Atomic file swapping

Sadly, this is not a nuclear-powered utility to swap files.

atomicswap is a Python module that implements the swapping of two files on a filesystem in a single operation that can't be broken up; either the entire operation completes correctly or none of it completes. This prevents the filesystem from being left in an inconsistent state and avoids certain race conditions.

The API is very simple; only a single swap() function is provided. The function takes two file paths for the two files to be swapped. In the event that either path is a relative path, you may also provide file descriptors for directories that the relative paths should start from; if either is missing then the path is relative to the current working directory. Paths can be provided either as Python strings or pathlib paths.

Installation

The latest stable version of atomicswap can be installed from the Python Package Index using pip in the normal manner:

pip install atomicswap

If you want to try out the latest code then you can install it from the GitHub repository:

pip install atomicswap@git+https://github.com/nickovs/atomicswap.git

Example

Swapping the files /etc/something/active and /etc/something/standby in a single operation can be performed as follows:

from atomicswap import swap
...
swap("/etc/something/active", "/etc/something/standby")

Alternatively, if using Path objects, this could be implemented as:

from pathlib import Path
from atomicswap import swap
...
base_dir = Path("/etc/something")
swap(base_dir / "active", base_dir / "standby")

Platform support

Currently atomicswap supports Linux, macOS and Windows. Note that the Windows implementation does not support specifying the base directories for relative paths using directory file descriptors.

Dependencies

On both macOS and Linux atomicswap is not dependent on any non-standard libraries or third party packages. On Windows it requires pywin32.

Implementation details

Both Linux and macOS have kernel system calls that provide the simultaneous, atomic swapping of the names of two files. On Linux the system call is named renameat2 while on macOS it is named renameatx_np, but the operation is much the same: passing a specific flag to the extended version of the rename function causes it to swap the names of two existing files rather than just changing the name of one file. On macOS the renameatx_np is exposed in the standard C library and can be called directly. Not all Linux distributions expose the renameat2 system call in their C library so the syscall wrapper function is used instead.

While there is no equivalent single function to perform the same operation on Windows, it is possible to perform an atomic swap operation using the Windows Kernel Transaction Manager and Transactional NTFS. Unfortunately Microsoft have stated that "TxF may not be available in future versions of Microsoft Windows", which potentially limits the utility of this sort of implementation.

License

atomicswap is released under the MIT license. See LICENSE.md for details.