bug in handling of ignored traps

[Eric Blake <eblake@redhat.com>]

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Per POSIX requirements on trap, dash is properly refusing to let
non-interactive scripts reset signal handlers via trap if the shell was
started with an inherited ignored signal handler.  However, dash lies to
the user, making it impossible to tell if the user is invoking a shell
in an environment where a signal was inherited as ignored.  ksh and bash
are nicer about things, and at least let the user query whether a shell
is treating a particular signal as non-resettable.

This is important for shell scripts that WANT to guarantee a particular
behavior of SIGPIPE handling (such as this thread on writing a grep test
for covering the behavior of grep both with and without SIGPIPE
inherited as ignored:
https://lists.gnu.org/archive/html/bug-grep/2012-02/msg00016.html).

Bash behavior - you can learn about the ignored handler from the get-go:

$ (trap '' PIPE; bash -c 'trap; echo 0; trap "echo 1" PIPE; trap; \
     echo 2; kill -s PIPE $$; trap; echo 3')
trap -- '' SIGPIPE
0
trap -- '' SIGPIPE
2
trap -- '' SIGPIPE
3

Ksh behavior - you can learn about the ignored handler, but only after
trying to use something else:

$ (trap '' PIPE; ksh -c 'trap; echo 0; trap "echo 1" PIPE; trap; \
     echo 2; kill -s PIPE $$; trap; echo 3')
0
trap -- '' PIPE
2
trap -- '' PIPE
3

dash behavior - the trap is properly ignored, but you can't learn about
it (that is, trap lies and tells you that the handler was changed):

$ (trap '' PIPE; dash -c 'trap; echo 0; trap "echo 1" PIPE; trap; \
     echo 2; kill -s PIPE $$; trap; echo 3')
0
trap -- 'echo 1' PIPE
2
trap -- 'echo 1' PIPE
3

and proof that when the inheritance issue is not in play, the handler works:

$ (trap - PIPE; dash -c 'trap; echo 0; trap "echo 1" PIPE; trap; \
     echo 2; kill -s PIPE $$; trap; echo 3')
0
trap -- 'echo 1' PIPE
2
1
trap -- 'echo 1' PIPE
3

-- 
Eric Blake   eblake@redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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