Bug #50914 mysqlbinlog not handling drop of current default database

mysqlbinlog only prints "use $database" statements to its output stream
when the active default database changes between events. This will cause
"No Database Selected" error when dropping and recreating that database.

To fix the problem, we clear print_event_info->db when printing an event
of CREATE/DROP/ALTER database statements, so that the Query_log_event
after such statements will be printed with the use 'db' anyway except
transaction keywords.

mysql-test/r/mysqlbinlog.result

@@ -638,3 +638,23 @@ ROLLBACK /* added by mysqlbinlog */;
 /*!50003 SET COMPLETION_TYPE=@OLD_COMPLETION_TYPE*/;
 End of 5.0 tests
 End of 5.1 tests
+RESET MASTER;
+CREATE DATABASE test1;
+USE test1;
+CREATE TABLE t1(id int);
+DROP DATABASE test1;
+CREATE DATABASE test1;
+USE test1;
+CREATE TABLE t1(id int);
+DROP TABLE t1;
+DROP DATABASE test1;
+FLUSH LOGS;
+show binlog events in 'master-bin.000002' from <binlog_start>;
+Log_name	Pos	Event_type	Server_id	End_log_pos	Info
+master-bin.000002	#	Query	#	#	CREATE DATABASE test1
+master-bin.000002	#	Query	#	#	use `test1`; CREATE TABLE t1(id int)
+master-bin.000002	#	Query	#	#	DROP DATABASE test1
+master-bin.000002	#	Query	#	#	CREATE DATABASE test1
+master-bin.000002	#	Query	#	#	use `test1`; CREATE TABLE t1(id int)
+master-bin.000002	#	Query	#	#	use `test1`; DROP TABLE t1
+master-bin.000002	#	Query	#	#	DROP DATABASE test1

mysql-test/t/mysqlbinlog.test

@@ -474,3 +474,30 @@ diff_files $MYSQLTEST_VARDIR/tmp/mysqlbinlog.warn $MYSQLTEST_VARDIR/tmp/mysqlbin
 # Cleanup for this part of test
 remove_file $MYSQLTEST_VARDIR/tmp/mysqlbinlog.warn.empty;
 remove_file $MYSQLTEST_VARDIR/tmp/mysqlbinlog.warn;
+
+#
+# BUG#50914
+# This test verifies if the approach of the mysqlbinlog prints
+# "use $database" statements to its output stream will cause
+# "No Database Selected" error when dropping and recreating
+# that database.
+#
+RESET MASTER;
+CREATE DATABASE test1;
+USE test1;
+CREATE TABLE t1(id int);
+DROP DATABASE test1;
+CREATE DATABASE test1;
+USE test1;
+CREATE TABLE t1(id int);
+DROP TABLE t1;
+DROP DATABASE test1;
+let $master_binlog= query_get_value(SHOW MASTER STATUS, File, 1);
+FLUSH LOGS;
+
+let $MYSQLD_DATADIR= `SELECT @@datadir`;
+exec $MYSQL_BINLOG $MYSQLD_DATADIR/$master_binlog | $MYSQL test 2>&1;
+
+let $binlog_file= query_get_value(SHOW MASTER STATUS, File, 1);
+source include/show_binlog_events.inc;
+

sql/log_event.cc

@@ -2894,7 +2894,12 @@ void Query_log_event::print_query_header(IO_CACHE* file,
                 error_code);
   }
 
-  if (!(flags & LOG_EVENT_SUPPRESS_USE_F) && db)
+  if ((flags & LOG_EVENT_SUPPRESS_USE_F))
+  {
+    if (!is_trans_keyword())
+      print_event_info->db[0]= '\0';
+  }
+  else if (db)
   {
     different_db= memcmp(print_event_info->db, db, db_len + 1);
     if (different_db)