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                              WORKLOG TASK
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TASK...........: table lock wait timeouts
CREATION DATE..: Wed, 13 Apr 2011, 17:09
SUPERVISOR.....: 
IMPLEMENTOR....: 
COPIES TO......: 
CATEGORY.......: Server-RawIdeaBin
TASK ID........: 197 (http://askmonty.org/worklog/?tid=197)
VERSION........: WorkLog-4.0
STATUS.........: Un-Assigned
PRIORITY.......: 60
WORKED HOURS...: 0
ESTIMATE.......: 0 (hours remain)
ORIG. ESTIMATE.: 0

PROGRESS NOTES:



DESCRIPTION:

MySQL has table_lock_wait_timeout but that doesn't do anything. For some apps it 
would help to timeout sessions blocked on a table lock. For example, when ALTER 
TABLE runs it implicitly gets an X lock on the table cache entry. Queries that 
follow are blocked until it finishes. The client-side read timeout might expire 
and disconnect the client but the server-side thread will continue blocking and 
the server can fill with blocked sessions.

Right now the stack when the client blocks is open_tables -> open_table -> 
wait_for_condition. I implemented a demo for this by:
1) use pthread_cond_timedwait in wait_for_condition
2) check for timeout expiration in open_tables and error out

Without the change in 2) the session will loop within open_tables on calls to 
open_table.


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