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Re: Bash 4.3 no longer handles traps when reading input with read


From: Chet Ramey
Subject: Re: Bash 4.3 no longer handles traps when reading input with read
Date: Sun, 13 Oct 2013 14:29:03 -0400
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.8; rv:24.0) Gecko/20100101 Thunderbird/24.0

On 10/11/13 6:19 PM, konsolebox wrote:
> On Fri, Oct 11, 2013 at 10:06 PM, Chet Ramey <address@hidden
> <mailto:address@hidden>> wrote:
> 
>     The SIGCHLD handler was particularly bad, since children can die and
>     interrupt
>     the shell at any time.  As an example, glibc uses internal locks
>     extensively, especially within its malloc implementation, and running trap
>     handlers when a signal arrives (or worse, longjmping back to saved state)
>     caused deadlocks.
> 
>     The bash signal and trap handling is now much closer to the Posix model:
>     minimal work done when the signal arrives, traps run when it is safe to
>     do so.  If the arriving signal causes certain system calls to return
>     -1/EINTR, for example if a SIGINT interrupts read(2), bash will notice and
>     check for pending traps.  SIGCHLD doesn't behave like that.
> 
> 
> Thanks for the reply.
> 
> I understand the possible effect of receiving multiple signals at once, but
> how are other signals different from it? If one would trap signals like
> SIGQUIT, SIGABRT, SIGINT, SIGHUP and SIGTERM for safe exitting of an
> interactive script e.g. during shutdown some of those signals could arrive
> quickly as well.

It's not a question of receiving multiple signals at once.  It's a question
of running too much in a signal handler.  Since Posix says you can only do
a certain small number of things in a signal handler, bash needs to do as
little as possible -- for both trapped signals and ones bash handles
internally -- and defer the real work until it's `safe'.  In the case of
traps, this is generally at command boundaries.

Bash-4.3 handles signals and traps when a read run by the read builtin is
interrupted by a  signal.  I'm not  sure that's the best thing to do,
since traps should really be run when commands finish, but we're going to
try it.

Since bash-4.2, bash has installed its SIGCHLD signal handler with
SA_RESTART, so a child death does not unexpectedly interrupt system calls.
There were several bug reports concerning this.  So yes, SIGCHLD is
special.

Chet
-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, ITS, CWRU    address@hidden    http://cnswww.cns.cwru.edu/~chet/



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