More diff functionality (libgit2#629)

This PR adds

- The ability to apply a Diff object to the repo
- Support for git_apply_hunk_cb and git_apply_delta_cb callbacks in options for applying the diffs
- The ability to import a diff from a raw buffer (for example, one exported by ToBuf) into a Diff object associated with the repo
- Tests for the above

Author: mbfr

diff.go

@@ -3,6 +3,7 @@ package git
 /*
 #include <git2.h>
 
+extern void _go_git_populate_apply_cb(git_apply_options *options);
 extern int _go_git_diff_foreach(git_diff *diff, int eachFile, int eachHunk, int eachLine, void *payload);
 extern void _go_git_setup_diff_notify_callbacks(git_diff_options* opts);
 extern int _go_git_diff_blobs(git_blob *old, const char *old_path, git_blob *new, const char *new_path, git_diff_options *opts, int eachFile, int eachHunk, int eachLine, void *payload);
@@ -550,7 +551,7 @@ const (
 	DiffFindRemoveUnmodified            DiffFindOptionsFlag = C.GIT_DIFF_FIND_REMOVE_UNMODIFIED
 )
 
-//TODO implement git_diff_similarity_metric
+// TODO implement git_diff_similarity_metric
 type DiffFindOptions struct {
 	Flags                      DiffFindOptionsFlag
 	RenameThreshold            uint16
@@ -847,3 +848,174 @@ func DiffBlobs(oldBlob *Blob, oldAsPath string, newBlob *Blob, newAsPath string,
 
 	return nil
 }
+
+// ApplyHunkCallback is a callback that will be made per delta (file) when applying a patch.
+type ApplyHunkCallback func(*DiffHunk) (apply bool, err error)
+
+// ApplyDeltaCallback is a callback that will be made per hunk when applying a patch.
+type ApplyDeltaCallback func(*DiffDelta) (apply bool, err error)
+
+// ApplyOptions has 2 callbacks that are called for hunks or deltas
+// If these functions return an error, abort the apply process immediately.
+// If the first return value is true, the delta/hunk will be applied. If it is false, the  delta/hunk will not be applied. In either case, the rest of the apply process will continue.
+type ApplyOptions struct {
+	ApplyHunkCallback  ApplyHunkCallback
+	ApplyDeltaCallback ApplyDeltaCallback
+	Flags              uint
+}
+
+//export hunkApplyCallback
+func hunkApplyCallback(_hunk *C.git_diff_hunk, _payload unsafe.Pointer) C.int {
+	opts, ok := pointerHandles.Get(_payload).(*ApplyOptions)
+	if !ok {
+		panic("invalid apply options payload")
+	}
+
+	if opts.ApplyHunkCallback == nil {
+		return 0
+	}
+
+	hunk := diffHunkFromC(_hunk)
+
+	apply, err := opts.ApplyHunkCallback(&hunk)
+	if err != nil {
+		if gitError, ok := err.(*GitError); ok {
+			return C.int(gitError.Code)
+		}
+		return -1
+	} else if apply {
+		return 0
+	} else {
+		return 1
+	}
+}
+
+//export deltaApplyCallback
+func deltaApplyCallback(_delta *C.git_diff_delta, _payload unsafe.Pointer) C.int {
+	opts, ok := pointerHandles.Get(_payload).(*ApplyOptions)
+	if !ok {
+		panic("invalid apply options payload")
+	}
+
+	if opts.ApplyDeltaCallback == nil {
+		return 0
+	}
+
+	delta := diffDeltaFromC(_delta)
+
+	apply, err := opts.ApplyDeltaCallback(&delta)
+	if err != nil {
+		if gitError, ok := err.(*GitError); ok {
+			return C.int(gitError.Code)
+		}
+		return -1
+	} else if apply {
+		return 0
+	} else {
+		return 1
+	}
+}
+
+// DefaultApplyOptions returns default options for applying diffs or patches.
+func DefaultApplyOptions() (*ApplyOptions, error) {
+	opts := C.git_apply_options{}
+
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+
+	ecode := C.git_apply_options_init(&opts, C.GIT_APPLY_OPTIONS_VERSION)
+	if int(ecode) != 0 {
+
+		return nil, MakeGitError(ecode)
+	}
+
+	return applyOptionsFromC(&opts), nil
+}
+
+func (a *ApplyOptions) toC() *C.git_apply_options {
+	if a == nil {
+		return nil
+	}
+
+	opts := &C.git_apply_options{
+		version: C.GIT_APPLY_OPTIONS_VERSION,
+		flags:   C.uint(a.Flags),
+	}
+
+	if a.ApplyDeltaCallback != nil || a.ApplyHunkCallback != nil {
+		C._go_git_populate_apply_cb(opts)
+		opts.payload = pointerHandles.Track(a)
+	}
+
+	return opts
+}
+
+func applyOptionsFromC(opts *C.git_apply_options) *ApplyOptions {
+	return &ApplyOptions{
+		Flags: uint(opts.flags),
+	}
+}
+
+// ApplyLocation represents the possible application locations for applying
+// diffs.
+type ApplyLocation int
+
+const (
+	// ApplyLocationWorkdir applies the patch to the workdir, leaving the
+	// index untouched. This is the equivalent of `git apply` with no location
+	// argument.
+	ApplyLocationWorkdir ApplyLocation = C.GIT_APPLY_LOCATION_WORKDIR
+	// ApplyLocationIndex applies the patch to the index, leaving the working
+	// directory untouched. This is the equivalent of `git apply --cached`.
+	ApplyLocationIndex ApplyLocation = C.GIT_APPLY_LOCATION_INDEX
+	// ApplyLocationBoth applies the patch to both the working directory and
+	// the index. This is the equivalent of `git apply --index`.
+	ApplyLocationBoth ApplyLocation = C.GIT_APPLY_LOCATION_BOTH
+)
+
+// ApplyDiff appllies a Diff to the given repository, making changes directly
+// in the working directory, the index, or both.
+func (v *Repository) ApplyDiff(diff *Diff, location ApplyLocation, opts *ApplyOptions) error {
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+
+	cOpts := opts.toC()
+	ecode := C.git_apply(v.ptr, diff.ptr, C.git_apply_location_t(location), cOpts)
+	runtime.KeepAlive(v)
+	runtime.KeepAlive(diff)
+	runtime.KeepAlive(cOpts)
+	if ecode < 0 {
+		return MakeGitError(ecode)
+	}
+
+	return nil
+}
+
+// DiffFromBuffer reads the contents of a git patch file into a Diff object.
+//
+// The diff object produced is similar to the one that would be produced if you
+// actually produced it computationally by comparing two trees, however there
+// may be subtle differences. For example, a patch file likely contains
+// abbreviated object IDs, so the object IDs in a git_diff_delta produced by
+// this function will also be abbreviated.
+//
+// This function will only read patch files created by a git implementation, it
+// will not read unified diffs produced by the diff program, nor any other
+// types of patch files.
+func DiffFromBuffer(buffer []byte, repo *Repository) (*Diff, error) {
+	var diff *C.git_diff
+
+	cBuffer := C.CBytes(buffer)
+	defer C.free(unsafe.Pointer(cBuffer))
+
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+
+	ecode := C.git_diff_from_buffer(&diff, (*C.char)(cBuffer), C.size_t(len(buffer)))
+	if ecode < 0 {
+		return nil, MakeGitError(ecode)
+	}
+	runtime.KeepAlive(diff)
+
+	return newDiffFromC(diff, repo), nil
+}