go-gitdiff

A Go library for parsing and applying patches generated by git diff, git show, and git format-patch. It can also parse and apply unified diffs generated by the standard diff tool.

It supports standard line-oriented text patches and Git binary patches, and aims to parse anything accepted by the git apply command.

patch, err := os.Open("changes.patch")
if err != nil {
    log.Fatal(err)
}

// files is a slice of *gitdiff.File describing the files changed in the patch
// preamble is a string of the content of the patch before the first file
files, preamble, err := gitdiff.Parse(patch)
if err != nil {
    log.Fatal(err)
}

code, err := os.Open("code.go")
if err != nil {
    log.Fatal(err)
}

// apply the changes in the patch to a source file
var output bytes.Buffer
if err := gitdiff.Apply(&output, code, files[0]); err != nil {
    log.Fatal(err)
}

Development Status

The parsing API and types are complete and I expect will remain stable. As of March 2022, I am refactoring the apply API to address existing issues and to support non-strict application. Version 0.6.1 is the last release using the old API and the master branch may contain breaking changes. Please use a release version to avoid surprises.

Parsing and strict application are well-covered by unit tests and the library is used in a production application that parses and applies thousands of patches every day. However, the space of all possible patches is large, so there are likely undiscovered bugs.

The parsing code has also had a modest amount of fuzz testing.

Why another git/unified diff parser?

Several packages with similar functionality exist, so why did I write another?

1. No other packages I found support binary diffs, as generated with the --binary flag. This is the main reason for writing a new package, as the format is pretty different from line-oriented diffs and is unique to Git.

2. Most other packages only parse patches, so you need additional code to apply them (and if applies are supported, it is only for text files.)

3. This package aims to accept anything that git apply accepts, and closely follows the logic in apply.c.

4. It seemed like a fun project and a way to learn more about Git.

Differences From Git

1. Certain types of invalid input that are accepted by git apply generate errors. These include:

   • Numbers immediately followed by non-numeric characters
   • Trailing characters on a line after valid or expected content
   • Malformed file header lines (lines that start with diff --git)

2. Errors for invalid input are generally more verbose and specific than those from git apply.

3. The translation from C to Go may have introduced inconsistencies in the way Unicode file names are handled; these are bugs, so please report any issues of this type.

4. When reading headers, there is no validation that OIDs present on an index line are shorter than or equal to the maximum hash length, as this requires knowing if the repository used SHA1 or SHA256 hashes.

5. When reading "traditional" patches (those not produced by git), prefixes are not stripped from file names; git apply attempts to remove prefixes that match the current repository directory/prefix.

6. Patches can only be applied in "strict" mode, where the line numbers and context of each fragment must exactly match the source file; git apply implements a search algorithm that tries different lines and amounts of context, with further options to normalize or ignore whitespace changes.