showstack: also handle SIGALRM
This is looking *very* handy when debugging mysterious hangs in a
test: you can wrap a hanging invocation in
`perl -e 'alarm shift @ARGV; exec @ARGV' 1`
for example, a hanging `hg pull` becomes
`perl -e 'alarm shift @ARGV; exec @ARGV' 1 hg pull`
where the `1` is the timeout in seconds before the process will be hit
with SIGALRM. After making that edit to the test file, you can then
use --extra-config-opt on run-tests.py to globaly enable showstack
during the test run, so you'll get full stack traces as you force your
hg to exit.
I wonder (but only a little, not enough to take action just yet) if we
should wire up some scaffolding in run-tests itself to automatically
wrap all commands in alarm(3) somehow to avoid hangs in the future?
Differential Revision: https://phab.mercurial-scm.org/D4870
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