[Android] Fork a detached child process from adb_logcat_monitor.py.

This way, we can invoke adb_logcat_monitor.py directly from
recipes. We cannot spawn a process which runs
adb_logcat_monitor.py (like we have been via
buildbot/bb_device_steps.py) from recipes as we cannot import
subprocess et al. from recipes.

BUG=286509

Review URL: https://codereview.chromium.org/24456002

git-svn-id: http://src.chromium.org/svn/trunk/src/build@226151 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

android/adb_logcat_monitor.py

@@ -96,6 +96,15 @@ def GetAttachedDevices(adb_cmd):
 
 def main(base_dir, adb_cmd='adb'):
   """Monitor adb forever.  Expects a SIGINT (Ctrl-C) to kill."""
+  # Spawn a detached child process.
+  pid = os.fork()
+  if pid > 0:
+    os._exit(os.EX_OK)
+  elif pid < 0:
+    sys.exit('Unable to spawn a detached child process.')
+  os.setsid()
+  # The rest happens in the child process.
+
   # We create the directory to ensure 'run once' semantics
   if os.path.exists(base_dir):
     print 'adb_logcat_monitor: %s already exists? Cleaning' % base_dir

android/buildbot/bb_device_steps.py

@@ -285,7 +285,7 @@ def EscapeBuilderName(builder_name):
 
 def SpawnLogcatMonitor():
   shutil.rmtree(LOGCAT_DIR, ignore_errors=True)
-  bb_utils.SpawnCmd([
+  bb_utils.RunCmd([
       os.path.join(CHROME_SRC_DIR, 'build', 'android', 'adb_logcat_monitor.py'),
       LOGCAT_DIR])