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

[Alain O'Dea]

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Hi folks:

I could really use an expert eye on this.

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:
https://gist.github.com/AlainODea/dc841046524cd7833da8

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:

http://hackage.haskell.org/package/base-4.7.0.0/docs/src/GHC-IO-Handle-Text.html#maybeFillReadBuffer

Is it expected that runInteractiveProcess would return non-blocking
Handles for stdout and stderr?

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

Thanks,
Alain
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