Add flock and timeout to infra/libs.

infra/libs/service_utils/daemon.py

+# Copyright 2015 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.
+
+"""Locking, timeout, and other process management functions."""
+
+import contextlib
+import fcntl
+import os
+import sys
+import tempfile
+
+
+@contextlib.contextmanager
+def _auto_closing_fd(*args, **kwargs):
+  """Opens a file, yields its fd, and closes it when done."""
+
+  fd = os.open(*args, **kwargs)
+  yield fd
+  os.close(fd)
+
+
+@contextlib.contextmanager
+def flock(lockfile, lockdir=None):
+  """Keeps a critical section from executing concurrently using a file lock.

[tandrii(chromium), 2015/04/23 13:02:47]

You should document that this lock only gives exclusivity to processes, not
threads. That is, two threads in the same processes will both happily acquire
lock at the same time.

+
+  Implementation based on http://goo.gl/dNf7fv (see John Mudd's comment) and
+  http://stackoverflow.com/a/18745264/3984761. This implementation creates the
+  lockfile if it doesn't exist and removes it when the critical section exits.
+  It yields True if the lock was acquired, and False if it wasn't. You must
+  check if the yielded value is True if you want to prevent concurrent critical
+  sections.
+
+  Example usage:
+
+  with daemon.flock('toaster') as acquired:

[tandrii(chromium), 2015/04/23 13:02:47]

IMO, it's more Pythonic to raise exception instead of yielding False.

[agable, 2015/04/27 18:48:33]

So then the construct would be

try:
  with daemon.flock('toaster') as acq:
    put_bread_in_toaster()
except LockAlreadyLocked:
  print 'toaster is occupied!'

? That seems fine to me. I agree that it is more pythonic.

[tandrii(chromium), 2015/04/27 21:17:09]

Yep, and you don't even need "as acq" part :)

[ghost stip (do not use), 2015/04/27 22:03:47]

Done.

+    if acquired:
+      put_bread_in_toaster()
+    else:
+      print 'toaster is occupied!'
+  """
+
+  if sys.platform.startswith('win'):  # pragma: no cover
+    raise NotImplementedError
+
+  lockdir = lockdir or tempfile.gettempdir()
+  full_lockfile = os.path.join(lockdir, lockfile)
+  lock_acquired = False
+
+  with _auto_closing_fd(
+      full_lockfile, os.O_CREAT | os.O_TRUNC | os.O_WRONLY) as fd:
+    try:
+      # Request exclusive (EX) non-blocking (NB) advisory lock.
+      fcntl.lockf(fd, fcntl.LOCK_EX | fcntl.LOCK_NB)
+
+      lock_acquired = True
+    except IOError:
+      # Could not obtain lock.
+      lock_acquired = False
+
+    if lock_acquired:
+      try:
+        held_inode = os.fstat(fd).st_ino
+        file_inode = os.stat(full_lockfile).st_ino
+
+        if held_inode != file_inode:
+          # The file was deleted under us, another process has created it again
+          # and may get a lock on it. That process doesn't know about the lock
+          # we have on the (now deleted) file, so we need to bail.
+          lock_acquired = False
+      except OSError:
+       # File has been deleted under us. We have to exit because another process
+       # might try to create it and obtain a lock, not knowing that we had a
+       # lock on the (now deleted) file.
+       lock_acquired = False
+
+    yield lock_acquired
+
+    try:
+      # The order of these two operations is very important. We need to delete
+      # the file before we release the lock. If we release the lock before we
+      # delete the file, we run the risk of another process obtaining a lock on
+      # the file we're about to delete. If the delete happens while the other
+      # critical section is running, a third process could create the file, get
+      # a lock on it, and run a second critical section simultaneously. Deleting
+      # before unlocking prevents this scenario.
+      os.unlink(full_lockfile)
+      fcntl.lockf(fd, fcntl.LOCK_UN)
+    except OSError:
+      # If the file was deleted for some other reason, don't sweat it.
+      pass
+
+
+def add_timeout(cmd, timeout_secs):
+  """Adds a timeout to a command."""

[agable, 2015/04/27 18:48:33]

Note that it does so using the linux /bin/timeout command.

[ghost stip (do not use), 2015/04/27 22:03:47]

Done.

+
+  if sys.platform.startswith('win') or sys.platform.startswith('darwin'):
+    raise NotImplementedError  # pragma: no cover
+
+  return ['timeout', str(timeout_secs)] + cmd