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

build/android/adb_logcat_monitor.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.
 
 """Saves logcats from all connected devices.
 
 Usage: adb_logcat_monitor.py <base_dir> [<adb_binary_path>]
 
@@ skipping 78 matching lines @@
     return []
   except (IOError, OSError):
     logging.exception('Exception from "adb devices"')
     return []
   finally:
     signal.alarm(0)
 
 
 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:
+    return 0

[Isaac (away), 2013/09/26 18:31:08]

yikes.  I believe this needs to be os._exit(0)

[Siva Chandra, 2013/09/26 20:24:13]

I think in principle this should be the other way: child process should exit
os._exit(...). However, in this case, we are always doing this right at the
beginning and always guaranteed that the parent process dies before the child
without creating any shared resources. Hence, exit from parent and child seem
safe enough. This is based on my understanding. You can explain more and help me
understand better if I got it wrong.

+  elif pid < 0:
+    sys.exit('Unable to spawn a detached child process.')
+  os.setsid()

[iannucci, 2013/09/26 18:15:57]

How does this process get cleaned up?

[Siva Chandra, 2013/09/26 18:30:06]

There is another step which is always executed. That step cleans this up.

[Isaac (away), 2013/09/26 18:31:08]

Another process reads PID from a file and kills it.  An alternative here would
be to revamp the way the logger works -- which might less error prone than using
a raw fork (which again, is really dangerous).

[navabi, 2013/09/26 19:56:00]

Where does that happen again? I don't see the PID being written to a file.
Can you point me to the step that is executed and cleans this up? Is it in a
finally block?

[Siva Chandra, 2013/09/26 20:29:44]

See below.

+  # 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
     shutil.rmtree(base_dir, ignore_errors=True)
 
   os.makedirs(base_dir)
   logging.basicConfig(filename=os.path.join(base_dir, 'eventlog'),
                       level=logging.INFO,
                       format='%(asctime)-2s %(levelname)-8s %(message)s')
 
   # Set up the alarm for calling 'adb devices'. This is to ensure
   # our script doesn't get stuck waiting for a process response
   def TimeoutHandler(_, unused_frame):
     raise TimeoutException()
   signal.signal(signal.SIGALRM, TimeoutHandler)
 
   # Handle SIGTERMs to ensure clean shutdown
   def SigtermHandler(_, unused_frame):
     raise SigtermError()
   signal.signal(signal.SIGTERM, SigtermHandler)
 
   logging.info('Started with pid %d', os.getpid())
   pid_file_path = os.path.join(base_dir, 'LOGCAT_MONITOR_PID')
 
   try:
     with open(pid_file_path, 'w') as f:

[Siva Chandra, 2013/09/26 20:29:44]

The PID gets written to a file here.

       f.write(str(os.getpid()))
     while True:
       for device_id in GetAttachedDevices(adb_cmd):
         if not device_id in devices:
           subprocess.call([adb_cmd, '-s', device_id, 'logcat', '-c'])
           devices[device_id] = (None, 0)
 
       for device in devices:
         # This will spawn logcat watchers for any device ever detected
         StartLogcatIfNecessary(device, adb_cmd, base_dir)
 
       time.sleep(5)
   except SigtermError:
     logging.info('Received SIGTERM, shutting down')

[Siva Chandra, 2013/09/26 20:29:44]

adb_logcat_printer.py:133 sends SIGTERM which is caught here.

   except:
     logging.exception('Unexpected exception in main.')
   finally:
     for process, _ in devices.itervalues():
       if process:
         try:
           process.terminate()
         except OSError:
           pass
     os.remove(pid_file_path)

[Siva Chandra, 2013/09/26 20:29:44]

The PID file is removed here.

 
 
 if __name__ == '__main__':
   if 2 <= len(sys.argv) <= 3:
     print 'adb_logcat_monitor: Initializing'
     sys.exit(main(*sys.argv[1:3]))
 
   print 'Usage: %s <base_dir> [<adb_binary_path>]' % sys.argv[0]