[PATCH 1 of 2 STABLE] worker: be silent if killed by the main process

[Jun Wu]

# HG changeset patch
# User Jun Wu <quark at fb.com>
# Date 1492704007 25200
#      Thu Apr 20 09:00:07 2017 -0700
# Node ID f0bd43ca0d5659a1e49bf4948a9d3d0f30a69e34
# Parent  59afb0750aecaff6c2b2e4edaab04eb91eca8a88
# Available At https://bitbucket.org/quark-zju/hg-draft
#              hg pull https://bitbucket.org/quark-zju/hg-draft -r f0bd43ca0d56
worker: be silent if killed by the main process

Previously, worker.py will send SIGTERM to remaining workers once a failure
of another worker is noticed. Workers receiving SIGTERM will go through
scmutil.callcatch, and may have extra outputs like "killed!" and a
traceback, which breaks test-worker.t.

Besides, it's hard to predicate how many "killed!" or traceback should be
printed if we keep that behavior.

Since the main process will notice a worker failure and handle it
accordingly, it makes sense to kill the remaining workers silently.
Therefore this patch adds a backdoor signal - SIGUSR2 to allow a silent
SIGTERM kill.

This makes test-worker.t less flaky. The next patch will make test-worker.t
stronger (and easier to break without this patch).

diff --git a/mercurial/worker.py b/mercurial/worker.py
--- a/mercurial/worker.py
+++ b/mercurial/worker.py
@@ -100,4 +100,6 @@ def _posixworker(ui, func, staticargs, a
         for p in pids:
             try:
+                # SIGUSR2 makes a worker silent for the upcoming SIGTERM
+                os.kill(p, signal.SIGUSR2)
                 os.kill(p, signal.SIGTERM)
             except OSError as err:
@@ -135,4 +137,11 @@ def _posixworker(ui, func, staticargs, a
     oldchldhandler = signal.signal(signal.SIGCHLD, sigchldhandler)
     ui.flush()
+    # when the main process notices an error (non-0 exit code), remaining
+    # workers will be killed without causing noisy output ("killed!" and a
+    # traceback). this is done by a special SIGUSR2.
+    def silentsigterm(signum, frame):
+        # avoid scmutil.callcatch code path which could be noisy
+        signal.signal(signal.SIGTERM, signal.SIG_DFL)
+    oldusr2handler = signal.signal(signal.SIGUSR2, silentsigterm)
     for pargs in partition(args, workers):
         pid = os.fork()
@@ -167,4 +176,5 @@ def _posixworker(ui, func, staticargs, a
                 os._exit(ret & 255)
         pids.add(pid)
+    signal.signal(signal.SIGUSR2, oldusr2handler)
     os.close(wfd)
     fp = os.fdopen(rfd, pycompat.sysstr('rb'), 0)