Add API support for creating a PR with an empty commit

applier.go

@@ -32,7 +32,8 @@ type Applier struct {
 	entries     map[string]*github.TreeEntry
 	uncommitted bool
 
-	applyOptions []gitdiff.ApplyOption
+	applyOptions      []gitdiff.ApplyOption
+	allowEmptyCommits bool
 }
 
 // NewApplier creates a new Applier for a repository. The Applier applies
@@ -53,6 +54,13 @@ func (a *Applier) SetApplyOptions(opts ...gitdiff.ApplyOption) {
 	a.applyOptions = opts
 }
 
+// SetAllowEmptyCommits controls whether Commit may create a commit without
+// pending changes. Empty commits reuse the current tree. The default is false.
+// This setting is preserved across calls to Commit and Reset.
+func (a *Applier) SetAllowEmptyCommits(allow bool) {
+	a.allowEmptyCommits = allow
+}
+
 // Apply applies the changes in a file, adds the result to the list of pending
 // tree entries, and returns the entry. If the application succeeds, Apply
 // creates a blob in the repository with the modified content.
@@ -231,8 +239,8 @@ func (a *Applier) CreateTree(ctx context.Context) (*github.Tree, error) {
 
 // Commit commits the latest tree, optionally using the details in tmpl and
 // header. If there are pending tree entries, it calls CreateTree before
-// creating the commit. It returns an error if there are no pending trees or
-// tree entries.
+// creating the commit. If there are no pending trees or tree entries, it returns
+// an error unless empty commits are enabled with [Applier.SetAllowEmptyCommits].
 //
 // If tmpl is not nil, Commit uses it as a template for the new commit,
 // overwriting fields as needed. If header is not nil, Commit uses it to set
@@ -243,7 +251,7 @@ func (a *Applier) CreateTree(ctx context.Context) (*github.Tree, error) {
 // message, the current time, and the authenticated user as needed for the
 // commit details.
 func (a *Applier) Commit(ctx context.Context, tmpl *github.Commit, header *gitdiff.PatchHeader) (*github.Commit, error) {
-	if !a.uncommitted && len(a.entries) == 0 {
+	if !a.allowEmptyCommits && !a.uncommitted && len(a.entries) == 0 {
 		return nil, errors.New("no pending tree or tree entries")
 	}
 	if len(a.entries) > 0 {

applier_test.go

@@ -53,6 +53,35 @@ func TestApplier(t *testing.T) {
 	}
 }
 
+func TestApplierSupportsEmptyCommits(t *testing.T) {
+	tctx := prepareTestContext(t)
+
+	t.Logf("Test ID: %s", tctx.ID)
+	t.Logf("Test Repository: %s", tctx.Repo.String())
+
+	createBranch(t, tctx)
+	defer cleanupBranches(t, tctx)
+
+	applier := NewApplier(tctx.Client, tctx.Repo, tctx.BaseCommit)
+	applier.SetAllowEmptyCommits(true)
+
+	commit, err := applier.Commit(tctx, nil, &gitdiff.PatchHeader{Title: "empty"})
+	if err != nil {
+		t.Fatalf("error committing changes form empty patch: %v", err)
+	}
+	// calling "Commit" on the applier without making any calls to "Apply" to apply patches should create a new empty
+	// commit. The returned commit should be different from the base commit, its tree should be the same.
+	if commit.GetSHA() == tctx.BaseCommit.GetSHA() {
+		t.Errorf("empty commit has same SHA as base commit: %s", commit.GetSHA())
+	}
+	if len(commit.Parents) != 1 || commit.Parents[0].GetSHA() != tctx.BaseCommit.GetSHA() {
+		t.Errorf("empty commit does not have base commit as its parent: %v", commit.Parents)
+	}
+	if commit.GetTree().GetSHA() != tctx.BaseTree.GetSHA() {
+		t.Errorf("empty commit changed tree: expected %s, actual %s", tctx.BaseTree.GetSHA(), commit.GetTree().GetSHA())
+	}
+}
+
 type TestContext struct {
 	context.Context