MDEV-5262, MDEV-5914, MDEV-5941, MDEV-6020: Deadlocks during parallel

replication causing replication to fail.

In parallel replication, we run transactions from the master in parallel, but
force them to commit in the same order they did on the master. If we force T1
to commit before T2, but T2 holds eg. a row lock that is needed by T1, we get
a deadlock when T2 waits until T1 has committed.

Usually, we do not run T1 and T2 in parallel if there is a chance that they
can have conflicting locks like this, but there are certain edge cases where
it can occasionally happen (eg. MDEV-5914, MDEV-5941, MDEV-6020). The bug was
that this would cause replication to hang, eventually getting a lock timeout
and causing the slave to stop with error.

With this patch, InnoDB will report back to the upper layer whenever a
transactions T1 is about to do a lock wait on T2. If T1 and T2 are parallel
replication transactions, and T2 needs to commit later than T1, we can thus
detect the deadlock; we then kill T2, setting a flag that causes it to catch
the kill and convert it to a deadlock error; this error will then cause T2 to
roll back and release its locks (so that T1 can commit), and later T2 will be
re-tried and eventually also committed.

The kill happens asynchroneously in a slave background thread; this is
necessary, as the reporting from InnoDB about lock waits happen deep inside
the locking code, at a point where it is not possible to directly call
THD::awake() due to mutexes held.

Deadlock is assumed to be (very) rarely occuring, so this patch tries to
minimise the performance impact on the normal case where no deadlocks occur,
rather than optimise the handling of the occasional deadlock.

Also fix transaction retry due to deadlock when it happens after a transaction
already signalled to later transactions that it started to commit. In this
case we need to undo this signalling (and later redo it when we commit again
during retry), so following transactions will not start too early.

Also add a missing thd->send_kill_message() that got triggered during testing
(this corrects an incorrect fix for MySQL Bug#58933).

1 files changed, 115 insertions, 20 deletions

diff --git a/sql/rpl_parallel.cc b/sql/rpl_parallel.cc
index 67d61b7cf11..65461b3f990 100644
--- a/sql/rpl_parallel.cc
+++ b/sql/rpl_parallel.cc
@@ -156,6 +156,7 @@ finish_event_group(THD *thd, uint64 sub_id, rpl_parallel_entry *entry,
   mysql_mutex_unlock(&entry->LOCK_parallel_entry);
 
   thd->clear_error();
+  thd->reset_killed();
   thd->get_stmt_da()->reset_diagnostics_area();
   wfc->wakeup_subsequent_commits(rgi->worker_error);
 }
@@ -188,6 +189,25 @@ unlock_or_exit_cond(THD *thd, mysql_mutex_t *lock, bool *did_enter_cond,
 }
 
 
+static void
+register_wait_for_prior_event_group_commit(rpl_group_info *rgi,
+                                           rpl_parallel_entry *entry)
+{
+  mysql_mutex_assert_owner(&entry->LOCK_parallel_entry);
+  if (rgi->wait_commit_sub_id > entry->last_committed_sub_id)
+  {
+    /*
+      Register that the commit of this event group must wait for the
+      commit of the previous event group to complete before it may
+      complete itself, so that we preserve commit order.
+    */
+    wait_for_commit *waitee=
+      &rgi->wait_commit_group_info->commit_orderer;
+    rgi->commit_orderer.register_wait_for_prior_commit(waitee);
+  }
+}
+
+
 #ifndef DBUG_OFF
 static int
 dbug_simulate_tmp_error(rpl_group_info *rgi, THD *thd)
@@ -205,6 +225,40 @@ dbug_simulate_tmp_error(rpl_group_info *rgi, THD *thd)
 #endif
 
 
+/*
+  If we detect a deadlock due to eg. storage engine locks that conflict with
+  the fixed commit order, then the later transaction will be killed
+  asynchroneously to allow the former to complete its commit.
+
+  In this case, we convert the 'killed' error into a deadlock error, and retry
+  the later transaction.  */
+static void
+convert_kill_to_deadlock_error(rpl_group_info *rgi)
+{
+  THD *thd= rgi->thd;
+
+  if (thd->get_stmt_da()->sql_errno() == ER_QUERY_INTERRUPTED &&
+      rgi->killed_for_retry)
+  {
+    thd->clear_error();
+    thd->get_stmt_da()->reset_diagnostics_area();
+    my_error(ER_LOCK_DEADLOCK, MYF(0));
+    rgi->killed_for_retry= false;
+    thd->reset_killed();
+  }
+}
+
+
+static bool
+is_group_ending(Log_event *ev, Log_event_type event_type)
+{
+  return event_type == XID_EVENT ||
+         (event_type == QUERY_EVENT &&
+          (((Query_log_event *)ev)->is_commit() ||
+           ((Query_log_event *)ev)->is_rollback()));
+}
+
+
 static int
 retry_event_group(rpl_group_info *rgi, rpl_parallel_thread *rpt,
                   rpl_parallel_thread::queued_event *orig_qev)
@@ -221,11 +275,46 @@ retry_event_group(rpl_group_info *rgi, rpl_parallel_thread *rpt,
   ulonglong cur_offset, old_offset;
   char log_name[FN_REFLEN];
   THD *thd= rgi->thd;
+  rpl_parallel_entry *entry= rgi->parallel_entry;
   ulong retries= 0;
 
 do_retry:
   event_count= 0;
   err= 0;
+
+  /*
+    If we already started committing before getting the deadlock (or other
+    error) that caused us to need to retry, we have already signalled
+    subsequent transactions that we have started committing. This is
+    potentially a problem, as now we will rollback, and if subsequent
+    transactions would start to execute now, they could see an unexpected
+    state of the database and get eg. key not found or duplicate key error.
+
+    However, to get a deadlock in the first place, there must have been
+    another earlier transaction that is waiting for us. Thus that other
+    transaction has _not_ yet started to commit, and any subsequent
+    transactions will still be waiting at this point.
+
+    So here, we decrement back the count of transactions that started
+    committing (if we already incremented it), undoing the effect of an
+    earlier mark_start_commit(). Then later, when the retry succeeds and we
+    commit again, we can do a new mark_start_commit() and eventually wake up
+    subsequent transactions at the proper time.
+
+    We need to do the unmark before the rollback, to be sure that the
+    transaction we deadlocked with will not signal that it started to commit
+    until after the unmark.
+  */
+  rgi->unmark_start_commit();
+
+  /*
+    We might get the deadlock error that causes the retry during commit, while
+    sitting in wait_for_prior_commit(). If this happens, we will have a
+    pending error in the wait_for_commit object. So clear this by
+    unregistering (and later re-registering) the wait.
+  */
+  if(thd->wait_for_commit_ptr)
+    thd->wait_for_commit_ptr->unregister_wait_for_prior_commit();
   rgi->cleanup_context(thd, 1);
 
   mysql_mutex_lock(&rli->data_lock);
@@ -233,6 +322,10 @@ do_retry:
   statistic_increment(slave_retried_transactions, LOCK_status);
   mysql_mutex_unlock(&rli->data_lock);
 
+  mysql_mutex_lock(&entry->LOCK_parallel_entry);
+  register_wait_for_prior_event_group_commit(rgi, entry);
+  mysql_mutex_unlock(&entry->LOCK_parallel_entry);
+
   strcpy(log_name, ir->name);
   if ((fd= open_binlog(&rlog, log_name, &errmsg)) <0)
   {
@@ -319,6 +412,9 @@ do_retry:
       err= 1;
       goto err;
     }
+    if (is_group_ending(ev, event_type))
+      rgi->mark_start_commit();
+
     err= rpt_handle_event(qev, rpt);
     ++event_count;
     mysql_mutex_lock(&rpt->LOCK_rpl_thread);
@@ -332,6 +428,7 @@ do_retry:
                     err= dbug_simulate_tmp_error(rgi, thd););
     if (err)
     {
+      convert_kill_to_deadlock_error(rgi);
       if (has_temporary_error(thd))
       {
         ++retries;
@@ -599,17 +696,9 @@ handle_rpl_parallel_thread(void *arg)
 
         if (unlikely(entry->stop_on_error_sub_id <= rgi->wait_commit_sub_id))
           skip_event_group= true;
-        else if (rgi->wait_commit_sub_id > entry->last_committed_sub_id)
-        {
-          /*
-            Register that the commit of this event group must wait for the
-            commit of the previous event group to complete before it may
-            complete itself, so that we preserve commit order.
-          */
-          wait_for_commit *waitee=
-            &rgi->wait_commit_group_info->commit_orderer;
-          rgi->commit_orderer.register_wait_for_prior_commit(waitee);
-        }
+        else
+          register_wait_for_prior_event_group_commit(rgi, entry);
+
         unlock_or_exit_cond(thd, &entry->LOCK_parallel_entry,
                             &did_enter_cond, &old_stage);
 
@@ -651,10 +740,7 @@ handle_rpl_parallel_thread(void *arg)
         }
       }
 
-      group_ending= event_type == XID_EVENT ||
-         (event_type == QUERY_EVENT &&
-          (((Query_log_event *)events->ev)->is_commit() ||
-           ((Query_log_event *)events->ev)->is_rollback()));
+      group_ending= is_group_ending(events->ev, event_type);
       if (group_ending && likely(!rgi->worker_error))
       {
         DEBUG_SYNC(thd, "rpl_parallel_before_mark_start_commit");
@@ -674,8 +760,12 @@ handle_rpl_parallel_thread(void *arg)
         delete_or_keep_event_post_apply(rgi, event_type, events->ev);
         DBUG_EXECUTE_IF("rpl_parallel_simulate_temp_err_gtid_0_x_100",
                         err= dbug_simulate_tmp_error(rgi, thd););
-        if (err && has_temporary_error(thd))
-          err= retry_event_group(rgi, rpt, events);
+        if (err)
+        {
+          convert_kill_to_deadlock_error(rgi);
+          if (has_temporary_error(thd) && slave_trans_retries > 0)
+            err= retry_event_group(rgi, rpt, events);
+        }
       }
       else
       {
@@ -691,10 +781,14 @@ handle_rpl_parallel_thread(void *arg)
       events->next= qevs_to_free;
       qevs_to_free= events;
 
-      if (unlikely(err) && !rgi->worker_error)
+      if (unlikely(err))
       {
-        slave_output_error_info(rgi, thd);
-        signal_error_to_sql_driver_thread(thd, rgi, err);
+        if (!rgi->worker_error)
+        {
+          slave_output_error_info(rgi, thd);
+          signal_error_to_sql_driver_thread(thd, rgi, err);
+        }
+        thd->reset_killed();
       }
       if (end_of_group)
       {
@@ -1096,6 +1190,7 @@ rpl_parallel_thread::get_rgi(Relay_log_info *rli, Gtid_log_event *gtid_ev,
   rgi->relay_log= rli->last_inuse_relaylog;
   rgi->retry_start_offset= rli->future_event_relay_log_pos-event_size;
   rgi->retry_event_count= 0;
+  rgi->killed_for_retry= false;
 
   return rgi;
 }