[lacros] Add helper to launch lacros-chrome with mojo connection

This CL adds a helper script to make it easy to launch lacros-chrome
with mojo connection established to ash-chrome, and the main use case
is to be able to debug lacos-chrome inside a debugger.

Bug: 1120582
Change-Id: Ifddc5fdad44e23dffe9d821424365d89b525dfbc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2402285
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813316}

build/lacros/mojo_connection_lacros_launcher.py

+#!/usr/bin/env vpython3
+#
+# Copyright 2020 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.
+"""Helps launch lacros-chrome with mojo connection established on Linux.
+
+  The main use case is to be able to launch lacros-chrome in a debugger.
+
+  Please first launch an ash-chrome in the background as usual except without
+  the '--lacros-chrome-path' argument and with an additional
+  '--lacros-mojo-socket-for-testing' argument pointing to a socket path:
+
+  XDG_RUNTIME_DIR=/tmp/ash_chrome_xdg_runtime ./out/ash/chrome \\
+      --user-data-dir=/tmp/ash-chrome --enable-wayland-server \\
+      --no-startup-window --enable-features=LacrosSupport \\
+      --lacros-mojo-socket-for-testing=/tmp/lacros.sock
+
+  Then, run this script with '-s' pointing to the same socket path used to
+  launch ash-chrome, followed by a command one would use to launch lacros-chrome
+  inside a debugger:
+
+  EGL_PLATFORM=surfaceless XDG_RUNTIME_DIR=/tmp/ash_chrome_xdg_runtime \\
+  ./build/lacros/mojo_connection_lacros_launcher.py -s /tmp/lacros.sock
+  gdb --args ./out/lacros-release/chrome --user-data-dir=/tmp/lacros-chrome
+"""
+
+import argparse
+import array
+import contextlib
+import os
+import pathlib
+import socket
+import sys
+import subprocess
+
+
+def _ReceiveFD(sock):
+  """Receives a FD from ash-chrome that will be used to launch lacros-chrome.
+
+  Args:
+    sock: A connected unix domain socket.
+
+  Returns:
+    An integer represeting the received file descriptor.
+  """
+  # This function is borrowed from with modifications:
+  # https://docs.python.org/3/library/socket.html#socket.socket.recvmsg
+  fds = array.array("i")  # Array of ints
+  _, ancdata, _, _ = sock.recvmsg(0, socket.CMSG_LEN(fds.itemsize))
+  for cmsg_level, cmsg_type, cmsg_data in ancdata:
+    if cmsg_level == socket.SOL_SOCKET and cmsg_type == socket.SCM_RIGHTS:
+      assert len(cmsg_data) == fds.itemsize, 'Expecting exactly 1 FD'
+      fds.frombytes(cmsg_data[:fds.itemsize])
+
+  assert len(list(fds)) == 1, 'Expecting exactly 1 FD'
+  return os.fdopen(list(fds)[0])
+
+
+def Main():
+  arg_parser = argparse.ArgumentParser()
+  arg_parser.usage = __doc__
+  arg_parser.add_argument(
+      '-s',
+      '--socket-path',
+      type=pathlib.Path,
+      required=True,
+      help='Absolute path to the socket that were used to start ash-chrome, '
+      'for example: "/tmp/lacros.socket"')
+  args = arg_parser.parse_known_args()
+
+  assert 'XDG_RUNTIME_DIR' in os.environ
+  assert os.environ.get('EGL_PLATFORM') == 'surfaceless'
+
+  with socket.socket(socket.AF_UNIX, socket.SOCK_STREAM) as sock:
+    sock.connect(args[0].socket_path.as_posix())
+    file_obj = _ReceiveFD(sock)
+    assert file_obj, ('Failed to connect to the socket: %s' %
+                      args[0].socket_path.as_posix())
+
+  with contextlib.closing(file_obj):
+    cmd = args[1] + ['--mojo-platform-channel-handle=%d' % file_obj.fileno()]
+    proc = subprocess.Popen(cmd, pass_fds=(file_obj.fileno(), ))
+
+  return proc.wait()
+
+
+if __name__ == '__main__':
+  sys.exit(Main())