Make merge command idempotent for safe retry after partial failures (bd-26)

- Added mergeResult struct to track operations (added vs skipped)
- Check if source issues already closed before attempting to close
- Track dependencies migrated vs already existed
- Count text references updated
- Display detailed breakdown of operations in output
- Updated help text to clarify idempotent behavior
- Added comprehensive tests for idempotent retry scenarios

cmd/bd/merge_test.go

@@ -180,3 +180,202 @@ func containsSubstring(s, substr string) bool {
 	}
 	return false
 }
+
+// TestPerformMergeIdempotent verifies that merge operations are idempotent
+func TestPerformMergeIdempotent(t *testing.T) {
+	tmpDir := t.TempDir()
+	dbFile := filepath.Join(tmpDir, ".beads", "issues.db")
+	if err := os.MkdirAll(filepath.Dir(dbFile), 0755); err != nil {
+		t.Fatalf("Failed to create test directory: %v", err)
+	}
+
+	testStore, err := sqlite.New(dbFile)
+	if err != nil {
+		t.Fatalf("Failed to create test storage: %v", err)
+	}
+	defer testStore.Close()
+
+	store = testStore
+	ctx := context.Background()
+
+	// Create test issues
+	issue1 := &types.Issue{
+		ID:          "bd-100",
+		Title:       "Target issue",
+		Description: "This is the target",
+		Priority:    1,
+		IssueType:   types.TypeTask,
+		Status:      types.StatusOpen,
+	}
+	issue2 := &types.Issue{
+		ID:          "bd-101",
+		Title:       "Source issue 1",
+		Description: "This mentions bd-100",
+		Priority:    1,
+		IssueType:   types.TypeTask,
+		Status:      types.StatusOpen,
+	}
+	issue3 := &types.Issue{
+		ID:          "bd-102",
+		Title:       "Source issue 2",
+		Description: "Another source",
+		Priority:    1,
+		IssueType:   types.TypeTask,
+		Status:      types.StatusOpen,
+	}
+
+	for _, issue := range []*types.Issue{issue1, issue2, issue3} {
+		if err := testStore.CreateIssue(ctx, issue, "test"); err != nil {
+			t.Fatalf("Failed to create issue %s: %v", issue.ID, err)
+		}
+	}
+
+	// Add a dependency from bd-101 to another issue
+	issue4 := &types.Issue{
+		ID:          "bd-103",
+		Title:       "Dependency target",
+		Description: "Dependency target",
+		Priority:    1,
+		IssueType:   types.TypeTask,
+		Status:      types.StatusOpen,
+	}
+	if err := testStore.CreateIssue(ctx, issue4, "test"); err != nil {
+		t.Fatalf("Failed to create issue4: %v", err)
+	}
+
+	dep := &types.Dependency{
+		IssueID:     "bd-101",
+		DependsOnID: "bd-103",
+		Type:        types.DepBlocks,
+	}
+	if err := testStore.AddDependency(ctx, dep, "test"); err != nil {
+		t.Fatalf("Failed to add dependency: %v", err)
+	}
+
+	// First merge - should complete successfully
+	result1, err := performMerge(ctx, "bd-100", []string{"bd-101", "bd-102"})
+	if err != nil {
+		t.Fatalf("First merge failed: %v", err)
+	}
+
+	if result1.issuesClosed != 2 {
+		t.Errorf("First merge: expected 2 issues closed, got %d", result1.issuesClosed)
+	}
+	if result1.issuesSkipped != 0 {
+		t.Errorf("First merge: expected 0 issues skipped, got %d", result1.issuesSkipped)
+	}
+	if result1.depsAdded == 0 {
+		t.Errorf("First merge: expected some dependencies added, got 0")
+	}
+
+	// Verify issues are closed
+	closed1, _ := testStore.GetIssue(ctx, "bd-101")
+	if closed1.Status != types.StatusClosed {
+		t.Errorf("bd-101 should be closed after first merge")
+	}
+	closed2, _ := testStore.GetIssue(ctx, "bd-102")
+	if closed2.Status != types.StatusClosed {
+		t.Errorf("bd-102 should be closed after first merge")
+	}
+
+	// Second merge (retry) - should be idempotent
+	result2, err := performMerge(ctx, "bd-100", []string{"bd-101", "bd-102"})
+	if err != nil {
+		t.Fatalf("Second merge (retry) failed: %v", err)
+	}
+
+	// All operations should be skipped
+	if result2.issuesClosed != 0 {
+		t.Errorf("Second merge: expected 0 issues closed, got %d", result2.issuesClosed)
+	}
+	if result2.issuesSkipped != 2 {
+		t.Errorf("Second merge: expected 2 issues skipped, got %d", result2.issuesSkipped)
+	}
+
+	// Dependencies should be skipped (already exist)
+	if result2.depsAdded != 0 {
+		t.Errorf("Second merge: expected 0 dependencies added, got %d", result2.depsAdded)
+	}
+
+	// Text references are naturally idempotent - count may vary
+	// (it will update again but result is the same)
+}
+
+// TestPerformMergePartialRetry tests retrying after partial failure
+func TestPerformMergePartialRetry(t *testing.T) {
+	tmpDir := t.TempDir()
+	dbFile := filepath.Join(tmpDir, ".beads", "issues.db")
+	if err := os.MkdirAll(filepath.Dir(dbFile), 0755); err != nil {
+		t.Fatalf("Failed to create test directory: %v", err)
+	}
+
+	testStore, err := sqlite.New(dbFile)
+	if err != nil {
+		t.Fatalf("Failed to create test storage: %v", err)
+	}
+	defer testStore.Close()
+
+	store = testStore
+	ctx := context.Background()
+
+	// Create test issues
+	issue1 := &types.Issue{
+		ID:          "bd-200",
+		Title:       "Target",
+		Description: "Target issue",
+		Priority:    1,
+		IssueType:   types.TypeTask,
+		Status:      types.StatusOpen,
+	}
+	issue2 := &types.Issue{
+		ID:          "bd-201",
+		Title:       "Source 1",
+		Description: "Source 1",
+		Priority:    1,
+		IssueType:   types.TypeTask,
+		Status:      types.StatusOpen,
+	}
+	issue3 := &types.Issue{
+		ID:          "bd-202",
+		Title:       "Source 2",
+		Description: "Source 2",
+		Priority:    1,
+		IssueType:   types.TypeTask,
+		Status:      types.StatusOpen,
+	}
+
+	for _, issue := range []*types.Issue{issue1, issue2, issue3} {
+		if err := testStore.CreateIssue(ctx, issue, "test"); err != nil {
+			t.Fatalf("Failed to create issue %s: %v", issue.ID, err)
+		}
+	}
+
+	// Simulate partial failure: manually close one source issue
+	if err := testStore.CloseIssue(ctx, "bd-201", "Manually closed", "test"); err != nil {
+		t.Fatalf("Failed to manually close bd-201: %v", err)
+	}
+
+	// Run merge - should handle one already-closed issue gracefully
+	result, err := performMerge(ctx, "bd-200", []string{"bd-201", "bd-202"})
+	if err != nil {
+		t.Fatalf("Merge with partial state failed: %v", err)
+	}
+
+	// Should skip the already-closed issue and close the other
+	if result.issuesClosed != 1 {
+		t.Errorf("Expected 1 issue closed, got %d", result.issuesClosed)
+	}
+	if result.issuesSkipped != 1 {
+		t.Errorf("Expected 1 issue skipped, got %d", result.issuesSkipped)
+	}
+
+	// Verify both are now closed
+	closed1, _ := testStore.GetIssue(ctx, "bd-201")
+	if closed1.Status != types.StatusClosed {
+		t.Errorf("bd-201 should remain closed")
+	}
+	closed2, _ := testStore.GetIssue(ctx, "bd-202")
+	if closed2.Status != types.StatusClosed {
+		t.Errorf("bd-202 should be closed")
+	}
+}