  if (!my_strcasecmp(system_charset_info, token, "NO_CLEAR_EVENT")) {
+    action->clear_event= false;
+    /* Get next token. If none follows, set action. */
+    if (!(ptr = debug_sync_token(&token, &token_length, ptr, action_end))) goto set_action;
+  }
+
   /*
     Try HIT_LIMIT.
   */
@@ -1591,8 +1603,10 @@ static void debug_sync_execute(THD *thd, st_debug_sync_action *action)
         }
         error= 0;
       }
-      // TODO conditional on clear-event
-      debug_sync_global.clear_signal(action->wait_for);
+
+      if (action->clear_event)
+        debug_sync_global.clear_signal(action->wait_for);
+
       DBUG_EXECUTE("debug_sync_exec",
                    if (thd->killed)
                      DBUG_PRINT("debug_sync_exec",