File size: 7,404 Bytes
a1af790 | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 | from __future__ import annotations
import os
import secrets
import stat
import sys
from contextlib import suppress
from errno import EACCES, EIO, EISDIR
from pathlib import Path
from typing import Final
def write_all(fd: int, data: bytes) -> None:
"""
Write the whole buffer to *fd*, looping over the short writes POSIX lets ``os.write`` make.
A caller that stopped after one short write would leave a truncated marker behind. A peer that reads between two
writes sees a prefix of the record; each caller has to tolerate that. Lock records need not survive a power loss, so
we skip ``fsync``.
:param fd: file descriptor open for writing.
:param data: bytes to write in full.
:raises OSError: if a write reports zero progress before the record is complete.
"""
remaining = memoryview(data)
while remaining:
if (written := os.write(fd, remaining)) == 0:
raise OSError(EIO, "os.write wrote 0 bytes before the record was complete")
remaining = remaining[written:]
def raise_on_not_writable_file(filename: str) -> None:
"""
Raise an exception if attempting to open the file for writing would fail.
Separates files that can never be written from files that are writable but currently locked.
:param filename: file to check
:raises OSError: as if the file was opened for writing.
"""
try:
# lstat, not stat: settles exists-and-writable in one syscall, and a hostile symlink at the lock path would
# make stat inspect the link target, letting an attacker turn a contended acquire into a misleading
# PermissionError / IsADirectoryError and probe that target's attributes. The real open passes O_NOFOLLOW and
# refuses the symlink anyway.
file_stat = os.lstat(filename)
except OSError:
return # does not exist, or an error the caller cannot act on
# No mtime guard: the old `if st_mtime != 0` skip covered NFS/Linux quirks where os.lstat returned an all-zero
# struct, which it no longer does. Skipping on mtime 0 let a read-only file or a directory at the lock path pass
# as missing, so acquire() blocked forever on an open that cannot succeed.
# Match open() credentials where supported; group permissions and ACLs can grant access without S_IWUSR.
if not (file_stat.st_mode & stat.S_IWUSR) and not os.access(
filename, os.W_OK, effective_ids=os.access in os.supports_effective_ids
):
try:
os.lstat(filename)
except FileNotFoundError:
# A holder may unlink its marker between lstat() and access(); let the caller attempt creation.
return
raise PermissionError(EACCES, "Permission denied", filename)
if stat.S_ISDIR(file_stat.st_mode):
if sys.platform == "win32": # pragma: win32 cover
raise PermissionError(EACCES, "Permission denied", filename)
raise IsADirectoryError(EISDIR, "Is a directory", filename) # pragma: win32 no cover
def ensure_directory_exists(filename: Path | str) -> None:
"""
Ensure the directory containing the file exists (create it if necessary).
:param filename: file.
"""
Path(filename).parent.mkdir(parents=True, exist_ok=True)
def break_lock_file(lock_file: str, mtime_before: float, ino_before: int) -> None:
"""
Remove the lock file a caller found stale with modification time *mtime_before* and inode *ino_before*.
Right before renaming we ``lstat`` the lock again and leave it alone if a peer recreated it since the caller judged
it stale, as Mercurial re-reads a lock under its break lock. We rename before unlinking so that, of several
processes racing to break the same lock, one takes the file and the rest get ``OSError``. A newer modification time
or another inode on the renamed file means a peer recreated the lock between that ``lstat`` and the rename, so we
hard-link it back and drop the break name. ``link`` never replaces a name, so a marker a third process created after
our rename wins and the live one stays under the break name. In the two syscalls between rename and link a third
process can acquire beside the live holder, a soft lock by creating its marker or a StrictSoftFileLock by
publishing its sentinel at the empty path. Without ``os.link``, or on a filesystem that refuses hard links, the live
marker stays under the break name. We compare inodes as well as modification times because NFS and FAT store
coarse modification times, and a peer that recreates the lock within that granularity leaves the old mtime on it.
We call ``lstat`` to avoid following a symlink a peer swaps in after our stale check, and put back only a regular
file because ``link`` would follow a symlink to its target.
We add a random token to the break name so other processes cannot guess it and two breakers in one process do not
share ``<lock>.break.<pid>``. With a shared name, the second breaker could rename a recreated live lock onto that
path between our ``lstat`` and ``unlink``, and our ``unlink`` would delete it.
:param lock_file: path to the lock file to break.
:param mtime_before: modification time the caller saw when it judged the lock stale.
:param ino_before: inode number the caller saw when it judged the lock stale.
:raises OSError: if the rename, the re-check or the restore fails, for example because the file vanished or another
user owns it in a sticky directory.
"""
if (current := os.lstat(lock_file)).st_mtime > mtime_before or current.st_ino != ino_before:
return
break_path: Final[str] = f"{lock_file}.break.{os.getpid()}.{secrets.token_hex(16)}"
Path(lock_file).rename(break_path)
if (st_after := os.lstat(break_path)).st_mtime <= mtime_before and st_after.st_ino == ino_before:
Path(break_path).unlink()
elif stat.S_ISREG(st_after.st_mode) and _HAS_LINK: # pragma: needs hard-link
with suppress(FileExistsError):
os.link(break_path, lock_file)
Path(break_path).unlink()
def touch(name: str, *, fd: int) -> None:
# Prefer the already-open, already-verified fd so a peer that swaps a symlink or a different file in at the
# path after our O_NOFOLLOW read cannot redirect the touch: utime then targets the inode behind the fd.
# Where the platform cannot utime an fd, fall back to a path-based touch that still refuses to follow a
# symlink where supported, matching the O_NOFOLLOW reads used elsewhere here.
if _SUPPORTS_UTIME_FD: # pragma: needs utime-fd
os.utime(fd, None)
return
os.utime(name, None, follow_symlinks=not _SUPPORTS_UTIME_NOFOLLOW) # pragma: lacks utime-fd
# Termux/Android CPython ships without os.link.
_HAS_LINK: Final[bool] = hasattr(os, "link")
# Retargeting os.utime to an open fd lets a heartbeat refresh the exact inode it verified instead of whatever the
# pathname now names.
_SUPPORTS_UTIME_FD: Final[bool] = sys.platform != "win32" and os.utime in os.supports_fd
# os.utime follows symlinks unless told not to; not every platform can refuse the follow, so probe support.
_SUPPORTS_UTIME_NOFOLLOW: Final[bool] = os.utime in os.supports_follow_symlinks
__all__ = [
"break_lock_file",
"ensure_directory_exists",
"raise_on_not_writable_file",
"touch",
"write_all",
]
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