BUG#50364: FLUSH LOGS crashes the server (rpl.rpl_heartbeat_basic

fails in PB sporadically)
      
The IO thread can concurrently access the relay log IO_CACHE
while another thread is performing an FLUSH LOGS procedure.
      
FLUSH LOGS closes and reopens the relay log and while doing so it
(re)initializes its IO_CACHE. During this procedure the IO_CACHE
mutex is also reinitialized, which can cause problems if some
other thread (namely the IO THREAD) is concurrently accessing it
at the time .
      
This patch fixes the problem by extending the interface of the
flush_master_info function to also include a second paramater, 
"need_relay_log_lock", stating whether the thread should grab the 
relay log lock or not before actually flushing the relay log. 
Also, IO thread now calls flush_master_info with this flag set 
when it flushes master info with in the event read_event loop.

Finally, we also increase loop time in rpl_heartbeat_basic test 
case, so that the number of calls to flush logs doubles, stressing
this part of the code a little more.

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

@@ -382,7 +382,7 @@ let $slave_param_comparison= =;
 let $rcvd_heartbeats_before= query_get_value(SHOW STATUS LIKE 'slave_received_heartbeats', Value, 1);
 # Flush logs every 0.1 second during 5 sec
 --disable_query_log
-let $i=50;
+let $i=100;
 while ($i) {
   FLUSH LOGS;
   dec $i;

sql/repl_failsafe.cc

@@ -977,7 +977,7 @@ bool load_master_data(THD* thd)
           host was specified; there could have been a problem when replication
           started, which led to relay log's IO_CACHE to not be inited.
         */
-        if (flush_master_info(active_mi, 0))
+        if (flush_master_info(active_mi, FALSE, FALSE))
           sql_print_error("Failed to flush master info file");
       }
       mysql_free_result(master_status_res);

sql/rpl_mi.cc

@@ -387,7 +387,7 @@ file '%s')", fname);
   mi->rli.is_relay_log_recovery= FALSE;
   // now change cache READ -> WRITE - must do this before flush_master_info
   reinit_io_cache(&mi->file, WRITE_CACHE, 0L, 0, 1);
-  if ((error=test(flush_master_info(mi, 1))))
+  if ((error=test(flush_master_info(mi, TRUE, TRUE))))
     sql_print_error("Failed to flush master info file");
   pthread_mutex_unlock(&mi->data_lock);
   DBUG_RETURN(error);
@@ -413,7 +413,9 @@ file '%s')", fname);
      1 - flush master info failed
      0 - all ok
 */
-int flush_master_info(Master_info* mi, bool flush_relay_log_cache)
+int flush_master_info(Master_info* mi, 
+                      bool flush_relay_log_cache, 
+                      bool need_lock_relay_log)
 {
   IO_CACHE* file = &mi->file;
   char lbuf[22];
@@ -436,8 +438,19 @@ int flush_master_info(Master_info* mi, bool flush_relay_log_cache)
   */
   if (flush_relay_log_cache)
   {
+    pthread_mutex_t *log_lock= mi->rli.relay_log.get_log_lock();
     IO_CACHE *log_file= mi->rli.relay_log.get_log_file();
-    if (flush_io_cache(log_file))
+
+    if (need_lock_relay_log)
+      pthread_mutex_lock(log_lock);
+
+    safe_mutex_assert_owner(log_lock);
+    err= flush_io_cache(log_file);
+
+    if (need_lock_relay_log)
+      pthread_mutex_unlock(log_lock);
+
+    if (err)
       DBUG_RETURN(2);
   }
   

sql/rpl_mi.h

@@ -120,7 +120,9 @@ int init_master_info(Master_info* mi, const char* master_info_fname,
 		     bool abort_if_no_master_info_file,
 		     int thread_mask);
 void end_master_info(Master_info* mi);
-int flush_master_info(Master_info* mi, bool flush_relay_log_cache);
+int flush_master_info(Master_info* mi, 
+                      bool flush_relay_log_cache, 
+                      bool need_lock_relay_log);
 int change_master_server_id_cmp(ulong *id1, ulong *id2);
 
 #endif /* HAVE_REPLICATION */

sql/rpl_rli.cc

@@ -121,7 +121,7 @@ int init_relay_log_info(Relay_log_info* rli,
   /*
     The relay log will now be opened, as a SEQ_READ_APPEND IO_CACHE.
     Note that the I/O thread flushes it to disk after writing every
-    event, in flush_master_info(mi, 1).
+    event, in flush_master_info(mi, 1, ?).
   */
 
   /*

sql/slave.cc

@@ -1726,7 +1726,7 @@ static void write_ignored_events_info_to_relay_log(THD *thd, Master_info *mi)
                    " to the relay log, SHOW SLAVE STATUS may be"
                    " inaccurate");
       rli->relay_log.harvest_bytes_written(&rli->log_space_total);
-      if (flush_master_info(mi, 1))
+      if (flush_master_info(mi, TRUE, FALSE))
         sql_print_error("Failed to flush master info file");
       delete ev;
     }
@@ -3047,7 +3047,7 @@ Stopping slave I/O thread due to out-of-memory error from master");
         goto err;
       }
 
-      if (flush_master_info(mi, 1))
+      if (flush_master_info(mi, TRUE, TRUE))
       {
         sql_print_error("Failed to flush master info file");
         goto err;

sql/sql_repl.cc

@@ -1532,7 +1532,7 @@ bool change_master(THD* thd, Master_info* mi)
     Relay log's IO_CACHE may not be inited, if rli->inited==0 (server was never
     a slave before).
   */
-  if (flush_master_info(mi, 0))
+  if (flush_master_info(mi, FALSE, FALSE))
   {
     my_error(ER_RELAY_LOG_INIT, MYF(0), "Failed to flush master info file");
     ret= TRUE;