[Haskell-cafe] Killing threads in foreign calls.

Jason Dusek jason.dusek at gmail.com
Sun Apr 17 21:31:11 CEST 2011

  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:


  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':


  This in turn calls a `safe' binding, `c_PQexec', to `PQexec'
  in the C library:


  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
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