I use the Unison file synchronization tool to keep
files between multiple machines and servers up to date. However, I wanted to customize some
aspects of Unison’s behavior. Because I did not want to make invasive changes to its
codebase, I decided to amend Unison’s functionality from the outside by intercepting its use
of the C-level libSystem (libc on Linux and other Unixes) APIs.
This project provides a libintercept.dylib library, which re-exports all of libSystem
and can therefore replace it. Selected API calls are replaced or extended with my own
functionality.
If you compile the project either in Xcode or using xcodebuild, make sure to point the
UNISON_PATH build setting to your Unison.app bundle. By default, the build system is
configured to my personal needs. A release build will place libintercept.dylib inside the
Unison bundle and modify Unison with install_name_tool to link against
libintercept.dylib instead of libSystem. The build will also attempt to sign the
resulting executables.
For Linux and other Unixes, the enclosed Makefile builds and installs an equivalent
libintercept.so, which can be activated by setting the LD_PRELOAD environment variable
when launching Unison.
Currently, six intercept layers are provided, which add the following features to Unison:
nocache
Cause all writes performed by Unison to bypass the buffer cache. This has two advantages: It
avoids polluting the cache if Unison handles large files and it improves data safety, as the
subsequent read check done by Unison will read from the physical storage medium and not from
the cache.
config
As Unison reads its configuration files, this intercept layer parses them and extracts
additional configuration options used by other intercepts. All additional options start with
# and therefore look like comments to the normal Unison parser.
encrypt
Files are encrypted after local reads and decrypted before local writes. This ensures that
Unison operates on encrypted data when transferring file content to servers. The encryption
key can be configured using #encrypt = Path PATH -> aes-256-gcm:SECRET directives.
prepost
Runs pre and post processing commands. Global pre and post commands, which execute once
synchronization starts and completes, are configured as #precmd = COMMAND and
#postcmd = COMMAND. Lines of the form #post = Path PATH -> COMMAND cause a command to be
executed whenever a specific file has been changed.
symlink
Creates symlinks for a specified path name before they are traversed by Unison. The path and
link content are configured using #symlink = Path PATH -> TARGET. Symlinks are only
created within the first Unison root located below the current home directory.
umask
Files created in the user’s home directory employ a umask of 0700. This restriction does
not apply to subdirectories or explicit permission changes with chmod.
This work is licensed under the GNU GPLv3.