Bug#4692 - DISABLE/ENABLE KEYS waste a space

Post-pushbuild fix

Added a purecov comment and a test for coverage of parallel
enable keys.

myisam/mi_check.c

@@ -1487,7 +1487,7 @@ static int mi_drop_all_indexes(MI_CHECK *param, MI_INFO *info, my_bool force)
   /* Remove all key blocks of this index file from key cache. */
   if ((error= flush_key_blocks(share->key_cache, share->kfile,
                                FLUSH_IGNORE_CHANGED)))
-    goto end;
+    goto end; /* purecov: inspected */
 
   /* Clear index root block pointers. */
   for (i= 0; i < share->base.keys; i++)

mysql-test/r/myisam.result

@@ -1830,5 +1830,13 @@ ALTER TABLE t1 ENABLE KEYS;
 SHOW TABLE STATUS LIKE 't1';
 Name	Engine	Version	Row_format	Rows	Avg_row_length	Data_length	Max_data_length	Index_length	Data_free	Auto_increment	Create_time	Update_time	Check_time	Collation	Checksum	Create_options	Comment
 t1	MyISAM	10	Fixed	1	#	#	#	3072	#	#	#	#	#	#	#		
+# Enable keys with parallel repair
+SET @@myisam_repair_threads=2;
+ALTER TABLE t1 DISABLE KEYS;
+ALTER TABLE t1 ENABLE KEYS;
+SET @@myisam_repair_threads=1;
+CHECK TABLE t1 EXTENDED;
+Table	Op	Msg_type	Msg_text
+test.t1	check	status	OK
 DROP TABLE t1;
 End of 5.0 tests

mysql-test/t/myisam.test

@@ -1185,6 +1185,12 @@ SHOW TABLE STATUS LIKE 't1';
 #--exec ls -log var/master-data/test/t1.MYI
 #--exec myisamchk -dvv var/master-data/test/t1.MYI
 #--exec myisamchk -iev var/master-data/test/t1.MYI
+--echo # Enable keys with parallel repair
+SET @@myisam_repair_threads=2;
+ALTER TABLE t1 DISABLE KEYS;
+ALTER TABLE t1 ENABLE KEYS;
+SET @@myisam_repair_threads=1;
+CHECK TABLE t1 EXTENDED;
 DROP TABLE t1;
 
 --echo End of 5.0 tests