Description:
When a backup or restore operation runs, the thread structure (THD *) is not removed from the list of processes. Thus, when the operation is complete the process list shows a zombie process.
This was discovered while diagnosing BUG#32970.
The zombie thread doesn't seem to cause any problems -- just incorrectly reports a process still running that isn't. Interestingly, it is killed (removed) on subsequent backup operations but they too won't die and leave a (NULL, NULL) in the state, info columns of SHOW PROCESSLIST.
How to repeat:
* Start the server.
* Start and connect 2 clients.
* In client 1, issue the statements:
SET GLOBAL debug="d,backup_debug:d,backup";
# Note: any valid breakpoint for backup will do
SELECT get_lock("locking_thread_added", 100);
* In client 2, issue the statement:
#Note: any database will do
BACKUP DATABASE bup_ddl_blocker TO 'b.bak';
* In client 1, issue the query:
# Show the processlist
SELECT * FROM INFORMATION_SCHEMA.PROCESSLIST \G
Notice the output for the backup command:
ID: 1
USER: root
HOST: localhost:50473
DB: test
COMMAND: Query
TIME: 14
STATE: debug_sync_point: locking_thread_added
INFO: backup database test to 'b`.bak'
# release the breakpoint
SELECT release_lock("locking_thread_added");
# Show the processlist - BACKUP command should not be there
SELECT * FROM INFORMATION_SCHEMA.PROCESSLIST \G
Notice the output:
ID: 1
USER: root
HOST: localhost:50473
DB: test
COMMAND: Sleep
TIME: 41
STATE: NULL
INFO: NULL
2 rows in set (0.01 sec)
Repeated calls to show the processlist from either client result in the same output.
Where did he go?
Suggested fix:
Examine the code dealing with the THD * structure for the backup operation and correct any abnormal or missing method calls or attribute state changes.
