[3.10] bpo-26228: Fix pty EOF handling (GH-12049)

[miss-islington]

On non-Linux POSIX platforms, like FreeBSD or macOS,
the FD used to read a forked PTY may signal its exit not
by raising an error but by sending empty data to the read
syscall. This case wasn't handled, leading to hanging
pty.spawn calls.

Co-authored-by: Reilly Tucker Siemens reilly@tuckersiemens.com
Co-authored-by: Łukasz Langa lukasz@langa.pl
(cherry picked from commit 81ab8db)

Co-authored-by: Zephyr Shannon geoffpshannon@gmail.com

https://bugs.python.org/issue26228

[miss-islington]

@RadicalZephyr and @ambv: Status check is done, and it's a success ✅ .

[miss-islington]

@RadicalZephyr and @ambv: Status check is done, and it's a success ✅ .

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @ambv for commit 1f7fb15 🤖

If you want to schedule another build, you need to add the ":hammer: test-with-buildbots" label again.

[miss-islington]

@RadicalZephyr and @ambv: Status check is done, and it's a pending ❌ .

[miss-islington]

@RadicalZephyr and @ambv: Status check is done, and it's a pending ❌ .

[miss-islington]

@RadicalZephyr and @ambv: Status check is done, and it's a pending ❌ .

[miss-islington]

@RadicalZephyr and @ambv: Status check is done, and it's a pending ❌ .

[miss-islington]

@RadicalZephyr and @ambv: Status check is done, and it's a pending ❌ .

[miss-islington]

@RadicalZephyr and @ambv: Status check is done, and it's a pending ❌ .

[miss-islington]

@RadicalZephyr and @ambv: Status check is done, and it's a pending ❌ .

[miss-islington]

@RadicalZephyr and @ambv: Status check is done, and it's a pending ❌ .

[miss-islington]

@RadicalZephyr and @ambv: Status check is done, and it's a pending ❌ .

[miss-islington]

@RadicalZephyr and @ambv: Status check is done, and it's a pending ❌ .

[miss-islington]

@RadicalZephyr and @ambv: Status check is done, and it's a pending ❌ .

[miss-islington]

@RadicalZephyr and @ambv: Status check is done, and it's a pending ❌ .

[miss-islington]

@RadicalZephyr and @ambv: Status check is done, and it's a pending ❌ .

[miss-islington]

@RadicalZephyr and @ambv: Status check is done, and it's a pending ❌ .

[miss-islington]

@RadicalZephyr and @ambv: Status check is done, and it's a pending ❌ .

[miss-islington]

@RadicalZephyr and @ambv: Status check is done, and it's a pending ❌ .

[ambv]

@pablogsal, this is a bugfix we should have done a long time ago. If you feel messing with PTY at RC stage is too risky, we can merge this post-3.10.0. Both here, and on the original PR, I ran with test-with-buildbots and confirmed all's good manually on operating systems I have access to (Win10, Ubuntu, and macOS 10.15).

This backport has an outstanding buildbot/AMD64 FreeBSD Non-Debug PR job where the worker got disconnected. It would have been a valuable buildbot job, given BSD and non-pydebug, but it seems the worker's got trouble relatively often: https://buildbot.python.org/all/#/builders/387

In any case, we have to have this PR in at some point as it's one of the changes that will allow running -R: -uall on macOS.

[ambv]

This should say "macOS" but I have a follow-up PR for doc updates anyway so I'll fix that there.

[pablogsal]

I reviewed the change and I think is safe enough. Also we have another RC so we can still test it to some degree

[miss-islington]

@RadicalZephyr and @ambv: Status check is done, and it's a pending ❌ .

[miss-islington]

@RadicalZephyr and @ambv: Status check is done, and it's a success ✅ .

[miss-islington]

@RadicalZephyr and @ambv: Status check is done, and it's a pending ❌ .

[miss-islington]

@RadicalZephyr and @ambv: Status check is done, and it's a success ✅ .