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

[Jim Porter <jporterbugs@gmail.com>]

On 7/16/2022 11:03 PM, Eli Zaretskii wrote:
> Could you please describe the main ideas of the changeset?  It is hard
> to be sure I understand what you are trying to do by reading the
> patch.

Sure. I mentioned it briefly earlier in the thread, but a more-complete 
summary would probably help.

Normally, Eshell connects programs in a pipeline like "foo | bar" by 
setting a process filter for "foo", and inside that filter, (eventually) 
calling `process-send-string' for "bar". In most shells, you'd expect 
that connection to be a pipe, but in Eshell, the processes are created 
with a PTY connection by default.

My patch adds support for `make-process' to use a PTY only for the child 
process's stdin or its stdout (in addition to the preexisting behaviors 
of PTY for both or neither). This then lets Eshell request a pipe for 
foo's stdout and bar's stdin, while using PTYs for foo's stdin and bar's 
stdout:

   Before:
     [pty 1] -> foo -> [pty 1] -> Eshell -> [pty 2] -> bar -> [pty 2]

   After:
     [pty 1] -> foo -> [pipe] -> Eshell -> [pipe] -> bar -> [pty 2]

This should make Eshell behave quite a bit more similarly to other 
shells, which will hopefully reduce the number of bugs like this one. 
This change also allowed me to remove the workaround for bug#1388. In 
that bug, there was an issue where this command didn't work[1]:

   *echo 1+1 | bc

Before the fix for bug#1388, "bc" would have seen that its stdin was a 
PTY, and then started an interactive session. Bug#1388 fixed this by 
adding `eshell-needs-pipe-p' to identify specific programs that need a 
pipe connection when being piped to like the above. With my patch here, 
that workaround won't be necessary anymore, since programs in a pipeline 
will be connected via pipes. (Note that technically, this pipe 
connection is indirect, since there's one pipe from foo to Emacs, and 
another pipe from Emacs to bar.)

This patch should hopefully fix the issues on Cygwin (as described in 
this bug) because, when using pipes to connect programs, the behavior 
should be more consistent across multiple platforms.

[1] I'm using "*echo" here to use /bin/echo so that it writes a newline. 
Eshell's built-in echo doesn't write a newline by default.