MDEV-7402 'reset master' hangs, waits for signalled COND_xid_list

Using a boolean flag for 'there is a RESET MASTER in progress' doesn't
work very well for multiple concurrent RESET MASTER statements.
Changed to a counter.

mysql-test/suite/binlog/r/binlog_checkpoint.result

@@ -112,6 +112,18 @@ master-bin.000003	#
 master-bin.000004	#
 master-bin.000005	#
 master-bin.000006	#
+SET debug_sync = 'reset';
+*** MDEV-7402: 'reset master' hangs, waits for signalled COND_xid_list ***
+SET debug_sync="reset_logs_after_set_reset_master_pending SIGNAL reset_master_ready WAIT_FOR reset_master_cont";
+RESET MASTER;
+SET @old_dbug= @@global.DEBUG_DBUG;
+SET GLOBAL debug_dbug="+d,inject_binlog_background_thread_before_mark_xid_done";
+SET debug_sync="now WAIT_FOR reset_master_ready";
+RESET MASTER;
+SET debug_sync="now WAIT_FOR injected_binlog_background_thread";
+SET GLOBAL debug_dbug=@old_dbug;
+SET debug_sync="now SIGNAL reset_master_cont";
+SET debug_sync = 'reset';
 DROP TABLE t1, t2;
 SET GLOBAL max_binlog_size= @old_max_binlog_size;
 SET GLOBAL innodb_flush_log_at_trx_commit= @old_innodb_flush_log_at_trx_commit;

mysql-test/suite/binlog/t/binlog_checkpoint.test

@@ -138,8 +138,39 @@ SET DEBUG_SYNC= "now WAIT_FOR injected_binlog_background_thread";
 SET GLOBAL debug_dbug= @old_dbug;
 INSERT INTO t1 VALUES (31, REPEAT("x", 4100));
 --source include/show_binary_logs.inc
+SET debug_sync = 'reset';
+
+
+--echo *** MDEV-7402: 'reset master' hangs, waits for signalled COND_xid_list ***
+
+--source include/wait_for_binlog_checkpoint.inc
+
+connect(con3,localhost,root,,);
+# Make the binlog background thread wait before clearing the pending checkpoint.
+# The bug was that one RESET MASTER would clear the reset_master_pending
+# flag set by another RESET MASTER; this could cause the wakeup from the
+# binlog background thread not to be sent, and thus the second RESET MASTER
+# to wait infinitely.
+SET debug_sync="reset_logs_after_set_reset_master_pending SIGNAL reset_master_ready WAIT_FOR reset_master_cont";
+send RESET MASTER;
+
+--connection default
+SET @old_dbug= @@global.DEBUG_DBUG;
+SET GLOBAL debug_dbug="+d,inject_binlog_background_thread_before_mark_xid_done";
+SET debug_sync="now WAIT_FOR reset_master_ready";
+RESET MASTER;
+SET debug_sync="now WAIT_FOR injected_binlog_background_thread";
+SET GLOBAL debug_dbug=@old_dbug;
+SET debug_sync="now SIGNAL reset_master_cont";
+
+--connection con3
+REAP;
+
+--connection default
+SET debug_sync = 'reset';
 
 
+# Clean up.
 DROP TABLE t1, t2;
 SET GLOBAL max_binlog_size= @old_max_binlog_size;
 SET GLOBAL innodb_flush_log_at_trx_commit= @old_innodb_flush_log_at_trx_commit;

sql/log.cc

@@ -3031,7 +3031,7 @@ const char *MYSQL_LOG::generate_name(const char *log_name,
 
 
 MYSQL_BIN_LOG::MYSQL_BIN_LOG(uint *sync_period)
-  :reset_master_pending(false), mark_xid_done_waiting(0),
+  :reset_master_pending(0), mark_xid_done_waiting(0),
    bytes_written(0), file_id(1), open_count(1),
    group_commit_queue(0), group_commit_queue_busy(FALSE),
    num_commits(0), num_group_commits(0),
@@ -3867,12 +3867,13 @@ bool MYSQL_BIN_LOG::reset_logs(THD* thd, bool create_new_log,
       do this before we take the LOCK_log to not deadlock.
     */
     mysql_mutex_lock(&LOCK_xid_list);
-    reset_master_pending= true;
+    reset_master_pending++;
     while (mark_xid_done_waiting > 0)
       mysql_cond_wait(&COND_xid_list, &LOCK_xid_list);
     mysql_mutex_unlock(&LOCK_xid_list);
   }
 
+  DEBUG_SYNC(thd, "reset_logs_after_set_reset_master_pending");
   if (thd)
     ha_reset_logs(thd);
   /*
@@ -4054,7 +4055,7 @@ bool MYSQL_BIN_LOG::reset_logs(THD* thd, bool create_new_log,
       DBUG_ASSERT(b->xid_count == 0);
       my_free(binlog_xid_count_list.get());
     }
-    reset_master_pending= false;
+    reset_master_pending--;
     mysql_mutex_unlock(&LOCK_xid_list);
   }
 

sql/log.h

@@ -470,7 +470,7 @@ class MYSQL_BIN_LOG: public TC_LOG, private MYSQL_LOG
     anyway). Instead we should signal COND_xid_list whenever a new binlog
     checkpoint arrives - when all have arrived, RESET MASTER will complete.
   */
-  bool reset_master_pending;
+  uint reset_master_pending;
   ulong mark_xid_done_waiting;
 
   /* LOCK_log and LOCK_index are inited by init_pthread_objects() */