lib/_emerge/AsynchronousLock.py

# Copyright 2010-2018 Gentoo Foundation
# Distributed under the terms of the GNU General Public License v2

import fcntl
import logging
import sys

try:
	import dummy_threading
except ImportError:
	dummy_threading = None

try:
	import threading
except ImportError:
	threading = dummy_threading

import portage
from portage import os
from portage.exception import TryAgain
from portage.localization import _
from portage.locks import lockfile, unlockfile
from portage.util import writemsg_level
from _emerge.AbstractPollTask import AbstractPollTask
from _emerge.AsynchronousTask import AsynchronousTask
from _emerge.SpawnProcess import SpawnProcess

class AsynchronousLock(AsynchronousTask):
	"""
	This uses the portage.locks module to acquire a lock asynchronously,
	using either a thread (if available) or a subprocess.

	The default behavior is to use a process instead of a thread, since
	there is currently no way to interrupt a thread that is waiting for
	a lock (notably, SIGINT doesn't work because python delivers all
	signals to the main thread).
	"""

	__slots__ = ('path',) + \
		('_imp', '_force_async', '_force_dummy', '_force_process', \
		'_force_thread', '_unlock_future')

	_use_process_by_default = True

	def _start(self):

		if not self._force_async:
			try:
				self._imp = lockfile(self.path,
					wantnewlockfile=True, flags=os.O_NONBLOCK)
			except TryAgain:
				pass
			else:
				self.returncode = os.EX_OK
				self._async_wait()
				return

		if self._force_process or \
			(not self._force_thread and \
			(self._use_process_by_default or threading is dummy_threading)):
			self._imp = _LockProcess(path=self.path, scheduler=self.scheduler)
		else:
			self._imp = _LockThread(path=self.path,
				scheduler=self.scheduler,
				_force_dummy=self._force_dummy)

		self._imp.addExitListener(self._imp_exit)
		self._imp.start()

	def _imp_exit(self, imp):
		# call exit listeners
		self.returncode = imp.returncode
		self._async_wait()

	def _cancel(self):
		if isinstance(self._imp, AsynchronousTask):
			self._imp.cancel()

	def _poll(self):
		if isinstance(self._imp, AsynchronousTask):
			self._imp.poll()
		return self.returncode

	def async_unlock(self):
		"""
		Release the lock asynchronously. Release notification is available
		via the add_done_callback method of the returned Future instance.

		@returns: Future, result is None
		"""
		if self._imp is None:
			raise AssertionError('not locked')
		if self._unlock_future is not None:
			raise AssertionError("already unlocked")
		if isinstance(self._imp, (_LockProcess, _LockThread)):
			unlock_future = self._imp.async_unlock()
		else:
			unlockfile(self._imp)
			unlock_future = self.scheduler.create_future()
			self.scheduler.call_soon(unlock_future.set_result, None)
		self._imp = None
		self._unlock_future = unlock_future
		return unlock_future


class _LockThread(AbstractPollTask):
	"""
	This uses the portage.locks module to acquire a lock asynchronously,
	using a background thread. After the lock is acquired, the thread
	writes to a pipe in order to notify a poll loop running in the main
	thread.

	If the threading module is unavailable then the dummy_threading
	module will be used, and the lock will be acquired synchronously
	(before the start() method returns).
	"""

	__slots__ = ('path',) + \
		('_force_dummy', '_lock_obj', '_thread', '_unlock_future')

	def _start(self):
		self._registered = True
		threading_mod = threading
		if self._force_dummy:
			threading_mod = dummy_threading
		self._thread = threading_mod.Thread(target=self._run_lock)
		self._thread.daemon = True
		self._thread.start()

	def _run_lock(self):
		self._lock_obj = lockfile(self.path, wantnewlockfile=True)
		# Thread-safe callback to EventLoop
		self.scheduler.call_soon_threadsafe(self._run_lock_cb)

	def _run_lock_cb(self):
		self._unregister()
		self.returncode = os.EX_OK
		self._async_wait()

	def _cancel(self):
		# There's currently no way to force thread termination.
		pass

	def _unlock(self):
		if self._lock_obj is None:
			raise AssertionError('not locked')
		if self.returncode is None:
			raise AssertionError('lock not acquired yet')
		if self._unlock_future is not None:
			raise AssertionError("already unlocked")
		self._unlock_future = self.scheduler.create_future()
		unlockfile(self._lock_obj)
		self._lock_obj = None

	def async_unlock(self):
		"""
		Release the lock asynchronously. Release notification is available
		via the add_done_callback method of the returned Future instance.

		@returns: Future, result is None
		"""
		self._unlock()
		self.scheduler.call_soon(self._unlock_future.set_result, None)
		return self._unlock_future

	def _unregister(self):
		self._registered = False

		if self._thread is not None:
			self._thread.join()
			self._thread = None

class _LockProcess(AbstractPollTask):
	"""
	This uses the portage.locks module to acquire a lock asynchronously,
	using a subprocess. After the lock is acquired, the process
	writes to a pipe in order to notify a poll loop running in the main
	process. The unlock() method notifies the subprocess to release the
	lock and exit.
	"""

	__slots__ = ('path',) + \
		('_acquired', '_kill_test', '_proc', '_files', '_unlock_future')

	def _start(self):
		in_pr, in_pw = os.pipe()
		out_pr, out_pw = os.pipe()
		self._files = {}
		self._files['pipe_in'] = in_pr
		self._files['pipe_out'] = out_pw

		fcntl.fcntl(in_pr, fcntl.F_SETFL,
			fcntl.fcntl(in_pr, fcntl.F_GETFL) | os.O_NONBLOCK)

		# FD_CLOEXEC is enabled by default in Python >=3.4.
		if sys.hexversion < 0x3040000:
			try:
				fcntl.FD_CLOEXEC
			except AttributeError:
				pass
			else:
				fcntl.fcntl(in_pr, fcntl.F_SETFD,
					fcntl.fcntl(in_pr, fcntl.F_GETFD) | fcntl.FD_CLOEXEC)

		self.scheduler.add_reader(in_pr, self._output_handler)
		self._registered = True
		self._proc = SpawnProcess(
			args=[portage._python_interpreter,
				os.path.join(portage._bin_path, 'lock-helper.py'), self.path],
				env=dict(os.environ, PORTAGE_PYM_PATH=portage._pym_path),
				fd_pipes={0:out_pr, 1:in_pw, 2:sys.__stderr__.fileno()},
				scheduler=self.scheduler)
		self._proc.addExitListener(self._proc_exit)
		self._proc.start()
		os.close(out_pr)
		os.close(in_pw)

	def _proc_exit(self, proc):

		if self._files is not None:
			# Close pipe_out if it's still open, since it's useless
			# after the process has exited. This helps to avoid
			# "ResourceWarning: unclosed file" since Python 3.2.
			try:
				pipe_out = self._files.pop('pipe_out')
			except KeyError:
				pass
			else:
				os.close(pipe_out)

		if proc.returncode != os.EX_OK:
			# Typically, this will happen due to the
			# process being killed by a signal.

			if not self._acquired:
				# If the lock hasn't been aquired yet, the
				# caller can check the returncode and handle
				# this failure appropriately.
				if not (self.cancelled or self._kill_test):
					writemsg_level("_LockProcess: %s\n" % \
						_("failed to acquire lock on '%s'") % (self.path,),
						level=logging.ERROR, noiselevel=-1)
				self._unregister()
				self.returncode = proc.returncode
				self._async_wait()
				return

			if not self.cancelled and \
				self._unlock_future is None:
				# We don't want lost locks going unnoticed, so it's
				# only safe to ignore if either the cancel() or
				# unlock() methods have been previously called.
				raise AssertionError("lock process failed with returncode %s" \
					% (proc.returncode,))

		if self._unlock_future is not None:
			self._unlock_future.set_result(None)

	def _cancel(self):
		if self._proc is not None:
			self._proc.cancel()

	def _poll(self):
		if self._proc is not None:
			self._proc.poll()
		return self.returncode

	def _output_handler(self):
		buf = self._read_buf(self._files['pipe_in'])
		if buf:
			self._acquired = True
			self._unregister()
			self.returncode = os.EX_OK
			self._async_wait()

		return True

	def _unregister(self):
		self._registered = False

		if self._files is not None:
			try:
				pipe_in = self._files.pop('pipe_in')
			except KeyError:
				pass
			else:
				self.scheduler.remove_reader(pipe_in)
				os.close(pipe_in)

	def _unlock(self):
		if self._proc is None:
			raise AssertionError('not locked')
		if not self._acquired:
			raise AssertionError('lock not acquired yet')
		if self.returncode != os.EX_OK:
			raise AssertionError("lock process failed with returncode %s" \
				% (self.returncode,))
		if self._unlock_future is not None:
			raise AssertionError("already unlocked")
		self._unlock_future = self.scheduler.create_future()
		os.write(self._files['pipe_out'], b'\0')
		os.close(self._files['pipe_out'])
		self._files = None

	def async_unlock(self):
		"""
		Release the lock asynchronously. Release notification is available
		via the add_done_callback method of the returned Future instance.

		@returns: Future, result is None
		"""
		self._unlock()
		return self._unlock_future