[Haskell-cafe] Killing threads in foreign calls.

[Jason Dusek]

  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
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