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132 lines
4.0 KiB
Python
132 lines
4.0 KiB
Python
# Copyright 2020 The Chromium Authors. All rights reserved.
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# Use of this source code is governed by a BSD-style license that can be
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# found in the LICENSE file.
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"""Exclusive filelocking for all supported platforms."""
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import contextlib
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import logging
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import os
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import sys
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import time
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class LockError(Exception):
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pass
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if sys.platform.startswith('win'):
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# Windows implementation
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import win32imports
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BYTES_TO_LOCK = 1
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def _open_file(lockfile):
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return win32imports.Handle(
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win32imports.CreateFileW(
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lockfile, # lpFileName
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win32imports.GENERIC_WRITE, # dwDesiredAccess
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0, # dwShareMode=prevent others from opening file
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None, # lpSecurityAttributes
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win32imports.CREATE_ALWAYS, # dwCreationDisposition
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win32imports.FILE_ATTRIBUTE_NORMAL, # dwFlagsAndAttributes
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None # hTemplateFile
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))
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def _close_file(handle, unlock):
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if unlock:
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# Locks are released *before* the CloseHandle function is finished
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# processing:
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# - https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-unlockfileex#remarks
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pass
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win32imports.CloseHandle(handle)
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def _lock_file(handle):
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ret = win32imports.LockFileEx(
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handle, # hFile
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win32imports.LOCKFILE_FAIL_IMMEDIATELY
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| win32imports.LOCKFILE_EXCLUSIVE_LOCK, # dwFlags
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0, #dwReserved
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BYTES_TO_LOCK, # nNumberOfBytesToLockLow
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0, # nNumberOfBytesToLockHigh
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win32imports.Overlapped() # lpOverlapped
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)
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# LockFileEx returns result as bool, which is converted into an integer
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# (1 == successful; 0 == not successful)
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if ret == 0:
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error_code = win32imports.GetLastError()
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raise OSError('Failed to lock handle (error code: %d).' %
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error_code)
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else:
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# Unix implementation
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import fcntl
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def _open_file(lockfile):
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open_flags = (os.O_CREAT | os.O_WRONLY)
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return os.open(lockfile, open_flags, 0o644)
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def _close_file(fd, unlock):
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# "man 2 fcntl" states that closing any file descriptor referring to
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# the lock file will release all the process locks on the file, but
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# there is no guarantee that the locks will be released atomically
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# before the closure.
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#
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# It's necessary to release the lock before the file close to avoid
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# possible race conditions.
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if unlock:
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fcntl.flock(fd, fcntl.LOCK_UN)
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os.close(fd)
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def _lock_file(fd):
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fcntl.flock(fd, fcntl.LOCK_EX | fcntl.LOCK_NB)
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def _try_lock(lockfile):
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f = _open_file(lockfile)
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try:
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_lock_file(f)
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except Exception:
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_close_file(f, unlock=False)
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raise
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return lambda: _close_file(f, unlock=True)
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def _lock(path, timeout=0):
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"""_lock returns function to release the lock if locking was successful.
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_lock also implements simple retry logic.
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NOTE: timeout value doesn't include time it takes to aquire lock, just
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overall sleep time."""
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elapsed = 0
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sleep_time = 0.1
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while True:
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try:
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return _try_lock(path + '.locked')
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except (OSError, IOError) as e:
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if elapsed < timeout:
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logging.info(
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'Could not create git cache lockfile; '
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'will retry after sleep(%d).', sleep_time)
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elapsed += sleep_time
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time.sleep(sleep_time)
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continue
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raise LockError("Error locking %s (err: %s)" % (path, str(e)))
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@contextlib.contextmanager
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def lock(path, timeout=0):
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"""Get exclusive lock to path.
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Usage:
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import lockfile
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with lockfile.lock(path, timeout):
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# Do something
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pass
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"""
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release_fn = _lock(path, timeout)
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try:
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yield
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finally:
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release_fn()
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