Re: weird interaction between builtin cat and trap

[Peng Yu]

On 4/14/22, Chet Ramey <chet.ramey@case.edu> wrote:
> On 4/14/22 11:14 AM, Jesse Hathaway wrote:
>> On Thu, Apr 14, 2022 at 9:37 AM Peng Yu <pengyu.ut@gmail.com> wrote:
>>> This code is the even simpler to reproduce the error.
>>>
>>> $ cat ./main.sh
>>> #!/usr/bin/env bash
>>> # vim: set noexpandtab tabstop=2:
>>>
>>> enable -f "$BASH_LOADABLES_PATH"/cat cat
>>> trap 'echo EXIT' EXIT
>>> seq 1000000 | builtin cat | {
>>>          :
>>> } {fd}<&0
>>> $  ./main.sh
>>> cat: write error: Broken pipe
>>> EXIT
>>
>> I think this is a difference in implementation, rather than a bug.
>> The builtin cat writes any errors to stdout and exits 1, whereas
>> cat from coreutils sets its exit code to 141 on receiving a
>> Broken Pipe(32), but does not write anything to stderr.
>
> More or less. The trap has something to do with it, too.
>
> The builtin cat catches and reports write errors before it checks for fatal
> signals, including SIGPIPE. The coreutils cat just dies due to SIGPIPE
> (exit status 141).
>
> The trap EXIT causes bash to install a signal handler for the fatal
> signals, including SIGPIPE. (All it does is clean up, run the exit trap,
> and resend the signal to itself. And in this case the child running
> cat doesn't run the exit trap, so it's just a passthrough.)
>
> The coreutils cat doesn't do that. It may or may not check for write
> errors, but it certainly doesn't catch SIGPIPE. It terminates before it
> even has a chance to check for write errors.

Shouldn't you make the builtin cat behave the same as the external
cat? I'd expect the built-in command to be a drop-in replacement of
the corresponding external command, as using builtin is to speed up
the bash program but not to change its behavior.

https://simplicable.com/new/bug-vs-feature
- Bug: Something that can be reasonably viewed as a problem based on
current requirements....
- Feature: Something that hasn't been requested in current requirements....

According to the above definition of bug vs feature, it should be
considered as a bug meaning the behavior is not consistent with the
corresponding external program.

-- 
Regards,
Peng