forked from pytest-dev/pytest
/
atomic_writes.py
205 lines (166 loc) · 6.34 KB
/
atomic_writes.py
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"""
Module copied over from https://github.com/untitaker/python-atomicwrites, which has become
unmaintained.
Since then, we have made changes to simplify the code, focusing on pytest's use-case.
"""
import contextlib
import os
import sys
import tempfile
_proper_fsync = os.fsync
if sys.platform != "win32":
import fcntl
if hasattr(fcntl, "F_FULLFSYNC"):
def _proper_fsync(fd):
# https://lists.apple.com/archives/darwin-dev/2005/Feb/msg00072.html
# https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man2/fsync.2.html
# https://github.com/untitaker/python-atomicwrites/issues/6
fcntl.fcntl(fd, fcntl.F_FULLFSYNC)
def _sync_directory(directory):
# Ensure that filenames are written to disk
fd = os.open(directory, 0)
try:
_proper_fsync(fd)
finally:
os.close(fd)
def _replace_atomic(src, dst):
os.rename(src, dst)
_sync_directory(os.path.normpath(os.path.dirname(dst)))
def _move_atomic(src, dst):
os.link(src, dst)
os.unlink(src)
src_dir = os.path.normpath(os.path.dirname(src))
dst_dir = os.path.normpath(os.path.dirname(dst))
_sync_directory(dst_dir)
if src_dir != dst_dir:
_sync_directory(src_dir)
else:
from ctypes import windll, WinError
_MOVEFILE_REPLACE_EXISTING = 0x1
_MOVEFILE_WRITE_THROUGH = 0x8
_windows_default_flags = _MOVEFILE_WRITE_THROUGH
def _handle_errors(rv):
if not rv:
raise WinError()
def _replace_atomic(src, dst):
_handle_errors(
windll.kernel32.MoveFileExW(
src,
dst,
_windows_default_flags | _MOVEFILE_REPLACE_EXISTING,
)
)
def _move_atomic(src, dst):
_handle_errors(windll.kernel32.MoveFileExW(src, dst, _windows_default_flags))
def replace_atomic(src, dst):
"""
Move ``src`` to ``dst``. If ``dst`` exists, it will be silently
overwritten.
Both paths must reside on the same filesystem for the operation to be
atomic.
"""
return _replace_atomic(src, dst)
def move_atomic(src, dst):
"""
Move ``src`` to ``dst``. There might a timewindow where both filesystem
entries exist. If ``dst`` already exists, :py:exc:`FileExistsError` will be
raised.
Both paths must reside on the same filesystem for the operation to be
atomic.
"""
return _move_atomic(src, dst)
class AtomicWriter:
"""
A helper class for performing atomic writes. Usage::
with AtomicWriter(path).open() as f:
f.write(...)
:param path: The destination filepath. May or may not exist.
:param mode: The filemode for the temporary file. This defaults to `wb` in
Python 2 and `w` in Python 3.
:param overwrite: If set to false, an error is raised if ``path`` exists.
Errors are only raised after the file has been written to. Either way,
the operation is atomic.
:param open_kwargs: Keyword-arguments to pass to the underlying
:py:func:`open` call. This can be used to set the encoding when opening
files in text-mode.
If you need further control over the exact behavior, you are encouraged to
subclass.
"""
def __init__(self, path, mode="w", overwrite=False, **open_kwargs):
if "a" in mode:
raise ValueError(
"Appending to an existing file is not supported, because that "
"would involve an expensive `copy`-operation to a temporary "
"file. Open the file in normal `w`-mode and copy explicitly "
"if that's what you're after."
)
if "x" in mode:
raise ValueError("Use the `overwrite`-parameter instead.")
if "w" not in mode:
raise ValueError("AtomicWriters can only be written to.")
self._path = os.fspath(path)
self._mode = mode
self._overwrite = overwrite
self._open_kwargs = open_kwargs
def open(self):
"""Open the temporary file."""
return self._open(self.get_fileobject)
@contextlib.contextmanager
def _open(self, get_fileobject):
f = None # make sure f exists even if get_fileobject() fails
try:
success = False
with get_fileobject(**self._open_kwargs) as f:
yield f
self.sync(f)
self.commit(f)
success = True
finally:
if not success:
try:
self.rollback(f)
except Exception:
pass
def get_fileobject(
self, suffix="", prefix=tempfile.gettempprefix(), dir=None, **kwargs
):
"""Return the temporary file to use."""
if dir is None:
dir = os.path.normpath(os.path.dirname(self._path))
descriptor, name = tempfile.mkstemp(suffix=suffix, prefix=prefix, dir=dir)
# io.open() will take either the descriptor or the name, but we need
# the name later for commit()/replace_atomic() and couldn't find a way
# to get the filename from the descriptor.
os.close(descriptor)
kwargs["mode"] = self._mode
kwargs["file"] = name
return open(**kwargs)
def sync(self, f):
"""
Responsible for clearing as many file caches as possible before
commit.
"""
f.flush()
_proper_fsync(f.fileno())
def commit(self, f):
"""Move the temporary file to the target location."""
if self._overwrite:
replace_atomic(f.name, self._path)
else:
move_atomic(f.name, self._path)
def rollback(self, f):
"""Clean up all temporary resources."""
os.unlink(f.name)
def atomic_write(path, writer_cls=AtomicWriter, **cls_kwargs):
"""
Simple atomic writes. This wraps :py:class:`AtomicWriter`::
with atomic_write(path) as f:
f.write(...)
:param path: The target path to write to.
:param writer_cls: The writer class to use. This parameter is useful if you
subclassed :py:class:`AtomicWriter` to change some behavior and want to
use that new subclass.
Additional keyword arguments are passed to the writer class. See
:py:class:`AtomicWriter`.
"""
return writer_cls(path, **cls_kwargs).open()