feat: avoid pushing release branch only for rebasing (#114)

Right now releaser-pleaser pushes the branch even if it is only for a "rebase",
this wastes CI resources. Instead, it should only push when there are changes
to the files it owns.

- **Old**: Push when there is a diff origin/release-pr..release-pr
- **New**: Push when the these two diffs are not the same:
  
      origin/main..release-pr
      $(git merge-base origin/main origin/release-pr)..release-pr

Closes #92

go.mod

@@ -6,6 +6,7 @@ toolchain go1.24.4
 
 require (
 	github.com/blang/semver/v4 v4.0.0
+	github.com/go-git/go-billy/v5 v5.6.2
 	github.com/go-git/go-git/v5 v5.16.2
 	github.com/google/go-github/v72 v72.0.0
 	github.com/leodido/go-conventionalcommits v0.12.0
@@ -25,7 +26,6 @@ require (
 	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/emirpasic/gods v1.18.1 // indirect
 	github.com/go-git/gcfg v1.5.1-0.20230307220236-3a3c6141e376 // indirect
-	github.com/go-git/go-billy/v5 v5.6.2 // indirect
 	github.com/golang/groupcache v0.0.0-20241129210726-2c02b8208cf8 // indirect
 	github.com/google/go-querystring v1.1.0 // indirect
 	github.com/hashicorp/go-cleanhttp v0.5.2 // indirect

internal/git/git.go

@@ -193,8 +193,27 @@ func (r *Repository) Commit(_ context.Context, message string, author Author) (C
 	}, nil
 }
 
-func (r *Repository) HasChangesWithRemote(ctx context.Context, branch string) (bool, error) {
+// HasChangesWithRemote checks if the following two diffs are equal:
-	remoteRef, err := r.r.Reference(plumbing.NewRemoteReferenceName(remoteName, branch), false)
+//
+// - **Local**:                                 remote/main..branch
+// - **Remote**: (git merge-base remote/main remote/branch)..remote/branch
+//
+// This is done to avoid pushing when the only change would be a rebase of remote/branch onto the current remote/main.
+func (r *Repository) HasChangesWithRemote(ctx context.Context, mainBranch, prBranch string) (bool, error) {
+	return r.hasChangesWithRemote(ctx,
+		plumbing.NewRemoteReferenceName(remoteName, mainBranch),
+		plumbing.NewBranchReferenceName(prBranch),
+		plumbing.NewRemoteReferenceName(remoteName, prBranch),
+	)
+}
+
+func (r *Repository) hasChangesWithRemote(ctx context.Context, mainBranchRef, localPRBranchRef, remotePRBranchRef plumbing.ReferenceName) (bool, error) {
+	commitOnRemoteMain, err := r.commitFromRef(mainBranchRef)
+	if err != nil {
+		return false, err
+	}
+
+	commitOnRemotePRBranch, err := r.commitFromRef(remotePRBranchRef)
 	if err != nil {
 		if err.Error() == "reference not found" {
 			// No remote branch means that there are changes
@@ -204,29 +223,60 @@ func (r *Repository) HasChangesWithRemote(ctx context.Context, branch string) (b
 		return false, err
 	}
 
-	remoteCommit, err := r.r.CommitObject(remoteRef.Hash())
+	currentRemotePRMergeBase, err := r.mergeBase(commitOnRemoteMain, commitOnRemotePRBranch)
+	if err != nil {
+		return false, err
+	}
+	if currentRemotePRMergeBase == nil {
+		// If there is no merge base something weird has happened with the
+		// remote main branch, and we should definitely push updates.
+		return false, nil
+	}
+
+	remoteDiff, err := commitOnRemotePRBranch.PatchContext(ctx, currentRemotePRMergeBase)
 	if err != nil {
 		return false, err
 	}
 
-	localRef, err := r.r.Reference(plumbing.NewBranchReferenceName(branch), false)
+	commitOnLocalPRBranch, err := r.commitFromRef(localPRBranchRef)
 	if err != nil {
 		return false, err
 	}
 
-	localCommit, err := r.r.CommitObject(localRef.Hash())
+	localDiff, err := commitOnRemoteMain.PatchContext(ctx, commitOnLocalPRBranch)
 	if err != nil {
 		return false, err
 	}
 
-	diff, err := localCommit.PatchContext(ctx, remoteCommit)
+	return remoteDiff.String() == localDiff.String(), nil
+}
+
+func (r *Repository) commitFromRef(refName plumbing.ReferenceName) (*object.Commit, error) {
+	ref, err := r.r.Reference(refName, false)
 	if err != nil {
-		return false, err
+		return nil, err
 	}
 
-	hasChanges := len(diff.FilePatches()) > 0
+	commit, err := r.r.CommitObject(ref.Hash())
+	if err != nil {
+		return nil, err
+	}
 
-	return hasChanges, nil
+	return commit, nil
+}
+
+func (r *Repository) mergeBase(a, b *object.Commit) (*object.Commit, error) {
+	mergeBases, err := a.MergeBase(b)
+	if err != nil {
+		return nil, err
+	}
+
+	if len(mergeBases) == 0 {
+		return nil, nil
+	}
+
+	// :shrug: We dont really care which commit we pick, at worst we do an unnecessary push.
+	return mergeBases[0], nil
 }
 
 func (r *Repository) ForcePush(ctx context.Context, branch string) error {

internal/git/git_test.go

@@ -1,12 +1,15 @@
 package git
 
 import (
+	"context"
 	"reflect"
 	"strconv"
 	"testing"
 	"time"
 
+	"github.com/go-git/go-git/v5/plumbing"
 	"github.com/go-git/go-git/v5/plumbing/object"
+	"github.com/stretchr/testify/assert"
 )
 
 func TestAuthor_signature(t *testing.T) {
@@ -44,3 +47,126 @@ func TestAuthor_String(t *testing.T) {
 		})
 	}
 }
+
+const testMainBranch = "main"
+const testPRBranch = "releaser-pleaser"
+
+func TestRepository_HasChangesWithRemote(t *testing.T) {
+	// go-git/v5 has a bug where it tries to delete the repo root dir (".") multiple times if there is no file left in it.
+	// this happens while switching branches in worktree.go rmFileAndDirsIfEmpty.
+	// TODO: Fix bug upstream
+	// For now I just make sure that there is always at least one file left in the dir by adding an empty "README.md" in the test util.
+
+	mainBranchRef := plumbing.NewBranchReferenceName(testMainBranch)
+	localPRBranchRef := plumbing.NewBranchReferenceName(testPRBranch)
+	remotePRBranchRef := plumbing.NewBranchReferenceName("remote/" + testPRBranch)
+
+	tests := []struct {
+		name    string
+		repo    TestRepo
+		want    bool
+		wantErr assert.ErrorAssertionFunc
+	}{
+		{
+			name: "no remote pr branch",
+			repo: WithTestRepo(
+				WithCommit(
+					"chore: release v1.0.0",
+					WithFile("VERSION", "v1.0.0"),
+				),
+				WithCommit(
+					"chore: release v1.1.0",
+					OnBranch(mainBranchRef),
+					AsNewBranch(localPRBranchRef),
+					WithFile("VERSION", "v1.1.0"),
+				),
+			),
+			want:    true,
+			wantErr: assert.NoError,
+		},
+		{
+			name: "remote pr branch matches local",
+			repo: WithTestRepo(
+				WithCommit(
+					"chore: release v1.0.0",
+					WithFile("VERSION", "v1.0.0"),
+				),
+				WithCommit(
+					"chore: release v1.1.0",
+					OnBranch(mainBranchRef),
+					AsNewBranch(remotePRBranchRef),
+					WithFile("VERSION", "v1.1.0"),
+				),
+				WithCommit(
+					"chore: release v1.1.0",
+					OnBranch(mainBranchRef),
+					AsNewBranch(localPRBranchRef),
+					WithFile("VERSION", "v1.1.0"),
+				),
+			),
+			want:    false,
+			wantErr: assert.NoError,
+		},
+		{
+			name: "remote pr only needs rebase",
+			repo: WithTestRepo(
+				WithCommit(
+					"chore: release v1.0.0",
+					WithFile("VERSION", "v1.0.0"),
+				),
+				WithCommit(
+					"chore: release v1.1.0",
+					OnBranch(mainBranchRef),
+					AsNewBranch(remotePRBranchRef),
+					WithFile("VERSION", "v1.1.0"),
+				),
+				WithCommit(
+					"feat: new feature on remote",
+					OnBranch(mainBranchRef),
+					WithFile("feature", "yes"),
+				),
+				WithCommit(
+					"chore: release v1.1.0",
+					OnBranch(mainBranchRef),
+					AsNewBranch(localPRBranchRef),
+					WithFile("VERSION", "v1.1.0"),
+				),
+			),
+			want:    false,
+			wantErr: assert.NoError,
+		},
+		{
+			name: "needs update",
+			repo: WithTestRepo(
+				WithCommit(
+					"chore: release v1.0.0",
+					WithFile("VERSION", "v1.0.0"),
+				),
+				WithCommit(
+					"chore: release v1.1.0",
+					OnBranch(mainBranchRef),
+					AsNewBranch(remotePRBranchRef),
+					WithFile("VERSION", "v1.1.0"),
+				),
+				WithCommit(
+					"chore: release v1.2.0",
+					OnBranch(mainBranchRef),
+					AsNewBranch(localPRBranchRef),
+					WithFile("VERSION", "v1.2.0"),
+				),
+			),
+			want:    false,
+			wantErr: assert.NoError,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			repo := tt.repo(t)
+			got, err := repo.hasChangesWithRemote(context.Background(), mainBranchRef, localPRBranchRef, remotePRBranchRef)
+			if !tt.wantErr(t, err) {
+				return
+			}
+			assert.Equal(t, tt.want, got)
+		})
+	}
+}

internal/git/util_test.go

@@ -0,0 +1,189 @@
+package git
+
+import (
+	"fmt"
+	"io"
+	"log/slog"
+	"os"
+	"testing"
+	"time"
+
+	"github.com/go-git/go-billy/v5/memfs"
+	"github.com/go-git/go-git/v5"
+	"github.com/go-git/go-git/v5/plumbing"
+	"github.com/go-git/go-git/v5/plumbing/object"
+	"github.com/go-git/go-git/v5/storage/memory"
+	"github.com/stretchr/testify/require"
+)
+
+var (
+	author = &object.Signature{
+		Name: "releaser-pleaser",
+		When: time.Date(2020, 01, 01, 01, 01, 01, 01, time.UTC),
+	}
+)
+
+type CommitOption func(*commitOptions)
+type commitOptions struct {
+	cleanFiles bool
+	files      []commitFile
+	tags       []string
+	newRef     plumbing.ReferenceName
+	parentRef  plumbing.ReferenceName
+}
+type commitFile struct {
+	path    string
+	content string
+}
+
+type TestCommit func(*testing.T, *Repository) error
+type TestRepo func(*testing.T) *Repository
+
+func WithCommit(message string, options ...CommitOption) TestCommit {
+	return func(t *testing.T, repo *Repository) error {
+		t.Helper()
+
+		require.NotEmpty(t, message, "commit message is required")
+
+		opts := &commitOptions{}
+		for _, opt := range options {
+			opt(opts)
+		}
+
+		wt, err := repo.r.Worktree()
+		require.NoError(t, err)
+
+		if opts.parentRef != "" {
+			checkoutOptions := &git.CheckoutOptions{}
+
+			if opts.newRef != "" {
+				parentRef, err := repo.r.Reference(opts.parentRef, false)
+				require.NoError(t, err)
+
+				checkoutOptions.Create = true
+				checkoutOptions.Hash = parentRef.Hash()
+				checkoutOptions.Branch = opts.newRef
+			} else {
+				checkoutOptions.Branch = opts.parentRef
+			}
+
+			err = wt.Checkout(checkoutOptions)
+			require.NoError(t, err)
+		}
+
+		// Yeet all files
+		if opts.cleanFiles {
+			files, err := wt.Filesystem.ReadDir(".")
+			require.NoError(t, err, "failed to get current files")
+
+			for _, fileInfo := range files {
+				err = wt.Filesystem.Remove(fileInfo.Name())
+				require.NoError(t, err, "failed to remove file %q", fileInfo.Name())
+			}
+		}
+
+		// Create new files
+		for _, fileInfo := range opts.files {
+			file, err := wt.Filesystem.Create(fileInfo.path)
+			require.NoError(t, err, "failed to create file %q", fileInfo.path)
+
+			_, err = file.Write([]byte(fileInfo.content))
+			_ = file.Close()
+			require.NoError(t, err, "failed to write content to file %q", fileInfo.path)
+
+			_, err = wt.Add(fileInfo.path)
+			require.NoError(t, err, "failed to stage changes to file %q", fileInfo.path)
+
+		}
+
+		// Commit
+		commitHash, err := wt.Commit(message, &git.CommitOptions{
+			All:               true,
+			AllowEmptyCommits: true,
+			Author:            author,
+			Committer:         author,
+		})
+		require.NoError(t, err, "failed to commit")
+
+		// Create tags
+		for _, tagName := range opts.tags {
+			_, err = repo.r.CreateTag(tagName, commitHash, nil)
+			require.NoError(t, err, "failed to create tag %q", tagName)
+		}
+
+		return nil
+	}
+}
+
+func WithFile(path, content string) CommitOption {
+	return func(opts *commitOptions) {
+		opts.files = append(opts.files, commitFile{path: path, content: content})
+	}
+}
+
+// WithCleanFiles removes all previous files from the repo. Make sure to leave at least one file in the root
+// directory when switching branches!
+func WithCleanFiles() CommitOption {
+	return func(opts *commitOptions) {
+		opts.cleanFiles = true
+	}
+}
+
+func AsNewBranch(ref plumbing.ReferenceName) CommitOption {
+	return func(opts *commitOptions) {
+		opts.newRef = ref
+	}
+}
+
+func OnBranch(ref plumbing.ReferenceName) CommitOption {
+	return func(opts *commitOptions) {
+		opts.parentRef = ref
+	}
+}
+
+func WithTag(name string) CommitOption {
+	return func(opts *commitOptions) {
+		opts.tags = append(opts.tags, name)
+	}
+}
+
+// Can be useful to debug git issues by using it in a terminal
+const useOnDiskTestRepository = false
+
+func WithTestRepo(commits ...TestCommit) TestRepo {
+	return func(t *testing.T) *Repository {
+		t.Helper()
+
+		repo := &Repository{
+			logger: slog.New(slog.NewTextHandler(io.Discard, nil)),
+		}
+
+		var err error
+
+		initOptions := git.InitOptions{DefaultBranch: plumbing.Main}
+
+		if useOnDiskTestRepository {
+			dir, err := os.MkdirTemp(os.TempDir(), "rp-test-repo-")
+			require.NoError(t, err, "failed to create temp directory")
+
+			repo.r, err = git.PlainInitWithOptions(dir, &git.PlainInitOptions{InitOptions: initOptions})
+			require.NoError(t, err, "failed to create fs repository")
+
+			fmt.Printf("using temp directory: %s", dir)
+		} else {
+			repo.r, err = git.InitWithOptions(memory.NewStorage(), memfs.New(), initOptions)
+			require.NoError(t, err, "failed to create in-memory repository")
+		}
+
+		// Make initial commit
+		err = WithCommit("chore: init", WithFile("README.md", "# git test util"))(t, repo)
+		require.NoError(t, err, "failed to create init commit")
+
+		for i, commit := range commits {
+			err = commit(t, repo)
+			require.NoError(t, err, "failed to create commit %d", i)
+		}
+
+		return repo
+	}
+}

releaserpleaser.go

@@ -269,7 +269,7 @@ func (rp *ReleaserPleaser) runReconcileReleasePR(ctx context.Context) error {
 	logger.InfoContext(ctx, "created release commit", "commit.hash", releaseCommit.Hash, "commit.message", releaseCommit.Message, "commit.author", releaseCommitAuthor)
 
 	// Check if anything changed in comparison to the remote branch (if exists)
-	newReleasePRChanges, err := repo.HasChangesWithRemote(ctx, rpBranch)
+	newReleasePRChanges, err := repo.HasChangesWithRemote(ctx, rp.targetBranch, rpBranch)
 	if err != nil {
 		return err
 	}