bug#56025: 29.0.50; em-extpipe-test-2 times out on EMBA and Cygwin

[Jim Porter <jporterbugs@gmail.com>]

On 6/27/2022 2:19 PM, Ken Brown wrote:
> On 6/27/2022 3:18 PM, Jim Porter wrote:
>> I'd be very hesitant to do this, since as you mention above, this 
>> seems like a timing issue, and it's entirely possible that there are 
>> other, more widespread issues on Cygwin here. We'd also want to check 
>> the system that the process is actually running on; otherwise, 
>> remoting into a Cygwin system (via Tramp) would still exhibit the 
>> problem. I'll see if I can get a Cygwin environment up to test things 
>> out in the next week-ish.
> 
> OK, thanks.  Let me know if you need any help with that.

Ok, I've got Cygwin set up (though I'm just using the prebuilt Cygwin 
Emacs for now). I can confirm that the following hangs until I send 
another EOF via `C-c C-d':

   echo hi | rev

However, if I evaluate the following first, the above command works just 
fine:

   (add-to-list 'eshell-needs-pipe "rev")

Normally, Eshell starts each process using ptys to control them. 
However, the above Elisp code tells Eshell to use a pipe for "rev"[1]. 
I'm not totally clear on all the subtleties here, but it seems to me 
that it would make more sense for rev to use a pipe for its stdin and a 
pty for its stdout. That's not possible with subprocesses in Emacs 
though (as far as I know).

However, I don't think this fully answers things, since I also see 
inconsistent results if I run "echo hi | rev" a bunch of times. 
Sometimes it prints "ih" and then I need to hit `C-c C-d` once to stop 
it. Other times it doesn't print anything and I need to hit `C-c C-d' 
twice. There must be some kind of race condition, maybe in Emacs's 
src/process.c?

Hopefully this helps get us closer to a proper answer here though...

[1] Only when rev is being piped *to* in an Eshell pipeline.