Reverse & Dry Run

Both are flags on Options:

type Options struct {
	Reverse bool // undo the patch instead of applying it
	DryRun  bool // validate everything, write nothing
}

Dry run

A dry run does all of phase 1 — reads every target and places every hunk in memory — then stops before writing. A nil error means a real apply would succeed; the Result describes what would happen.

res, err := patchapply.Apply(fsys, files, &patchapply.Options{DryRun: true})
if err != nil {
	log.Fatal("won't apply cleanly:", err) // ErrConflict, ErrMissing, ...
}
for _, f := range res.Files {
	fmt.Printf("would %s %s\n", f.Status, f.Path)
}

Use it to check a mailed patch against a working tree before committing to it, or to show the user a preview.

Reverse

Reverse inverts each change before applying, so applying with Reverse: true undoes the same patch:

• additions become deletions and vice versa;
• a hunk's old/new line ranges swap;
• Added ↔ Deleted; a Renamed's paths swap; undoing a Copied removes the copy.

// Apply, then roll back.
patchapply.Apply(fsys, files, nil)
patchapply.Apply(fsys, files, &patchapply.Options{Reverse: true})
// fsys is back to its original state.

Reverse + dry run compose: Options{Reverse: true, DryRun: true} checks whether a patch can be cleanly rolled back without changing anything.