py3: suppress unraisable exceptions in test-worker.t

Python 3.8 calls sys.unraisablehook when an unraisable
exception is encountered. The default behavior is to print a
warning.

test-worker.t was triggering this hook due to a race between
a newly forked process exiting and that process's
_os.register_at_fork handlers running. I was seeing the
stdlib's random module in the stack re-seeding itself. Although
there could be other after-fork handlers in the mix.

This commit defines sys.unraisablehook to effectively no-op.
This suppresses the warning and makes test output on Python 3.8
consistent with prior versions. test-worker.t now passes on
Python 3.8.

Differential Revision: https://phab.mercurial-scm.org/D7949

tests/test-worker.t

@@ -1,130 +1,134
 Test UI worker interaction
 
   $ cat > t.py <<EOF
   > from __future__ import absolute_import, print_function
+  > import sys
   > import time
   > from mercurial import (
   >     error,
   >     registrar,
   >     ui as uimod,
   >     worker,
   > )
+  > sys.unraisablehook = lambda x: None
   > def abort(ui, args):
   >     if args[0] == 0:
   >         # by first worker for test stability
   >         raise error.Abort(b'known exception')
   >     return runme(ui, [])
   > def exc(ui, args):
   >     if args[0] == 0:
   >         # by first worker for test stability
   >         raise Exception('unknown exception')
   >     return runme(ui, [])
   > def runme(ui, args):
   >     for arg in args:
   >         ui.status(b'run\n')
   >         yield 1, arg
   >     time.sleep(0.1) # easier to trigger killworkers code path
   > functable = {
   >     b'abort': abort,
   >     b'exc': exc,
   >     b'runme': runme,
   > }
   > cmdtable = {}
   > command = registrar.command(cmdtable)
   > @command(b'test', [], b'hg test [COST] [FUNC]')
   > def t(ui, repo, cost=1.0, func=b'runme'):
   >     cost = float(cost)
   >     func = functable[func]
   >     ui.status(b'start\n')
   >     runs = worker.worker(ui, cost, func, (ui,), range(8))
   >     for n, i in runs:
   >         pass
   >     ui.status(b'done\n')
   > EOF
   $ abspath=`pwd`/t.py
   $ hg init
 
 Run tests with worker enable by forcing a heigh cost
 
   $ hg --config "extensions.t=$abspath" test 100000.0
   start
   run
   run
   run
   run
   run
   run
   run
   run
   done
 
 Run tests without worker by forcing a low cost
 
   $ hg --config "extensions.t=$abspath" test 0.0000001
   start
   run
   run
   run
   run
   run
   run
   run
   run
   done
 
 #if no-windows
 
 Known exception should be caught, but printed if --traceback is enabled
 
   $ hg --config "extensions.t=$abspath" --config 'worker.numcpus=8' \
   > test 100000.0 abort 2>&1
   start
   abort: known exception
   [255]
 
   $ hg --config "extensions.t=$abspath" --config 'worker.numcpus=8' \
   > test 100000.0 abort --traceback 2>&1 | egrep '(SystemExit|Abort)'
       raise error.Abort(b'known exception')
   mercurial.error.Abort: known exception (py3 !)
   Abort: known exception (no-py3 !)
   SystemExit: 255
 
 Traceback must be printed for unknown exceptions
 
   $ hg --config "extensions.t=$abspath" --config 'worker.numcpus=8' \
   > test 100000.0 exc 2>&1 | grep '^Exception'
   Exception: unknown exception
 
 Workers should not do cleanups in all cases
 
   $ cat > $TESTTMP/detectcleanup.py <<EOF
   > from __future__ import absolute_import
   > import atexit
   > import os
+  > import sys
   > import time
+  > sys.unraisablehook = lambda x: None
   > oldfork = os.fork
   > count = 0
   > parentpid = os.getpid()
   > def delayedfork():
   >     global count
   >     count += 1
   >     pid = oldfork()
   >     # make it easier to test SIGTERM hitting other workers when they have
   >     # not set up error handling yet.
   >     if count > 1 and pid == 0:
   >         time.sleep(0.1)
   >     return pid
   > os.fork = delayedfork
   > def cleanup():
   >     if os.getpid() != parentpid:
   >         os.write(1, 'should never happen\n')
   > atexit.register(cleanup)
   > EOF
 
   $ hg --config "extensions.t=$abspath" --config worker.numcpus=8 --config \
   > "extensions.d=$TESTTMP/detectcleanup.py" test 100000 abort
   start
   abort: known exception
   [255]
 
 #endif