Re: [PATCH v2] Allow trap to un-ignore SIGINT in asynchronous subshells

[Johannes Altmanninger <aclopte@xxxxxxxxx>]

On Fri, Mar 29, 2024 at 02:50:39PM +0100, Jilles Tjoelker wrote:
> On Fri, Mar 29, 2024 at 12:24:00PM +0100, Johannes Altmanninger wrote:
> > [*]: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html
> > [**]: https://lists.gnu.org/archive/html/bug-bash/2023-01/msg00050.html
> 
> There is definitely a good reason for S_HARD_IGN: it implements the
> following requirement of the trap builtin (XCU 2.14 Special Built-In
> Utilities -> trap):
> 
> ] Signals that were ignored on entry to a non-interactive shell cannot
> ] be trapped or reset, although no error need be reported when
> ] attempting to do so.

The wording in [*] is extremely confusing.  To me, "If job control
is disabled (see the description of set -m) when the shell executes
an asynchronous list, the commands in the list shall inherit from
the shell a signal action of ignored (SIG_IGN) for the SIGINT and
SIGQUIT signals." sounds like those signals are ignored on entry,
so it sounds like "cannot be trapped or reset" applies.

I guess we are dealing with two different sets of inherited signal
handlers, the specific SIGINT/SIGQUIT for asynchronous subshells,
and the others that were inherited normally.

> ] An interactive shell may reset or catch signals ] ignored on entry.
> 
> The change that should be made is that the automatic ignore of SIGINT
> and SIGQUIT in background subshells should not use hard ignore.
> Hard ignore still applies if the shell inherits ignored signals
> from its parent.

Oh, I had thought, ignoresig(SIGINT) and ignoresig(SIGQUIT) on
background subshell entry were our only uses of S_HARD_IGN.
So this means that this logic from setsignal()

	if (act.sa_handler == SIG_IGN) {
		if (mflag && (signo == SIGTSTP ||
		     signo == SIGTTIN || signo == SIGTTOU)) {
			tsig = S_IGN;	/* don't hard ignore these */
		} else
			tsig = S_HARD_IGN;
	}

interprets all inherited-as-ignored signals as being hard-ignored,
but that doesn't include SIGINT/SIGQUIT because they were inherited
in a different way, via ignoresig()

Will fix.