feat: avoid pushing release branch only for rebasing

internal/git/git.go

@@ -170,8 +170,19 @@ func (r *Repository) Commit(_ context.Context, message string) (Commit, error) {
 	}, nil
 }
 
-func (r *Repository) HasChangesWithRemote(ctx context.Context, branch string) (bool, error) {
-	remoteRef, err := r.r.Reference(plumbing.NewRemoteReferenceName(remoteName, branch), false)
+// HasChangesWithRemote checks if the following two diffs are equal:
+//
+// - **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) {
+	commitOnRemoteMain, err := r.commitFromRef(plumbing.NewRemoteReferenceName(remoteName, mainBranch))
+	if err != nil {
+		return false, err
+	}
+
+	commitOnRemotePRBranch, err := r.commitFromRef(plumbing.NewRemoteReferenceName(remoteName, prBranch))
 	if err != nil {
 		if err.Error() == "reference not found" {
 			// No remote branch means that there are changes
@@ -181,29 +192,57 @@ 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
+	}
 
-	localRef, err := r.r.Reference(plumbing.NewBranchReferenceName(branch), false)
+	remoteDiff, err := currentRemotePRMergeBase.PatchContext(ctx, commitOnRemotePRBranch)
 	if err != nil {
 		return false, err
 	}
 
-	localCommit, err := r.r.CommitObject(localRef.Hash())
+	commitOnLocalPRBranch, err := r.commitFromRef(plumbing.NewBranchReferenceName(prBranch))
+
+	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 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
+	}
 
-	return hasChanges, 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

@@ -0,0 +1,131 @@
+package git
+
+import (
+	"context"
+	"testing"
+
+	"github.com/go-git/go-git/v5/plumbing"
+	"github.com/stretchr/testify/assert"
+)
+
+const testMainBranch = "main"
+const testPRBranch = "releaser-pleaser"
+
+func TestRepository_HasChangesWithRemote(t *testing.T) {
+	tests := []struct {
+		name    string
+		repo    TestRepo
+		want    bool
+		wantErr assert.ErrorAssertionFunc
+	}{
+		{
+			name: "no remote pr branch",
+			repo: WithTestRepo(
+				WithCommit(
+					"chore: release v1.0.0",
+					OnBranch(plumbing.NewBranchReferenceName(testMainBranch)),
+					AsNewBranch(plumbing.NewRemoteReferenceName(remoteName, testMainBranch)),
+					WithFile("VERSION", "v1.0.0"),
+				),
+				WithCommit(
+					"chore: release v1.1.0",
+					OnBranch(plumbing.NewRemoteReferenceName(remoteName, testMainBranch)),
+					AsNewBranch(plumbing.NewBranchReferenceName(testPRBranch)),
+					WithFile("VERSION", "v1.1.0"),
+				),
+			),
+			want:    true,
+			wantErr: assert.NoError,
+		},
+		{
+			name: "remote pr branch matches local",
+			repo: WithTestRepo(
+				WithCommit(
+					"chore: release v1.0.0",
+					OnBranch(plumbing.NewBranchReferenceName(testMainBranch)),
+					AsNewBranch(plumbing.NewRemoteReferenceName(remoteName, testMainBranch)),
+					WithFile("VERSION", "v1.0.0"),
+				),
+				WithCommit(
+					"chore: release v1.1.0",
+					OnBranch(plumbing.NewRemoteReferenceName(remoteName, testMainBranch)),
+					AsNewBranch(plumbing.NewBranchReferenceName(testPRBranch)),
+					WithFile("VERSION", "v1.1.0"),
+				),
+				WithCommit(
+					"chore: release v1.1.0",
+					OnBranch(plumbing.NewRemoteReferenceName(remoteName, testMainBranch)),
+					AsNewBranch(plumbing.NewRemoteReferenceName(remoteName, testPRBranch)),
+					WithFile("VERSION", "v1.1.0"),
+				),
+			),
+			want:    false,
+			wantErr: assert.NoError,
+		},
+		{
+			name: "remote pr only needs rebase",
+			repo: WithTestRepo(
+				WithCommit(
+					"chore: release v1.0.0",
+					OnBranch(plumbing.NewBranchReferenceName(testMainBranch)),
+					AsNewBranch(plumbing.NewRemoteReferenceName(remoteName, testMainBranch)),
+					WithFile("VERSION", "v1.0.0"),
+				),
+				WithCommit(
+					"chore: release v1.1.0",
+					OnBranch(plumbing.NewRemoteReferenceName(remoteName, testMainBranch)),
+					AsNewBranch(plumbing.NewRemoteReferenceName(remoteName, testPRBranch)),
+					WithFile("VERSION", "v1.1.0"),
+				),
+				WithCommit(
+					"feat: new feature on remote",
+					OnBranch(plumbing.NewRemoteReferenceName(remoteName, testMainBranch)),
+					WithFile("feature", "yes"),
+				),
+				WithCommit(
+					"chore: release v1.1.0",
+					OnBranch(plumbing.NewRemoteReferenceName(remoteName, testMainBranch)),
+					AsNewBranch(plumbing.NewBranchReferenceName(testPRBranch)),
+					WithFile("VERSION", "v1.1.0"),
+				),
+			),
+			want:    false,
+			wantErr: assert.NoError,
+		},
+		{
+			name: "needs update",
+			repo: WithTestRepo(
+				WithCommit(
+					"chore: release v1.0.0",
+					OnBranch(plumbing.NewBranchReferenceName(testMainBranch)),
+					AsNewBranch(plumbing.NewRemoteReferenceName(remoteName, testMainBranch)),
+					WithFile("VERSION", "v1.0.0"),
+				),
+				WithCommit(
+					"chore: release v1.1.0",
+					OnBranch(plumbing.NewRemoteReferenceName(remoteName, testMainBranch)),
+					AsNewBranch(plumbing.NewRemoteReferenceName(remoteName, testPRBranch)),
+					WithFile("VERSION", "v1.1.0"),
+				),
+				WithCommit(
+					"chore: release v1.2.0",
+					OnBranch(plumbing.NewRemoteReferenceName(remoteName, testMainBranch)),
+					AsNewBranch(plumbing.NewBranchReferenceName(testPRBranch)),
+					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(), testMainBranch, testPRBranch)
+			if !tt.wantErr(t, err) {
+				return
+			}
+			assert.Equal(t, got, tt.want)
+		})
+	}
+}