Choose Virtual Me2Me X session based on installed components.

remoting/tools/me2me_virtual_host.py

Index: remoting/tools/me2me_virtual_host.py
diff --git a/remoting/tools/me2me_virtual_host.py b/remoting/tools/me2me_virtual_host.py
index b295824466dec0be46e05f3b8fde3be4188eb7b0..87d1800355ffdd6d752a84918e5169522c6e08be 100755
--- a/remoting/tools/me2me_virtual_host.py
+++ b/remoting/tools/me2me_virtual_host.py
@@ -33,6 +33,13 @@ import uuid
 import gaia_auth
 import keygen
 
+# The X session command is normally determined automatically.  To use a

[Wez, 2012/07/12 21:18:14]

nit: "By default this script will try to determine the X session command
automatically."

[Lambros, 2012/07/13 00:30:18]

Done.

+# 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.
@@ -333,14 +340,13 @@ class Desktop:
 
   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)
@@ -439,6 +445,46 @@ class PidFile:
       os.remove(self.filename)
 
 
+def choose_x_session():
+  """Finds an appropriate choice of X session command for this system.

[Wez, 2012/07/12 21:18:14]

nit: "Chooses the most appropriate X session command for this system."

[Lambros, 2012/07/13 00:30:18]

Done.

+
+  If XSESSION_COMMAND is already set, its value is returned directly.
+  Otherwise, a session is chosen for this system.
+
+  This may print some information to stderr, so this function is called
+  before daemonization, and the result is assigned to XSESSION_COMMAND.  This
+  way, any problems or errors can be reported before daemonization occurs.

[Wez, 2012/07/12 21:18:14]

You print the information to stderr so that it will be displayed to the user
when they actually run the script?  (The way the comment is worded it sounds
like stderr output is an unavoidable side-effect rather than a desired
behaviour!)

[Lambros, 2012/07/13 00:30:18]

Removed the side-effect.

+
+  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
+
+  for candidate in [
+    "/usr/bin/gnome-session-fallback",
+    "/etc/gdm/Xsession"]:
+    if os.path.exists(candidate):
+      return candidate

[Wez, 2012/07/12 21:18:14]

You could structure this as a dictionary of {file-to-test-for:
command-line-to-run} pairs, so that you don't need to treat Unity-2D as a
special case.

[Lambros, 2012/07/13 00:30:18]

Done, but with a list instead of a dictionary, because
the order is important :)

+
+  # 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.
+  if os.path.exists("/usr/bin/unity-2d-panel"):
+    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")

[Wez, 2012/07/12 21:18:14]

It would be cleaner to have the calling code test the result of choose_x_session
and print a message to the user if Unity was chosen, I think, rather than have
it as a side-effect of this function.

e.g. if "unity-2d" in XSESSION_COMMAND: ...

[Lambros, 2012/07/13 00:30:18]

Done.

+    return ["/usr/bin/gnome-session", "--session=ubuntu-2d"]
+
+  return None
+
+
 def locate_executable(exe_name):
   for path in EXE_PATHS_TO_TRY:
     exe_path = os.path.join(SCRIPT_PATH, path, exe_name)
@@ -588,6 +634,12 @@ def main():
   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 "Unable to choose suitable X session command"
+    return 1
+
   atexit.register(cleanup)
 
   for s in [signal.SIGHUP, signal.SIGINT, signal.SIGTERM, signal.SIGUSR1]: