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

contrib/showstack.py

@@ -4,7 +4,7 b''
 """dump stack trace when receiving SIGQUIT (Ctrl-\) and SIGINFO (Ctrl-T on BSDs)
 """
 
-from __future__ import absolute_import
+from __future__ import absolute_import, print_function
 import signal
 import sys
 import traceback
@@ -14,8 +14,14 b' def sigshow(*args):'
     traceback.print_stack(args[1], limit=10, file=sys.stderr)
     sys.stderr.write("----\n")
 
+def sigexit(*args):
+    sigshow(*args)
+    print('alarm!')
+    sys.exit(1)
+
 def extsetup(ui):
     signal.signal(signal.SIGQUIT, sigshow)
+    signal.signal(signal.SIGALRM, sigexit)
     try:
         signal.signal(signal.SIGINFO, sigshow)
     except AttributeError: