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From: Denys Vlasenko <dvlasenk@redhat.com>
Newsgroups: gnu.bash.bug
Subject: Re: "wait" loses signals
Date: Fri, 21 Feb 2020 15:44:37 +0100
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Approved: bug-bash@gnu.org
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References: <750d460d-b8a4-4157-1488-9f4d9f973715@redhat.com> <35034c85-fd5a-5034-a2d5-e3903888069d@case.edu> <cf7adbec-e705-1071-34e1-50f8188f0edc@redhat.com> <d60f1dbc-990d-7291-e075-b67c07f61a86@case.edu> <00620c20-19ea-e71e-dc1b-926847901f82@redhat.com>
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Cc: Harald van Dijk <harald@gigawatt.nl>
To: chet.ramey@case.edu, bug-bash@gnu.org
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On 2/20/20 4:27 PM, Chet Ramey wrote:
> On 2/20/20 3:02 AM, Denys Vlasenko wrote:
>> On 2/19/20 9:30 PM, Chet Ramey wrote:
>>> On 2/19/20 5:29 AM, Denys Vlasenko wrote:
>>>> A bug report from Harald van Dijk:
>>>>
>>>> test2.sh:
>>>> trap 'kill $!; exit' TERM
>>>> { kill $$; exec sleep 9; } &
>>>> wait $!
>>>>
>>>> The above script ought exit quickly, and not leave a stray
>>>> "sleep" child:
>>>> (1) if "kill $$" signal is delivered before "wait",
>>>> then TERM trap will kill the child, and exit.
>>>
>>> This strikes me as a shaky assumption, dependent on when the shell receives
>>> the SIGTERM and when it runs traps.
>>
>> The undisputable fact is that after shell forks a child
>> to run the "{...} &" subshell, it will receive the SIGTERM signal.
>>
>> And since it has a trap for it, it should be run.
>>
>>> (There's nothing in POSIX that says
>>> when pending traps are processed. Bash runs them after commands.)
>>
>> Yes, and here we are "after command", specifically after "{...} &" command.
>> Since we got a trapped signal, we must run its trap.
> 
> Did you look at the scenario in my message?

What scenario?

As I said, there are just two possibilities:
signal is received before the point when shell checks for received
signals after "{...} &" command;
or signal is received after that point, and thus signal is
considered to be received "inside wait builtin".

In both cases, trap should be run.

> Keep in mind that you can't run the trap out of the signal handler.

Yes, running anything remotely complex out of signal handlers is
a bad idea: signals can arrive somewhere in the middle of stdio, or memory allocation,
or something similarly critical. Reentering one of those can deadlock.

Properly-written programs are careful to record
signal reception in a flag variable, or a pipe, etc,
then return from signal handler, and act on it later, not in a signal handler.