MDEV-4206 : log all slow statements (do not use filters), if log_slow_filter is empty.

mysql-test/r/log_slow.result

@@ -56,5 +56,18 @@ insert_id	int(11)	NO		NULL
 server_id	int(10) unsigned	NO		NULL	
 sql_text	mediumtext	NO		NULL	
 flush slow logs;
+set long_query_time=0.1;
+set log_slow_filter='';
+set global slow_query_log=1;
+set global log_output='TABLE';
+select sleep(0.5);
+sleep(0.5)
+0
+select count(*) FROM mysql.slow_log;
+count(*)
+1
+truncate mysql.slow_log;
+set @@long_query_time=default;
+set global slow_query_log=default;
 set @@log_slow_filter=default;
 set @@log_slow_verbosity=default;

mysql-test/t/log_slow.test

@@ -42,7 +42,18 @@ show fields from mysql.slow_log;
 
 flush slow logs;
 
-# Reset used variables
+# MDEV-4206 (empty filter should be no filter)
+set long_query_time=0.1;
+set log_slow_filter='';
+set global slow_query_log=1;
+set global log_output='TABLE';
+select sleep(0.5);
+select count(*) FROM mysql.slow_log;
+truncate mysql.slow_log;
+
 
+# Reset used variables
+set @@long_query_time=default;
+set global slow_query_log=default;
 set @@log_slow_filter=default;
 set @@log_slow_verbosity=default;

sql/sql_parse.cc

@@ -1497,7 +1497,8 @@ void log_slow_statement(THD *thd)
 
   /* Follow the slow log filter configuration. */ 
   if (!thd->enable_slow_log ||
-      !(thd->variables.log_slow_filter & thd->query_plan_flags))
+      (thd->variables.log_slow_filter
+        && !(thd->variables.log_slow_filter & thd->query_plan_flags)))
     DBUG_VOID_RETURN; 
  
   if (((thd->server_status & SERVER_QUERY_WAS_SLOW) ||