Bug #32970 Locking thread not added to process list properly

Description:
The backup locking thread in be_thread.cc is not added correctly to the list of processes when the command SELECT state, info FROM INFORMATION_SCHEMA.PROCESSLIST; is issued. In fact, the fill_schema_processlist() method in sql_show.cc @1839 will fail randomly with the following (or similar) error:

master.err: main.backup_ddl_blocker: safe_mutex: Trying to lock unitialized mutex at sql_show.cc, line 1839

The cause is a race condition between the locking thread startup and the show method. If the show method accesses the thread before it has completed its setup, the above error will result from the show command trying to lock the uninitialized mutex.

How to repeat:
Replace this code in create_new_thd() in be_thread.cc and run the backup_ddl_blocker test multiple times. It will eventually fail on most Linux machines (but not on Windows -- see my_thr_init() for details).

===== be_thread.cc 1.2 vs edited =====
--- 1.2/sql/backup/be_thread.cc	2007-11-28 22:13:08 -05:00
+++ edited/be_thread.cc	2007-12-04 11:14:40 -05:00
@@ -73,13 +73,6 @@
   thd->main_security_ctx.master_access= ~0;
   thd->main_security_ctx.priv_user= 0;
   thd->real_id= pthread_self();
-  /*
-    Making this thread visible to SHOW PROCESSLIST is useful for
-    troubleshooting a backup job (why does it stall etc).
-  */
-  pthread_mutex_lock(&LOCK_thread_count);
-  threads.append(thd);
-  pthread_mutex_unlock(&LOCK_thread_count);
   lex_start(thd);
   mysql_reset_thd_for_next_command(thd);
   DBUG_RETURN(thd);

Suggested fix:
Examine the locking thread setup and create methods to ensure the proper sequence is followed and that the thread is added to the global thread list with the proper attributes assigned with valid values.

Original code was correct but required additional code to allow thread to show in processlist. Patch forthcoming.

Patch ready for review. See http://lists.mysql.com/commits/40163.

Problem found in patch corrected. See new patch http://lists.mysql.com/commits/40212 .

Good to push.

Patch queued.

Pushed into 6.0.5-alpha

Noted in 6.0.5 changelog.

The Online Backup locking thread did not show up in the
server's process list information due to a race condition.

Correction: No changelog entry needed; this bug did not appear in any released version.