Is it an error to call hGetLine on a non-blocking Handle?

Bryan O'Sullivan bos at
Tue May 6 16:39:19 UTC 2014

On Mon, May 5, 2014 at 10:04 PM, Alain O'Dea <alain.odea at> wrote:

> I'm trying to get to the bottom of a very tenacious bug in the GHC
> Builder.
> The details of how this happens are documented separately for reference:
> The Handles involved are returned by
> System.Process.runInteractiveProcess.  They are non-blocking at the
> Illumos kernel level.
> It appears that maybeFillReadBuffer (called by hGetLine under the
> hood) treats EAGAIN as an error:
> Is it expected that runInteractiveProcess would return non-blocking
> Handles for stdout and stderr?

The non-blockingness should be hidden.

Your attempt to follow EAGAIN down the programmatic rabbit hole hasn't
gotten you nearly far enough, I'm afraid. You stopped at a layer where the
underlying non-blocking nature of the IO shouldn't be visible, so it would
be inappropriate to check for EAGAIN there. As you'll notice if you follow
the breadcrumbs further, it becomes quite difficult to spelunk through the
various vtables to see what's really going on.

I *think* that you should be looking at GHC.IO.FD.readRawBufferPtr, as
that's where the IO is ultimately happening. The trouble is, it
calls throwErrnoIfMinus1RetryMayBlock, which should already be doing the
right thing with EAGAIN. Sadly, I don't know what's going wrong here, but
I'm sure you will need to go down a level or two of abstraction from where
you are now before you'll find the source of the problem.
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