I agree with all the changes you made. I made one review comment.

It would be nice to add a test case to expose the problem. Correct me if I am wrong, but it doesn’t look like pty.spawn() is tested at all.

FWIW on Linux, reading from the master end seems to raise EIO if the slave has been closed. And writing to the master when the slave is closed seems to fill up a buffer and eventually blocks.

Ideally I think the best solution for handing exec() failure (Issue 17824) would be to eliminate fork-exec with posix_spawn(); see Issue 20104. But as you say, that’s a separate problem.