fix(sync): respect sync.branch config when no upstream tracking (#640)

* fix(sync): respect sync.branch config when no upstream tracking

When sync.branch is explicitly configured, bd sync should use worktree-based
sync even if the current branch has no upstream tracking. This fixes the
issue where jj (Jujutsu) colocated repos and git worktrees without upstream
would incorrectly fall back to --from-main mode.

The fix checks for sync.branch configuration BEFORE the upstream check,
allowing the configured sync branch to take precedence.

Affected workflows:
- jj colocated repos (always use detached HEAD)
- Git worktrees not tracking a remote
- Temporary checkouts for testing

Fixes #638

* test(sync): add regression tests for sync.branch priority over upstream check

Add tests verifying that when sync.branch is configured, bd sync does NOT
fall back to --from-main mode even without upstream tracking. This covers:

- sync.branch configured without upstream (should use worktree sync)
- No sync.branch and no upstream (should fallback to from-main)
- Detached HEAD with sync.branch (jj workflow, should use worktree sync)

These tests ensure the fix for GH#638 doesn't regress.

---------

Co-authored-by: Charles P. Cross <cpdata@users.noreply.github.com>

cmd/bd/sync.go

@@ -179,9 +179,20 @@ Use --merge to merge the sync branch back to main branch.`,
 			os.Exit(1)
 		}
 
+		// GH#638: Check sync.branch BEFORE upstream check
+		// When sync.branch is configured, we should use worktree-based sync even if
+		// the current branch has no upstream (e.g., detached HEAD in jj, git worktrees)
+		var hasSyncBranchConfig bool
+		if err := ensureStoreActive(); err == nil && store != nil {
+			if syncBranch, _ := syncbranch.Get(ctx, store); syncBranch != "" {
+				hasSyncBranchConfig = true
+			}
+		}
+
 		// Preflight: check for upstream tracking
 		// If no upstream, automatically switch to --from-main mode (gt-ick9: ephemeral branch support)
-		if !noPull && !gitHasUpstream() {
+		// GH#638: Skip this fallback if sync.branch is explicitly configured
+		if !noPull && !gitHasUpstream() && !hasSyncBranchConfig {
 			if hasGitRemote(ctx) {
 				// Remote exists but no upstream - use from-main mode
 				fmt.Println("→ No upstream configured, using --from-main mode")

cmd/bd/sync_branch_priority_test.go

@@ -0,0 +1,148 @@
+package main
+
+import (
+	"context"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"testing"
+
+	"github.com/steveyegge/beads/internal/storage/sqlite"
+	"github.com/steveyegge/beads/internal/syncbranch"
+)
+
+// TestSyncBranchConfigPriorityOverUpstream tests that when sync.branch is configured,
+// bd sync should NOT fall back to --from-main mode even if the current branch has no upstream.
+// This is the regression test for GH#638.
+func TestSyncBranchConfigPriorityOverUpstream(t *testing.T) {
+	if testing.Short() {
+		t.Skip("Skipping integration test in short mode")
+	}
+
+	ctx := context.Background()
+
+	t.Run("sync.branch configured without upstream should not fallback to from-main", func(t *testing.T) {
+		// Setup: Create a git repo with no upstream tracking
+		tmpDir, cleanup := setupGitRepo(t)
+		defer cleanup()
+
+		// Create beads database and configure sync.branch
+		beadsDir := filepath.Join(tmpDir, ".beads")
+		if err := os.MkdirAll(beadsDir, 0750); err != nil {
+			t.Fatalf("Failed to create .beads dir: %v", err)
+		}
+
+		dbPath := filepath.Join(beadsDir, "beads.db")
+		testStore, err := sqlite.New(ctx, dbPath)
+		if err != nil {
+			t.Fatalf("Failed to create test database: %v", err)
+		}
+		defer testStore.Close()
+
+		// Configure sync.branch
+		if err := syncbranch.Set(ctx, testStore, "beads-sync"); err != nil {
+			t.Fatalf("Failed to set sync.branch: %v", err)
+		}
+
+		// Verify sync.branch is configured
+		syncBranch, err := syncbranch.Get(ctx, testStore)
+		if err != nil {
+			t.Fatalf("Failed to get sync.branch: %v", err)
+		}
+		if syncBranch != "beads-sync" {
+			t.Errorf("Expected sync.branch='beads-sync', got %q", syncBranch)
+		}
+
+		// Verify we have no upstream
+		if gitHasUpstream() {
+			t.Skip("Test requires no upstream tracking")
+		}
+
+		// The key assertion: hasSyncBranchConfig should be true
+		// which prevents fallback to from-main mode
+		var hasSyncBranchConfig bool
+		if syncBranch != "" {
+			hasSyncBranchConfig = true
+		}
+
+		if !hasSyncBranchConfig {
+			t.Error("hasSyncBranchConfig should be true when sync.branch is configured")
+		}
+
+		// With the fix, this condition should be false (should NOT fallback)
+		shouldFallbackToFromMain := !gitHasUpstream() && !hasSyncBranchConfig
+		if shouldFallbackToFromMain {
+			t.Error("Should NOT fallback to from-main when sync.branch is configured")
+		}
+	})
+
+	t.Run("no sync.branch and no upstream should fallback to from-main", func(t *testing.T) {
+		// Setup: Create a git repo with no upstream tracking
+		_, cleanup := setupGitRepo(t)
+		defer cleanup()
+
+		// No sync.branch configured, no upstream
+		hasSyncBranchConfig := false
+
+		// Verify we have no upstream
+		if gitHasUpstream() {
+			t.Skip("Test requires no upstream tracking")
+		}
+
+		// With no sync.branch, should fallback to from-main
+		shouldFallbackToFromMain := !gitHasUpstream() && !hasSyncBranchConfig
+		if !shouldFallbackToFromMain {
+			t.Error("Should fallback to from-main when no sync.branch and no upstream")
+		}
+	})
+
+	t.Run("detached HEAD with sync.branch should not fallback", func(t *testing.T) {
+		// Setup: Create a git repo and detach HEAD (simulating jj workflow)
+		tmpDir, cleanup := setupGitRepo(t)
+		defer cleanup()
+
+		// Get current commit hash
+		cmd := exec.Command("git", "rev-parse", "HEAD")
+		output, err := cmd.Output()
+		if err != nil {
+			t.Fatalf("Failed to get HEAD: %v", err)
+		}
+		commitHash := string(output[:len(output)-1]) // trim newline
+
+		// Detach HEAD
+		cmd = exec.Command("git", "checkout", "--detach", commitHash)
+		if err := cmd.Run(); err != nil {
+			t.Fatalf("Failed to detach HEAD: %v", err)
+		}
+
+		// Create beads database and configure sync.branch
+		beadsDir := filepath.Join(tmpDir, ".beads")
+		if err := os.MkdirAll(beadsDir, 0750); err != nil {
+			t.Fatalf("Failed to create .beads dir: %v", err)
+		}
+
+		dbPath := filepath.Join(beadsDir, "beads.db")
+		testStore, err := sqlite.New(ctx, dbPath)
+		if err != nil {
+			t.Fatalf("Failed to create test database: %v", err)
+		}
+		defer testStore.Close()
+
+		// Configure sync.branch
+		if err := syncbranch.Set(ctx, testStore, "beads-sync"); err != nil {
+			t.Fatalf("Failed to set sync.branch: %v", err)
+		}
+
+		// Verify detached HEAD has no upstream
+		if gitHasUpstream() {
+			t.Error("Detached HEAD should not have upstream")
+		}
+
+		// With sync.branch configured, should NOT fallback
+		hasSyncBranchConfig := true
+		shouldFallbackToFromMain := !gitHasUpstream() && !hasSyncBranchConfig
+		if shouldFallbackToFromMain {
+			t.Error("Detached HEAD with sync.branch should NOT fallback to from-main")
+		}
+	})
+}