Description
In some Linux environments and on FreeBSD, I've observed frequent hangs due to Swift Concurrency task cancellation terminating a process via the teardown sequence, while reading the output hangs forever. I expect the present issues on GitHub Actions Linux CI are due to this as well, though strangely I haven't been able to reproduce it on my local aarch64 Linux VM.
I'm not sure why this doesn't seem to occur on macOS and Windows, though a likely explanation is that Subprocess's Windows implementation doesn't use DispatchIO, and Dispatch is very buggy on non-Apple platforms.
I/O reading needs to be cancellable so that the library doesn't hang. This is quite readily reproducible with testCancelProcessVeryEarlyOnStressTest on FreeBSD; sleep will get zombied from the cancellation but the library will sit waiting for output forever if it managed to get into DispatchIO.read before the cancellation took effect.