[FIX] server: support for systemd socket activation in threaded server

This patches fixes the untested and broken draft of inetd and systemd
activation support in the threaded server.

This patch also introduces a change of behaviour in the `_reexec()`
function by keeping the environment when Odoo is respawning during the
following events:

    - SIGHUP signal is received
    - one click install has been triggered
    - code reload needed when using `--dev=reload`

odoo/service/server.py

@@ -92,9 +92,6 @@ class RequestHandler(werkzeug.serving.WSGIRequestHandler):
         me = threading.currentThread()
         me.name = 'odoo.service.http.request.%s' % (me.ident,)
 
-# _reexec() should set LISTEN_* to avoid connection refused during reload time. It
-# should also work with systemd socket activation. This is currently untested
-# and not yet used.
 
 class ThreadedWSGIServerReloadable(LoggingBaseWSGIServerMixIn, werkzeug.serving.ThreadedWSGIServer):
     """ werkzeug Threaded WSGI Server patched to allow reusing a listen socket
@@ -106,11 +103,10 @@ class ThreadedWSGIServerReloadable(LoggingBaseWSGIServerMixIn, werkzeug.serving.
                                                            handler=RequestHandler)
 
     def server_bind(self):
-        envfd = os.environ.get('LISTEN_FDS')
-        if envfd and os.environ.get('LISTEN_PID') == str(os.getpid()):
+        SD_LISTEN_FDS_START = 3
+        if os.environ.get('LISTEN_FDS') == '1' and os.environ.get('LISTEN_PID') == str(os.getpid()):
             self.reload_socket = True
-            self.socket = socket.fromfd(int(envfd), socket.AF_INET, socket.SOCK_STREAM)
-            # should we os.close(int(envfd)) ? it seem python duplicate the fd.
+            self.socket = socket.fromfd(SD_LISTEN_FDS_START, socket.AF_INET, socket.SOCK_STREAM)
         else:
             self.reload_socket = False
             super(ThreadedWSGIServerReloadable, self).server_bind()
@@ -883,7 +879,8 @@ def _reexec(updated_modules=None):
         args += ["-u", ','.join(updated_modules)]
     if not args or args[0] != exe:
         args.insert(0, exe)
-    os.execv(sys.executable, args)
+    # We should keep the LISTEN_* environment variabled in order to support socket activation on reexec
+    os.execve(sys.executable, args, os.environ)
 
 def load_test_file_py(registry, test_file):
     # Locate python module based on its filename and run the tests