cowpatch

This is a "copy-on-write" patch application tool. Think "overlayfs for patching big source trees, but doesn't need Linux or root". Its intended audience is people who need to script building of large software with patches applied, especially for testing different iterations or combinations of patches. cowpatch allows you to keep a single "pristine" tree of the upstream source, and do all the work on ultrathin patched clones.

Please be aware that the "copy-on-write" property applies only at patch time. If the build process modifies its own tree in any way, it can and will clobber the pristine tree. Use of cowpatch is recommended only with software that's capable of "out-of-tree" builds.

Warning

THIS IS EXPERIMENTAL SOFTWARE. It does not perform any fancy checks like patch would do before acting on a file. Malformed or malicious diffs, or even just bugs, may cause it to perform destructive acts outsie of the tree you intend to patch.

Usage

cowpatch should be run with the same options you'd pass to patch (especially -pN as appropriate) in a directory initially populated with nothing but symlinks to entries in the pristine tree.

In addition, cowpatch-specific options -I and -S are available to initialize or manually split files in a copy-on-write tree.

Example

Do something like the following to make a new cow clone and apply a patch:

mkdir gcc-patched
cd gcc-patched
cowpatch.sh -I ../gcc-orig
cowpatch.sh -p1 < ../gcc-changes.diff

Here, the cowpatch.sh -I ../gcc-orig command behaves like ln -s ../gcc-orig/* . except that it correctly handles hidden files (ones whose names start with .).

Note that it's important to start with a directory full of symlinks, not just a symlink to the base directory. The following will NOT work:

ln -s gcc-orig gcc-patched
cd gcc-patched
...

because the cd simply changes into the symlink target directory.

To manually split a particular file (e.g. for editing) without a patch that modifies it, you can use the -S option. For example:

cowpatch.sh -S configure

Notes

You can apply more than one patch this way. Application of each additional patch will "cow" split only the directories/files it applies to.