procutil: always waiting on child processes to prevent zombies with 'hg serve' stable
authorRodrigo Damazio Bovendorp <rdamazio@google.com>
Thu, 07 May 2020 03:14:52 -0700
branchstable
changeset 44781 ed684a82e29b
parent 44780 f727939f3513
child 44782 3d5fb6cab832
procutil: always waiting on child processes to prevent zombies with 'hg serve' When runbgcommand is invoked by an extension with ensurestart=False, we never called waitpid - which is fine in most cases, except if that's happening on a command server (e.g. chg), in which case the child defunct process will just sit there for as long as the server is running. The actual semantics of SIGCHLD signal handling is a lot more complex than it seems, and the POSIX standard *seems* to read that it's ignored by default and everything would just work without the waitpid if we're not listening for it, but the truth is that it's only ignored if we *explicitly* set it to SIG_IGN. We further cannot set it to SIG_IGN or to a catch-all handler across all of 'hg serve', because Python's suprocess.Popen relies on that signal, and a few specific parts of hg also set custom handlers, so instead we wait for specific PIDs in dedicated threads. I did a poor-man's benchmark of the thread creation and it seems to take about 1ms, which is way better than the 20+ms from ensurestart=True. Differential Revision: https://phab.mercurial-scm.org/D8497
mercurial/utils/procutil.py
--- a/mercurial/utils/procutil.py	Thu May 07 15:00:33 2020 +0200
+++ b/mercurial/utils/procutil.py	Thu May 07 03:14:52 2020 -0700
@@ -16,6 +16,7 @@
 import signal
 import subprocess
 import sys
+import threading
 import time
 
 from ..i18n import _
@@ -604,6 +605,14 @@
             pid = os.fork()
             if pid:
                 if not ensurestart:
+                    # Even though we're not waiting on the child process,
+                    # we still must call waitpid() on it at some point so
+                    # it's not a zombie/defunct. This is especially relevant for
+                    # chg since the parent process won't die anytime soon.
+                    # We use a thread to make the overhead tiny.
+                    def _do_wait():
+                        os.waitpid(pid, 0)
+                    threading.Thread(target=_do_wait, daemon=True).start()
                     return
                 # Parent process
                 (_pid, status) = os.waitpid(pid, 0)