I am building an application that uses Postgres for storage.
If a query runs too long, I would like to kill the querying
thread, releasing its lock on the connection; if the
connection is a in a bad state -- for example, busy -- I would
like to clean up the connection.
Unfortunately, killing calls in to libpq seems not to work. I
have put a minimal example on hpaste:
http://hpaste.org/45774/minimal_pg_contention_example
If you install libpq with Cabal, you can run it. In the
example, the main thread spawns a worker thread that queries
Postgres, running "SELECT pg_sleep(10);"; the main thread
waits half a second and then tries to kill the worker.
Unfortunately, the worker always manages to get as far as
printing "complete".
In the code, I call `Database.PQ.exec':
http://hackage.haskell.org/packages/archive/libpq/0.4.1/doc/html/src/Database-PQ.html#exec
This in turn calls a `safe' binding, `c_PQexec', to `PQexec'
in the C library:
http://hackage.haskell.org/packages/archive/libpq/0.4.1/doc/html/src/Database-PQ.html#line-2222
There are async interfaces, too; they do not seem to be any
more killable then the sync ones.
Maybe the problem is that you can't kill a thread while it's
in a foreign call? I do not see any documentation to this
effect; but I may have missed it.
--
Jason Dusek
() ascii ribbon campaign - against html e-mail
/\ www.asciiribbon.org - against proprietary attachments
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
