GNU bug report logs - #79079
31.0.50; Piped command output is sometimes lost in Eshell

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From: Jim Porter <jporterbugs <at> gmail.com>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: mail <at> daniel-mendler.de, eggert <at> cs.ucla.edu, 79079 <at> debbugs.gnu.org
Subject: bug#79079: 31.0.50; Piped command output is sometimes lost in Eshell
Date: Sat, 26 Jul 2025 14:12:59 -0700

[Message part 1 (text/plain, inline)]

On 7/26/2025 10:23 AM, Jim Porter wrote:
> On 7/26/2025 3:15 AM, Eli Zaretskii wrote:
>>> Date: Thu, 24 Jul 2025 08:52:16 -0700
>>> Cc: mail <at> daniel-mendler.de, 79079 <at> debbugs.gnu.org
>>> From: Jim Porter <jporterbugs <at> gmail.com>
>>>
>> So calling the filter of the "head"s process when we get EPIPE should
>> fix that, no?
> 
> It would probably fix this particular case, but I'm not sure it's the 
> correct way to do it in general. I'll construct some test programs to 
> test out some of the corner cases here so that we can be sure whatever 
> implementation we do settle on works correctly in all cases.

After examining this more closely, there are three problems with the 
current Emacs code, of varying importance:

1. When we receive an EPIPE, we don't call our process-filter one last 
time (this is the original bug report). You can see this by running the 
following Eshell command repeatedly until it produces no output:

  git log | head -n 10 | cat

2. When we receive an EPIPE, we set the exit status to 256; that's not 
right, since the exit status should be 0. You can see this by running 
the following Eshell command until the prompt shows a non-zero status:

  git log | head -n 10 > #<my-buffer>

3. If the child process closes stdin and wants to keep doing some work 
(like write to stdout), it can't, since we called 'deactivate_process'. 
You can see this with the attached script by running (it's supposed to 
print the first line of "git log"):

  git log | ./test-script.sh

I've also attached two WIP patches for the different implementations. 
"bad-call-filter.diff" uses 'read_process_output' to call our process 
filter. It only solves issue #1 above. "good-close-pipe.diff" (which I 
posted previously) solves all three issues. While the latter is a 
slightly bigger change and requires some more testing on my end to make 
sure I've covered 100% of the cases, I think it's the more-correct 
solution. If we make the bigger change, it's probably worth calling it 
out in NEWS and on emacs-devel so that people can be on the lookout for 
any edge cases I haven't identified.

[bad-call-filter.diff (text/plain, attachment)]

[good-close-pipe.diff (text/plain, attachment)]

[test-script.sh (text/plain, attachment)]

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