streamingProcess does not work with non-threaded runtime

[nh2]

In the non-threaded runtime, the waitForProcess (waitpid()) call used in streamingProcess blocks the entire Haskell process, thus making the program block forever if the child process needs to be fed input by Haskell in order to terminate.

This is because waitpid() is not Fd-based and thus cannot use the non-blocking IO of the IO manager; see the waitForProcess docs here:

GHC Note: in order to call waitForProcess without blocking all the other threads in the system, you must compile the program with -threaded.

Detail repro with explanation: https://github.com/nh2/sourceProcessWithStreams-nonthreaded-problem

I use the nonthreaded runtime a lot, and I also use streamingProcess and conduits on top of it, so I'd like those to work with each other.

[nh2]

I have a proposed solution in the fix branch of my repro:

nh2/sourceProcessWithStreams-nonthreaded-problem@3709139...fix

The solution turns out to be a 3-line change, but I need some advice of backward compatibility in that package:

I'm currently using createPipeFd which according to https://www.stackage.org/lts-6.17/package/process-1.2.3.0 was added in 1.4.2.0 January 2016.

So I'm not sure I can use that function.

An alternative would be to use createPipe that works on Handle and then handleToFd, but I don't know if this function works on Windows.

I'll create a tentative PR using createPipe to see what the AppVeyor Windows CI thinks about it.