Add createPipeOpen

[ch1bo]

This allows, for example, to read a sub-process' stderr after the process has exited.

[snoyberg]

It's surprising that this is only in the internal module, does this not need to be exported elsewhere?

It would be great to have a test case to cover this, plus a minor version bump and changelog entry.

[ch1bo]

TBH I have not tried to use it through the package yet, I had this helper in my code base. Will make amends as you suggested!

[ch1bo]

I'm not sure why it did not work with createPipe originally. When trying to write tests for this:

    it "createPipe can not be read after process exit" $ do
        withProcessTerm_ (setStderr createPipe $ proc "sh" ["-c", "echo hello >&2"]) $ \p -> do
            let h = getStderr p
            waitExitCode p `shouldReturn` ExitSuccess
            S.hGetContents h `shouldReturn` "hello\n"

    it "createPipeOpen can be read after process exit" $ do
        withProcessTerm_ (setStderr createPipeOpen $ proc "sh" ["-c", "echo hello >&2"]) $ \p ->
          bracket (pure $ getStderr p) hClose $ \h -> do
            waitExitCode p `shouldReturn` ExitSuccess
            S.hGetContents h `shouldReturn` "hello\n"

Both were succeeding (I was expecting a EOF exception or so on the createPipe one).

I followed up by rewriting my workaround (using System.Process.createPipe and useHandleOpen) in terms of createPipe and it seems to do the trick for us there too.

So closing this as seemingly not needed.