Choose Virtual Me2Me X session based on installed components.

remoting/tools/me2me_virtual_host.py

 #!/usr/bin/env python
 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Virtual Me2Me implementation.  This script runs and manages the processes
 # required for a Virtual Me2Me desktop, which are: X server, X desktop
 # session, and Host process.
 # This script is intended to run continuously as a background daemon
 # process, running under an ordinary (non-root) user account.
@@ skipping 15 matching lines @@
 import sys
 import tempfile
 import time
 import urllib2
 import uuid
 
 # Local modules
 import gaia_auth
 import keygen
 
+# By default this script will try to determine the most appropriate X session
+# command for the system.  To use a specific session instead, set this variable
+# to the executable filename, or a list containing the executable and any
+# arguments, for example:
+# XSESSION_COMMAND = "/usr/bin/gnome-session-fallback"
+# XSESSION_COMMAND = ["/usr/bin/gnome-session", "--session=ubuntu-2d"]
+XSESSION_COMMAND = None
+
 REMOTING_COMMAND = "remoting_me2me_host"
 
 # Command-line switch for passing the config path to remoting_me2me_host.
 HOST_CONFIG_SWITCH_NAME = "host-config"
 
 # Needs to be an absolute path, since the current working directory is changed
 # when this process self-daemonizes.
 SCRIPT_PATH = os.path.dirname(sys.argv[0])
 if SCRIPT_PATH:
   SCRIPT_PATH = os.path.abspath(SCRIPT_PATH)
@@ skipping 280 matching lines @@
     # be started with -noreset otherwise it'll reset as soon as the command
     # completes, since there are no other X clients running yet.
     proc = subprocess.Popen("setxkbmap -rules evdev", env=self.child_env,
                             shell=True)
     pid, retcode = os.waitpid(proc.pid, 0)
     if retcode != 0:
       logging.error("Failed to set XKB to 'evdev'")
 
   def launch_x_session(self):
     # Start desktop session
-    # The /dev/null input redirection is necessary to prevent Xsession from
+    # The /dev/null input redirection is necessary to prevent the X session
     # reading from stdin.  If this code runs as a shell background job in a
     # terminal, any reading from stdin causes the job to be suspended.
     # Daemonization would solve this problem by separating the process from the
     # controlling terminal.
-    #
-    # This assumes that GDM is installed and configured on the system.
-    self.session_proc = subprocess.Popen("/etc/gdm/Xsession",
+    logging.info("Launching X session: %s" % XSESSION_COMMAND)
+    self.session_proc = subprocess.Popen(XSESSION_COMMAND,
                                          stdin=open(os.devnull, "r"),
                                          cwd=HOME_DIR,
                                          env=self.child_env)
     if not self.session_proc.pid:
       raise Exception("Could not start X session")
 
   def launch_host(self, host):
     # Start remoting host
     args = [locate_executable(REMOTING_COMMAND),
             "--%s=%s" % (HOST_CONFIG_SWITCH_NAME, host.config_file)]
@@ skipping 78 matching lines @@
 
   def delete_file(self):
     """Delete the PID file if it was created by this instance.
 
     This is called on process termination.
     """
     if self.created:
       os.remove(self.filename)
 
 
+def choose_x_session():
+  """Chooses the most appropriate X session command for this system.
+
+  If XSESSION_COMMAND is already set, its value is returned directly.
+  Otherwise, a session is chosen for this system.
+
+  Returns:
+    A string containing the command to run, or a list of strings containing
+    the executable program and its arguments, which is suitable for passing as
+    the first parameter of subprocess.Popen().  If a suitable session cannot
+    be found, returns None.
+  """
+  if XSESSION_COMMAND is not None:
+    return XSESSION_COMMAND
+
+  # Unity-2d would normally be the preferred choice on Ubuntu 12.04.  At the
+  # time of writing, this session does not work properly (missing launcher and
+  # panel), so gnome-session-fallback is used in preference.
+  # "unity-2d-panel" was chosen here simply because it appears in the TryExec
+  # line of the session's .desktop file; other choices might be just as good.
+  for test_file, command in [
+    ("/usr/bin/gnome-session-fallback", "/usr/bin/gnome-session-fallback"),
+    ("/etc/gdm/Xsession", "/etc/gdm/Xsession"),
+    ("/usr/bin/unity-2d-panel",
+      ["/usr/bin/gnome-session", "--session=ubuntu-2d"]),
+  ]:
+    if os.path.exists(test_file):
+      return command
+
+  return None
+
+
 def locate_executable(exe_name):
   for path in EXE_PATHS_TO_TRY:
     exe_path = os.path.join(SCRIPT_PATH, path, exe_name)
     if os.path.exists(exe_path):
       return exe_path
 
   raise Exception("Could not locate executable '%s'" % exe_name)
 
 
 def daemonize(log_filename):
@@ skipping 129 matching lines @@
     width = int(size_components[0])
     height = int(size_components[1])
 
     # Enforce minimum desktop size, as a sanity-check.  The limit of 100 will
     # detect typos of 2 instead of 3 digits.
     if width < 100 or height < 100:
       raise ValueError
   except ValueError:
     parser.error("Width and height should be 100 pixels or greater")
 
+  global XSESSION_COMMAND
+  XSESSION_COMMAND = choose_x_session()
+  if XSESSION_COMMAND is None:
+    print >> sys.stderr, "Unable to choose suitable X session command."
+    return 1
+
+  if "--session=ubuntu-2d" in XSESSION_COMMAND:
+    print >> sys.stderr, (
+      "The Unity 2D desktop session will be used.\n"
+      "If you encounter problems with this choice of desktop, please install\n"
+      "the gnome-session-fallback package, and restart this script.\n")
+
   atexit.register(cleanup)
 
   for s in [signal.SIGHUP, signal.SIGINT, signal.SIGTERM, signal.SIGUSR1]:
     signal.signal(s, signal_handler)
 
   # Ensure full path to config directory exists.
   if not os.path.exists(CONFIG_DIR):
     os.makedirs(CONFIG_DIR, mode=0700)
 
   if options.explicit_config:
@@ skipping 158 matching lines @@
         os.remove(host.config_file)
         return 0
       elif os.WEXITSTATUS(status) == 4:
         logging.info("OAuth credentials are invalid - exiting.")
         os.remove(auth.config_file)
         return 0
 
 if __name__ == "__main__":
   logging.basicConfig(level=logging.DEBUG)
   sys.exit(main())