fix: branch resolution in worktrees

cmd/post_checkout.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"fmt"
 	"os"
+	"path/filepath"
 	"strings"
 
 	"github.com/spf13/cobra"
@@ -20,7 +21,7 @@ func postCheckoutCmd() *cobra.Command {
 				return nil
 			}
 
-			branch, err := resolveBranch()
+			branch, err := resolveBranch(".")
 			if err != nil {
 				return err
 			}
@@ -38,10 +39,14 @@ func postCheckoutCmd() *cobra.Command {
 	}
 }
 
-func resolveBranch() (string, error) {
-	data, err := os.ReadFile(".git/HEAD")
+func resolveBranch(dir string) (string, error) {
+	gitDir, err := resolveGitDir(dir)
 	if err != nil {
-		return "", fmt.Errorf("reading .git/HEAD: %w", err)
+		return "", err
+	}
+	data, err := os.ReadFile(filepath.Join(gitDir, "HEAD"))
+	if err != nil {
+		return "", fmt.Errorf("reading HEAD: %w", err)
 	}
 	head := strings.TrimSpace(string(data))
 	const prefix = "ref: refs/heads/"
@@ -51,3 +56,27 @@ func resolveBranch() (string, error) {
 	// detached HEAD — return commit hash as-is
 	return head, nil
 }
+
+// resolveGitDir returns the path to the actual git directory.
+// In a worktree, .git is a file containing "gitdir: <path>" rather than a directory.
+func resolveGitDir(dir string) (string, error) {
+	dotGit := filepath.Join(dir, ".git")
+	info, err := os.Stat(dotGit)
+	if err != nil {
+		return "", fmt.Errorf("stat .git: %w", err)
+	}
+	if info.IsDir() {
+		return dotGit, nil
+	}
+	// worktree case: .git is a file pointing to the real git dir
+	data, err := os.ReadFile(dotGit)
+	if err != nil {
+		return "", fmt.Errorf("reading .git: %w", err)
+	}
+	line := strings.TrimSpace(string(data))
+	const prefix = "gitdir: "
+	if !strings.HasPrefix(line, prefix) {
+		return "", fmt.Errorf("unexpected .git file content: %q", line)
+	}
+	return line[len(prefix):], nil
+}

cmd/post_checkout_test.go

@@ -0,0 +1,124 @@
+package cmd
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestResolveGitDir_Directory(t *testing.T) {
+	dir := t.TempDir()
+	if err := os.Mkdir(filepath.Join(dir, ".git"), 0755); err != nil {
+		t.Fatal(err)
+	}
+
+	got, err := resolveGitDir(dir)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if got != filepath.Join(dir, ".git") {
+		t.Errorf("got %q, want %q", got, filepath.Join(dir, ".git"))
+	}
+}
+
+func TestResolveGitDir_Worktree(t *testing.T) {
+	dir := t.TempDir()
+	realGitDir := "/some/repo/.git/worktrees/my-worktree"
+	content := "gitdir: " + realGitDir + "\n"
+	if err := os.WriteFile(filepath.Join(dir, ".git"), []byte(content), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	got, err := resolveGitDir(dir)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if got != realGitDir {
+		t.Errorf("got %q, want %q", got, realGitDir)
+	}
+}
+
+func TestResolveGitDir_UnexpectedFileContent(t *testing.T) {
+	dir := t.TempDir()
+	if err := os.WriteFile(filepath.Join(dir, ".git"), []byte("not a gitdir pointer\n"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	_, err := resolveGitDir(dir)
+	if err == nil {
+		t.Fatal("expected error for unexpected .git file content, got nil")
+	}
+}
+
+func TestResolveBranch_Normal(t *testing.T) {
+	dir := t.TempDir()
+	if err := os.Mkdir(filepath.Join(dir, ".git"), 0755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(dir, ".git", "HEAD"), []byte("ref: refs/heads/my-feature\n"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	got, err := resolveBranch(dir)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if got != "my-feature" {
+		t.Errorf("got %q, want %q", got, "my-feature")
+	}
+}
+
+func TestResolveBranch_DetachedHead(t *testing.T) {
+	dir := t.TempDir()
+	hash := "abc1234def5678900000000000000000000000000"
+	if err := os.Mkdir(filepath.Join(dir, ".git"), 0755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(dir, ".git", "HEAD"), []byte(hash+"\n"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	got, err := resolveBranch(dir)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if got != hash {
+		t.Errorf("got %q, want %q", got, hash)
+	}
+}
+
+func TestResolveBranch_Worktree(t *testing.T) {
+	// simulate the worktree structure:
+	// <worktree>/.git  (file)  →  <realGitDir>/HEAD
+	realGitDir := t.TempDir()
+	if err := os.WriteFile(filepath.Join(realGitDir, "HEAD"), []byte("ref: refs/heads/worktree-branch\n"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	worktreeDir := t.TempDir()
+	gitFile := "gitdir: " + realGitDir + "\n"
+	if err := os.WriteFile(filepath.Join(worktreeDir, ".git"), []byte(gitFile), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	got, err := resolveBranch(worktreeDir)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if got != "worktree-branch" {
+		t.Errorf("got %q, want %q", got, "worktree-branch")
+	}
+}
+
+func TestResolveBranch_MissingHead(t *testing.T) {
+	dir := t.TempDir()
+	if err := os.Mkdir(filepath.Join(dir, ".git"), 0755); err != nil {
+		t.Fatal(err)
+	}
+	// no HEAD file written
+
+	_, err := resolveBranch(dir)
+	if err == nil {
+		t.Fatal("expected error for missing HEAD, got nil")
+	}
+}

cmd/run.go

@@ -17,7 +17,7 @@ func runCmd() *cobra.Command {
 		Short: "Manually apply env patching for the current branch",
 		Args:  cobra.NoArgs,
 		RunE: func(cmd *cobra.Command, args []string) error {
-			branch, err := resolveBranch()
+			branch, err := resolveBranch(".")
 			if err != nil {
 				return err
 			}