chgserver: auto exit after being idle for too long or lose the socket file

This is a part of the one server per config series. In multiple-server setup,
new server may be started for a temporary config change like in command line,
--config extensions.foo=bar.py. This may end up with a lot of not so useful
server processes. Other questions are about socket file and process management,
How to stop these processes? What if a new server wants to listen on a same
address, replacing the old one?

This patch introduces AutoExitMixIn, which will:

1. Exit after being idle for too long. So useless servers won't run forever.
2. Periodically check the ownership of socket file, exit if it is no longer
   owned. This brings strong consistency between the filesystem and the
   process, and handles some race conditions neatly.
   Since rename is atomic, a new server can just have a same server address
   with an old one and won't worry about how to make sure the old server is
   killed, address conflict, service downtime issues.
   The user can safely stop all servers by simply removing the socket files,
   without worrying about outdated or accidentally removed pidfiles.

hgext/chgserver.py

@@ -34,6 +34,8 b' import os'
 import re
 import signal
 import struct
+import threading
+import time
 import traceback
 
 from mercurial.i18n import _
@@ -380,20 +382,93 b' class _requesthandler(SocketServer.Strea'
             traceback.print_exc(file=sv.cerr)
             raise
 
+def _tempaddress(address):
+    return '%s.%d.tmp' % (address, os.getpid())
+
+class AutoExitMixIn:  # use old-style to comply with SocketServer design
+    lastactive = time.time()
+    idletimeout = 3600  # default 1 hour
+
+    def startautoexitthread(self):
+        # note: the auto-exit check here is cheap enough to not use a thread,
+        # be done in serve_forever. however SocketServer is hook-unfriendly,
+        # you simply cannot hook serve_forever without copying a lot of code.
+        # besides, serve_forever's docstring suggests using thread.
+        thread = threading.Thread(target=self._autoexitloop)
+        thread.daemon = True
+        thread.start()
+
+    def _autoexitloop(self, interval=1):
+        while True:
+            time.sleep(interval)
+            if not self.issocketowner():
+                _log('%s is not owned, exiting.\n' % self.server_address)
+                break
+            if time.time() - self.lastactive > self.idletimeout:
+                _log('being idle too long. exiting.\n')
+                break
+        self.shutdown()
+
+    def process_request(self, request, address):
+        self.lastactive = time.time()
+        return SocketServer.ForkingMixIn.process_request(
+            self, request, address)
+
+    def server_bind(self):
+        # use a unique temp address so we can stat the file and do ownership
+        # check later
+        tempaddress = _tempaddress(self.server_address)
+        self.socket.bind(tempaddress)
+        self._socketstat = os.stat(tempaddress)
+        # rename will replace the old socket file if exists atomically. the
+        # old server will detect ownership change and exit.
+        util.rename(tempaddress, self.server_address)
+
+    def issocketowner(self):
+        try:
+            stat = os.stat(self.server_address)
+            return (stat.st_ino == self._socketstat.st_ino and
+                    stat.st_mtime == self._socketstat.st_mtime)
+        except OSError:
+            return False
+
+    def unlinksocketfile(self):
+        if not self.issocketowner():
+            return
+        # it is possible to have a race condition here that we may
+        # remove another server's socket file. but that's okay
+        # since that server will detect and exit automatically and
+        # the client will start a new server on demand.
+        try:
+            os.unlink(self.server_address)
+        except OSError as exc:
+            if exc.errno != errno.ENOENT:
+                raise
+
 class chgunixservice(commandserver.unixservice):
     def init(self):
         # drop options set for "hg serve --cmdserver" command
         self.ui.setconfig('progress', 'assume-tty', None)
         signal.signal(signal.SIGHUP, self._reloadconfig)
-        class cls(SocketServer.ForkingMixIn, SocketServer.UnixStreamServer):
+        class cls(AutoExitMixIn, SocketServer.ForkingMixIn,
+                  SocketServer.UnixStreamServer):
             ui = self.ui
             repo = self.repo
         self.server = cls(self.address, _requesthandler)
+        self.server.idletimeout = self.ui.configint(
+            'chgserver', 'idletimeout', self.server.idletimeout)
+        self.server.startautoexitthread()
         # avoid writing "listening at" message to stdout before attachio
         # request, which calls setvbuf()
 
     def _reloadconfig(self, signum, frame):
         self.ui = self.server.ui = _renewui(self.ui)
 
+    def run(self):
+        try:
+            self.server.serve_forever()
+        finally:
+            self.server.unlinksocketfile()
+
 def uisetup(ui):
     commandserver._servicemap['chgunix'] = chgunixservice