from __future__ import annotations import os import sys import warnings from contextlib import suppress from errno import EACCES, EAGAIN, ENOSYS, EWOULDBLOCK from pathlib import Path from typing import Final, cast from ._api import BaseFileLock from ._util import ensure_directory_exists has_fcntl = False if sys.platform == "win32": # pragma: win32 cover class UnixFileLock(BaseFileLock): """Uses the :func:`fcntl.flock` to hard lock the lock file on unix systems.""" def _acquire(self) -> None: raise NotImplementedError def _release(self) -> None: raise NotImplementedError else: # pragma: win32 no cover try: import fcntl _ = (fcntl.flock, fcntl.LOCK_EX, fcntl.LOCK_NB, fcntl.LOCK_UN) except (ImportError, AttributeError): _FCNTL_UNAVAILABLE: Final[str] = "fcntl is unavailable" def _lock_fd_nonblocking(_fd: int) -> bool: raise OSError(ENOSYS, _FCNTL_UNAVAILABLE) def _unlock_fd(_fd: int) -> None: raise OSError(ENOSYS, _FCNTL_UNAVAILABLE) else: has_fcntl = True # Contention errnos for a nonblocking flock. EAGAIN/EWOULDBLOCK are the usual "held elsewhere" codes; some # filesystems report EACCES instead, so treat it as contention too rather than a permanent error. _CONTENTION_ERRNOS: Final[frozenset[int]] = frozenset({EACCES, EAGAIN, EWOULDBLOCK}) def _lock_fd_nonblocking(fd: int) -> bool: # One nonblocking exclusive flock attempt shared by UnixFileLock and lock_descriptor, so both contend on # the same lock and classify errors identically. The caller owns fd; this never closes it. try: fcntl.flock(fd, fcntl.LOCK_EX | fcntl.LOCK_NB) except OSError as exception: if exception.errno in _CONTENTION_ERRNOS: return False raise return True def _unlock_fd(fd: int) -> None: fcntl.flock(fd, fcntl.LOCK_UN) class UnixFileLock(BaseFileLock): """ Uses the :func:`fcntl.flock` to hard lock the lock file on unix systems. We leave the lock file in place after release. Unlinking a locked file on Unix splits waiters across inodes and breaks mutual exclusion for processes that coordinate via the same path. """ def _acquire(self) -> None: missing_flock = self._acquire_native() if missing_flock is not None: self._switch_to_soft_lock(*missing_flock) def _acquire_native(self) -> tuple[int, OSError] | None: ensure_directory_exists(self.lock_file) # Open without O_TRUNC and defer truncation and fchmod until after flock succeeds: a contender that loses # the lock must not truncate the holder's file (erasing caller diagnostics) or change its mode. The winner # truncates and normalizes mode once it owns the lock (#591). open_flags = os.O_RDWR if (o_nofollow := getattr(os, "O_NOFOLLOW", None)) is not None: open_flags |= o_nofollow open_flags |= os.O_CREAT open_mode = self._open_mode() try: fd = os.open(self.lock_file, open_flags, open_mode) except FileNotFoundError: # On FUSE/NFS, os.open(O_CREAT) is not atomic; a split LOOKUP + CREATE lets a concurrent unlink() # delete the file between them. For a valid path, treat ENOENT as transient contention. For an # invalid path (e.g. empty string), re-raise to avoid an infinite retry loop. if self.lock_file and Path(self.lock_file).parent.exists(): return None raise except PermissionError: # Sticky-bit dirs (e.g. /tmp): O_CREAT fails if the file is owned by another user (#317). # Fall back to opening the existing file without O_CREAT. if not Path(self.lock_file).exists(): raise try: fd = os.open(self.lock_file, open_flags & ~os.O_CREAT, open_mode) except FileNotFoundError: return None self._mark_descriptor_pending(fd) try: locked = _lock_fd_nonblocking(fd) except OSError as exception: if exception.errno != ENOSYS: self._mark_descriptor_released() os.close(fd) raise # contention returns False from _lock_fd_nonblocking, so any raise here is a real failure return fd, exception if locked: self._finalize_locked_fd(fd) else: self._mark_descriptor_released() os.close(fd) # contention; let the retry loop try again return None def _switch_to_soft_lock(self, fd: int, missing_flock: OSError) -> None: # The filesystem does not implement flock. Capture the opened file's identity before closing so the cleanup # below removes only this attempt's placeholder, not a peer's replacement. identity: tuple[int, int] | None = None with suppress(OSError): identity = (fstat := os.fstat(fd)).st_dev, fstat.st_ino self._mark_descriptor_released() os.close(fd) if not self._fallback_to_soft or self._preserve_lock_file or self._on_acquired is not None: # Fail closed: the caller opted out of existence-lock semantics (#603), asked to preserve the pathname # (#605), or set an on_acquired hook (#607), none of which a soft lock can honor. raise missing_flock with suppress(OSError): current = os.lstat(self.lock_file) if identity == (current.st_dev, current.st_ino): Path(self.lock_file).unlink() self._fallback_to_soft_lock() self._acquire() def _finalize_locked_fd(self, fd: int) -> None: # Runs with the flock held. Truncate and normalize mode under a guard so any failure closes fd rather than # leaking it and its lock. A concurrent _release() may have unlinked the inode between our open() and # flock() (st_nlink 0), leaving a useless dead-inode lock; drop it and let the retry loop start fresh. keep = False try: stat_result = os.fstat(fd) if stat_result.st_nlink != 0: os.ftruncate(fd, 0) self._apply_explicit_mode(fd) keep = True except OSError: self._mark_descriptor_released() os.close(fd) raise if keep: self._mark_descriptor_owned(fd, (stat_result.st_dev, stat_result.st_ino)) else: self._mark_descriptor_released() os.close(fd) def _apply_explicit_mode(self, fd: int) -> None: if self.has_explicit_mode: with suppress(PermissionError): os.fchmod(fd, self._context.mode) def _fallback_to_soft_lock(self) -> None: # Import lazily: this runs only on the rare flock fallback, and asyncio imports _unix, so a # module-level import of it here would cycle. from ._soft import SoftFileLock # ruff:ignore[import-outside-top-level] warnings.warn("flock not supported on this filesystem, falling back to SoftFileLock", stacklevel=2) from .asyncio import AsyncSoftFileLock, BaseAsyncFileLock # ruff:ignore[import-outside-top-level] self.__class__ = AsyncSoftFileLock if isinstance(self, BaseAsyncFileLock) else SoftFileLock def _release(self) -> None: fd = cast("int", self._context.lock_file_fd) # Retain the descriptor until flock succeeds: a failed unlock leaves the kernel lock held, so is_locked # must keep reporting held for a retry. Once flock commits, clear held state and close as post-unlock # cleanup; a close failure (EIO on FUSE/Docker bind mounts) does not make the kernel lock held again. _unlock_fd(fd) self._mark_descriptor_released() self._close_released_fd(fd, default_suppresses=True) if sys.platform == "win32": # pragma: win32 cover __all__ = ["UnixFileLock", "has_fcntl"] else: # pragma: win32 no cover __all__ = ["UnixFileLock", "_lock_fd_nonblocking", "_unlock_fd", "has_fcntl"]