Switch to the new operations for obtaining OS handles

[jeltsch]

There is an effort to move some GHC-specific modules out of base. Candidates for such moving are GHC.IO.Handle.Types and GHC.IO.Handle.Internals. These modules are often used for obtaining operating-system handles (file descriptors, Windows handles) from Haskell handles. Such uses are also present in the process package.

There is a proposal to add dedicated operations for operating-system handle acquisition to base. A corresponding draft implementation can be found in GHC merge request !14732. The present pull request for process changes the package to use those new operations.

Note that this pull request changes the behavior of System.Process.Common.mbFd in two ways:

1. The given handle is released between setting it to “blocking” and acquiring its file descriptor. It should be checked whether this may cause problems when another thread concurrently accesses the handle.
2. In the case that the given handle is a duplex handle, both ends are set to “blocking” instead of just the reading end, as it was before. This was easier to implement based on the withHandle family of operations from GHC.IO.Handle.Internals but may not be what is desired.

[jeltsch]

Note that the tests necessarily fail with currently released GHC versions, as they require the new module System.IO.OS, which is added by GHC ticket !14732. This is also the reason why I gave this pull request draft status.

[jeltsch]

I removed the draft status of this pull request for it not to get lost.

[tomjaguarpaw]

Thank you very much for this contribution. process seems to be undermaintained at the moment. I'm a technically a maintainer, but I see myself as a "stop gap" or "emergency" maintainer, since I don't have the expertise necessary to make meaningful contributions to this library.

I will try to clarify exactly who the maintainers of process are, and see if I can find someone with the knowledge to review your PR.

Perhaps @jeltsch, you'd like to become a maintainer of process?

[jeltsch]

Thanks a lot for moving this forward.

As for myself, I don’t see myself as a future maintainer of process, I’m afraid.

[tomjaguarpaw]

@bgamari, do you have the capacity to take on the review of this?