backfill ontoOldBase for deleted merged branches

Author: skarim

cmd/rebase.go

@@ -188,6 +188,17 @@ func runRebase(cfg *config.Config, opts *rebaseOptions) error {
 		return ErrSilent
 	}
 
+	// Backfill originalRefs for merged branches that were deleted locally.
+	// The rebase loop uses originalRefs[br.Branch] as ontoOldBase; without
+	// a valid entry the subsequent --onto rebase would receive an empty ref.
+	for _, b := range s.Branches {
+		if b.IsMerged() && !git.BranchExists(b.Branch) {
+			if b.Head != "" {
+				originalRefs[b.Branch] = b.Head
+			}
+		}
+	}
+
 	// Track --onto rebase state for merged branches.
 	needsOnto := false
 	var ontoOldBase string

cmd/rebase_test.go

@@ -1044,11 +1044,12 @@ func TestRebase_BranchDiverged_NoFF(t *testing.T) {
 
 func TestRebase_SkipsMergedBranchesNotExistingLocally(t *testing.T) {
 	// Simulates a stack where b1 is merged and its branch was auto-deleted
-	// from the remote, so it doesn't exist locally.
+	// from the remote, so it doesn't exist locally. The stored Head SHA is
+	// used as ontoOldBase for the next branch's --onto rebase.
 	s := stack.Stack{
 		Trunk: stack.BranchRef{Branch: "main"},
 		Branches: []stack.BranchRef{
-			{Branch: "b1", PullRequest: &stack.PullRequestRef{Number: 42, Merged: true}},
+			{Branch: "b1", Head: "b1-stored-head-sha", PullRequest: &stack.PullRequestRef{Number: 42, Merged: true}},
 			{Branch: "b2"},
 		},
 	}
@@ -1095,7 +1096,10 @@ func TestRebase_SkipsMergedBranchesNotExistingLocally(t *testing.T) {
 	assert.NoError(t, err)
 	assert.Contains(t, output, "Skipping b1")
 
-	// Only b2 should be rebased
+	// Only b2 should be rebased, and the rebase should use b1's stored
+	// Head SHA as oldBase so `git rebase --onto` receives valid arguments.
 	require.Len(t, rebaseCalls, 1)
 	assert.Equal(t, "b2", rebaseCalls[0].branch)
+	assert.Equal(t, "main", rebaseCalls[0].newBase)
+	assert.Equal(t, "b1-stored-head-sha", rebaseCalls[0].oldBase)
 }

cmd/sync.go

@@ -142,6 +142,20 @@ func runSync(cfg *config.Config, opts *syncOptions) error {
 		}
 		originalRefs, _ := git.RevParseMap(branchNames)
 
+		// Backfill originalRefs for merged branches that were deleted locally.
+		// The rebase loop uses originalRefs[br.Branch] as ontoOldBase; without
+		// a valid entry the subsequent --onto rebase would receive an empty ref.
+		for _, b := range s.Branches {
+			if b.IsMerged() && !git.BranchExists(b.Branch) {
+				if b.Head != "" {
+					if originalRefs == nil {
+						originalRefs = make(map[string]string)
+					}
+					originalRefs[b.Branch] = b.Head
+				}
+			}
+		}
+
 		needsOnto := false
 		var ontoOldBase string
 

cmd/sync_test.go

@@ -848,7 +848,7 @@ func TestSync_MergedBranchDeletedFromRemote(t *testing.T) {
 	s := stack.Stack{
 		Trunk: stack.BranchRef{Branch: "main"},
 		Branches: []stack.BranchRef{
-			{Branch: "b1", PullRequest: &stack.PullRequestRef{Number: 1, Merged: true}},
+			{Branch: "b1", Head: "b1-stored-head-sha", PullRequest: &stack.PullRequestRef{Number: 1, Merged: true}},
 			{Branch: "b2"},
 		},
 	}
@@ -915,7 +915,10 @@ func TestSync_MergedBranchDeletedFromRemote(t *testing.T) {
 	assert.NoError(t, err)
 	assert.Contains(t, output, "Skipping b1")
 
-	// Only b2 should be rebased
+	// Only b2 should be rebased, and the rebase should use b1's stored
+	// Head SHA as oldBase so `git rebase --onto` receives valid arguments.
 	require.Len(t, rebaseOntoCalls, 1)
 	assert.Equal(t, "b2", rebaseOntoCalls[0].branch)
+	assert.Equal(t, "main", rebaseOntoCalls[0].newBase)
+	assert.Equal(t, "b1-stored-head-sha", rebaseOntoCalls[0].oldBase)
 }