Fix Bug #20139 Infinite loop after "FLUSH" and "LOCK tabX, general_log"

Due to incorrect handling of FLUSH TABLES, log tables were marked for flush,
but not reopened. Later we started to wait for the log table to be closed
(disabled) after the flush. And as nobody disabled logs in concurrent treads,
the command lasted forever.
After internal consultations it was decided to skip logs during FLUSH TABLES.
The reasoning is that logging is done in the "log device", whatever it is
which is always active and controlled by FLUSH LOGS. So, to flush logs
one should use FLUSH LOGS, and not FLUSH TABLES.

mysql-test/r/log_tables.result

@@ -72,3 +72,8 @@ sleep(2)
 select * from mysql.slow_log;
 start_time	user_host	query_time	lock_time	rows_sent	rows_examined	db	last_insert_id	insert_id	server_id	sql_text
 TIMESTAMP	USER_HOST	QUERY_TIME	00:00:00	1	0	test	0	0	1	select sleep(2)
+flush tables with read lock;
+unlock tables;
+use mysql;
+lock tables general_log read local, help_category read local;
+unlock tables;

mysql-test/t/log_tables.test

@@ -171,6 +171,16 @@ select sleep(2);
 --replace_column 1 TIMESTAMP 2 USER_HOST 3 QUERY_TIME
 select * from mysql.slow_log;
 
+#
+# Bug #20139 Infinite loop after "FLUSH" and "LOCK tabX, general_log"
+#
+
+flush tables with read lock;
+unlock tables;
+use mysql;
+lock tables general_log read local, help_category read local;
+unlock tables;
+
 # kill all connections
 disconnect con1;
 disconnect con2;

sql/sql_base.cc

@@ -1997,17 +1997,17 @@ TABLE *open_table(THD *thd, TABLE_LIST *table_list, MEM_ROOT *mem_root,
        table= (TABLE*) hash_next(&open_cache, (byte*) key, key_length,
                                  &state))
   {
-    if (table->s->version != refresh_version)
+    /*
+      Here we flush tables marked for flush. However we never flush log
+      tables here. They are flushed only on FLUSH LOGS.
+    */
+    if (table->s->version != refresh_version && !table->s->log_table)
     {
       DBUG_PRINT("note",
                  ("Found table '%s.%s' with different refresh version",
                   table_list->db, table_list->table_name));
 
-      /*
-        Don't close tables if we are working with a log table or were
-        asked not to close the table explicitly
-      */
-      if (flags & MYSQL_LOCK_IGNORE_FLUSH || table->s->log_table)
+      if (flags & MYSQL_LOCK_IGNORE_FLUSH)
       {
         /* Force close at once after usage */
         thd->version= table->s->version;
@@ -2346,7 +2346,11 @@ void close_old_data_files(THD *thd, TABLE *table, bool abort_locks,
 
   for (; table ; table=table->next)
   {
-    if (table->s->version != refresh_version)
+    /*
+      Reopen marked for flush. But close log tables. They are flushed only
+      explicitly on FLUSH LOGS
+    */
+    if (table->s->version != refresh_version && !table->s->log_table)
     {
       found=1;
       if (table->db_stat)