[Haskell-cafe] Killing threads in foreign calls.

[Jason Dusek]

On Sun, Apr 17, 2011 at 20:26, Edward Z. Yang <ezyang at mit.edu> wrote:
> This is a fairly nontrivial problem.  First off, let me tell you
> what you do not /actually/ want to happen: you don't want the OS
> level thread performing the foreign call to actually be killed...

  From this I gather, one can not generally kill Haskell threads
  while they are in the midst of foreign calls. I guess
  interrupting execution to terminate the program is special
  since you don't expect anything to work properly afterward.

> If the asynchronous API has the ability to cancel a query given
> some handler, you instead want to set up a custom kill thread function
> that checks if a thread has an active query and then performs
> another FFI call to perform that cancellation.

  It turns out the PGcancel exists for this purpose. Is it safe
  and reasonable to make query cancellation a ThreadKilled
  handler in the query thread?

  I gather I need to write the busy loop for polling for data in
  Haskell. Although libpq has a procedure -- PGgetResult -- that
  polls for data, it would not respond to killThread.

--
Jason Dusek
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