MDEV-5509: Seconds_behind_master incorrect in parallel replication

The problem was a race between the SQL driver thread and the worker threads.
The SQL driver thread would set rli->last_master_timestamp to zero to
mark that it has caught up with the master, while the worker threads would
set it to the timestamp of the executed event. This can happen out-of-order
in parallel replication, causing the "caught up" status to be overwritten
and Seconds_Behind_Master to wrongly grow when the slave is idle.

To fix, introduce a separate flag rli->sql_thread_caught_up to mark that the
SQL driver thread is caught up. This avoids issues with worker threads
overwriting the SQL driver thread status. In parallel replication, we then
make SHOW SLAVE STATUS check in addition that all worker threads are idle
before showing Seconds_Behind_Master as 0 due to slave idle.

mysql-test/suite/rpl/r/rpl_parallel2.result

+include/rpl_init.inc [topology=1->2]
+*** MDEV-5509: Incorrect value for Seconds_Behind_Master if parallel replication ***
+SET @old_parallel_threads=@@GLOBAL.slave_parallel_threads;
+include/stop_slave.inc
+SET GLOBAL slave_parallel_threads=5;
+include/start_slave.inc
+CREATE TABLE t1 (a INT PRIMARY KEY, b INT);
+CALL mtr.add_suppression("Unsafe statement written to the binary log using statement format since BINLOG_FORMAT = STATEMENT. Statement is unsafe because it uses a system function that may return a different value on the slave");
+INSERT INTO t1 VALUES (1,sleep(2));
+Warnings:
+Note	1592	Unsafe statement written to the binary log using statement format since BINLOG_FORMAT = STATEMENT. Statement is unsafe because it uses a system function that may return a different value on the slave.
+Seconds_Behind_Master should be zero here because the slave is fully caught up and idle.
+Seconds_Behind_Master = '0'
+include/stop_slave.inc
+SET GLOBAL slave_parallel_threads=@old_parallel_threads;
+include/start_slave.inc
+DROP TABLE t1;
+include/rpl_end.inc

mysql-test/suite/rpl/t/rpl_parallel2.test

+--source include/have_binlog_format_statement.inc
+--let $rpl_topology=1->2
+--source include/rpl_init.inc
+
+--echo *** MDEV-5509: Incorrect value for Seconds_Behind_Master if parallel replication ***
+
+--connection server_2
+SET @old_parallel_threads=@@GLOBAL.slave_parallel_threads;
+--source include/stop_slave.inc
+SET GLOBAL slave_parallel_threads=5;
+--source include/start_slave.inc
+
+--connection server_1
+CREATE TABLE t1 (a INT PRIMARY KEY, b INT);
+CALL mtr.add_suppression("Unsafe statement written to the binary log using statement format since BINLOG_FORMAT = STATEMENT. Statement is unsafe because it uses a system function that may return a different value on the slave");
+--save_master_pos
+
+--connection server_2
+--sync_with_master
+
+--connection server_1
+INSERT INTO t1 VALUES (1,sleep(2));
+--save_master_pos
+
+--connection server_2
+--sync_with_master
+
+--echo Seconds_Behind_Master should be zero here because the slave is fully caught up and idle.
+--let $status_items= Seconds_Behind_Master
+--source include/show_slave_status.inc
+
+
+--connection server_2
+--source include/stop_slave.inc
+SET GLOBAL slave_parallel_threads=@old_parallel_threads;
+--source include/start_slave.inc
+
+--connection server_1
+DROP TABLE t1;
+
+--source include/rpl_end.inc

sql/rpl_parallel.cc

@@ -766,6 +766,27 @@ rpl_parallel::wait_for_done()
 }
 
 
+bool
+rpl_parallel::workers_idle()
+{
+  struct rpl_parallel_entry *e;
+  uint32 i, max_i;
+
+  max_i= domain_hash.records;
+  for (i= 0; i < max_i; ++i)
+  {
+    bool active;
+    e= (struct rpl_parallel_entry *)my_hash_element(&domain_hash, i);
+    mysql_mutex_lock(&e->LOCK_parallel_entry);
+    active= e->current_sub_id > e->last_committed_sub_id;
+    mysql_mutex_unlock(&e->LOCK_parallel_entry);
+    if (active)
+      break;
+  }
+  return (i == max_i);
+}
+
+
 /*
   do_event() is executed by the sql_driver_thd thread.
   It's main purpose is to find a thread that can execute the query.

sql/rpl_parallel.h

@@ -117,6 +117,7 @@ struct rpl_parallel {
   void reset();
   rpl_parallel_entry *find(uint32 domain_id);
   void wait_for_done();
+  bool workers_idle();
   bool do_event(rpl_group_info *serial_rgi, Log_event *ev,
                 ulonglong event_size);
 };

sql/rpl_rli.cc

@@ -56,7 +56,7 @@ Relay_log_info::Relay_log_info(bool is_slave_recovery)
    is_fake(FALSE),
 #endif
    group_master_log_pos(0), log_space_total(0), ignore_log_space_limit(0),
-   last_master_timestamp(0), slave_skip_counter(0),
+   last_master_timestamp(0), sql_thread_caught_up(true), slave_skip_counter(0),
    abort_pos_wait(0), slave_run_id(0), sql_driver_thd(),
    inited(0), abort_slave(0), slave_running(0), until_condition(UNTIL_NONE),
    until_log_pos(0), retried_trans(0), executed_entries(0),
@@ -1287,9 +1287,14 @@ void Relay_log_info::stmt_done(my_off_t event_master_log_pos,
       (probably ok - except in some very rare cases, only consequence
       is that value may take some time to display in
       Seconds_Behind_Master - not critical).
+
+      In parallel replication, we take care to not set last_master_timestamp
+      backwards, in case of out-of-order calls here.
     */
     if (!(event_creation_time == 0 &&
-          IF_DBUG(debug_not_change_ts_if_art_event > 0, 1)))
+          IF_DBUG(debug_not_change_ts_if_art_event > 0, 1)) &&
+        !(rgi->is_parallel_exec && event_creation_time <= last_master_timestamp)
+        )
         last_master_timestamp= event_creation_time;
   }
   DBUG_VOID_RETURN;

sql/rpl_rli.h

@@ -221,6 +221,12 @@ public:
   bool sql_force_rotate_relay;
 
   time_t last_master_timestamp;
+  /*
+    The SQL driver thread sets this true while it is waiting at the end of the
+    relay log for more events to arrive. SHOW SLAVE STATUS uses this to report
+    Seconds_Behind_Master as zero while the SQL thread is so waiting.
+  */
+  bool sql_thread_caught_up;
 
   void clear_until_condition();
 

sql/slave.cc

@@ -2619,8 +2619,24 @@ static bool send_show_master_info_data(THD *thd, Master_info *mi, bool full,
     if ((mi->slave_running == MYSQL_SLAVE_RUN_CONNECT) &&
         mi->rli.slave_running)
     {
-      long time_diff= ((long)(time(0) - mi->rli.last_master_timestamp)
-                       - mi->clock_diff_with_master);
+      long time_diff;
+      bool idle;
+      time_t stamp= mi->rli.last_master_timestamp;
+
+      if (!stamp)
+        idle= true;
+      else
+      {
+        idle= mi->rli.sql_thread_caught_up;
+        if (opt_slave_parallel_threads > 0 && idle &&
+            !mi->rli.parallel.workers_idle())
+          idle= false;
+      }
+      if (idle)
+        time_diff= 0;
+      else
+      {
+        time_diff= ((long)(time(0) - stamp) - mi->clock_diff_with_master);
       /*
         Apparently on some systems time_diff can be <0. Here are possible
         reasons related to MySQL:
@@ -2636,13 +2652,15 @@ static bool send_show_master_info_data(THD *thd, Master_info *mi, bool full,
         slave is 2. At SHOW SLAVE STATUS time, assume that the difference
         between timestamp of slave and rli->last_master_timestamp is 0
         (i.e. they are in the same second), then we get 0-(2-1)=-1 as a result.
-        This confuses users, so we don't go below 0: hence the max().
+        This confuses users, so we don't go below 0.
 
         last_master_timestamp == 0 (an "impossible" timestamp 1970) is a
         special marker to say "consider we have caught up".
       */
-      protocol->store((longlong)(mi->rli.last_master_timestamp ?
-                                 max(0, time_diff) : 0));
+        if (time_diff < 0)
+          time_diff= 0;
+      }
+      protocol->store((longlong)time_diff);
     }
     else
     {
@@ -6100,6 +6118,7 @@ static Log_event* next_event(rpl_group_info *rgi, ulonglong *event_size)
 
       if (hot_log)
         mysql_mutex_unlock(log_lock);
+      rli->sql_thread_caught_up= false;
       DBUG_RETURN(ev);
     }
     if (opt_reckless_slave)                     // For mysql-test
@@ -6137,12 +6156,10 @@ static Log_event* next_event(rpl_group_info *rgi, ulonglong *event_size)
           Seconds_Behind_Master would be zero only when master has no
           more updates in binlog for slave. The heartbeat can be sent
           in a (small) fraction of slave_net_timeout. Until it's done
-          rli->last_master_timestamp is temporarely (for time of
-          waiting for the following event) reset whenever EOF is
-          reached.
+          rli->sql_thread_caught_up is temporarely (for time of waiting for
+          the following event) set whenever EOF is reached.
         */
-        time_t save_timestamp= rli->last_master_timestamp;
-        rli->last_master_timestamp= 0;
+        rli->sql_thread_caught_up= true;
 
         DBUG_ASSERT(rli->relay_log.get_open_count() ==
                     rli->cur_log_old_open_count);
@@ -6268,7 +6285,7 @@ static Log_event* next_event(rpl_group_info *rgi, ulonglong *event_size)
         rli->relay_log.wait_for_update_relay_log(rli->sql_driver_thd);
         // re-acquire data lock since we released it earlier
         mysql_mutex_lock(&rli->data_lock);
-        rli->last_master_timestamp= save_timestamp;
+        rli->sql_thread_caught_up= false;
         continue;
       }
       /*