fix(git): refuse to sync a branch checked out in another worktree

Signed-off-by: bircni <bircni@icloud.com>

internal/git/sync.go

@@ -7,6 +7,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"strings"
 )
 
 // SyncOptions selects the local branch to fast-forward and the remote whose branch of the same name it follows.
@@ -29,7 +30,9 @@ type SyncResult struct {
 // SyncBranch fast-forwards the local branch to the remote's branch of the same name, creating it
 // when it is missing, and never changes which branch is checked out. The checked-out branch merges
 // with --ff-only and needs a clean working tree; another branch moves with update-ref, which fails
-// if the branch changed meanwhile. A branch that already contains the remote history is kept.
+// if the branch changed meanwhile. A branch that already contains the remote history is kept. A
+// branch checked out in another linked worktree is refused, since moving it would leave that
+// worktree's files behind its HEAD.
 func SyncBranch(ctx context.Context, runner Runner, opts SyncOptions) (SyncResult, error) {
 	run := func(args ...string) (string, error) { return runner.Output(ctx, opts.Directory, opts.Prompt, args...) }
 	if opts.Remote == "" || opts.Branch == "" {
@@ -47,6 +50,15 @@ func SyncBranch(ctx context.Context, runner Runner, opts SyncOptions) (SyncResul
 		return SyncResult{}, err
 	}
 	checkedOut := head == ref
+	if !checkedOut {
+		worktrees, err := run("worktree", "list", "--porcelain")
+		if err != nil {
+			return SyncResult{}, err
+		}
+		if path := worktreeWithBranch(worktrees, ref); path != "" {
+			return SyncResult{}, fmt.Errorf("%s is checked out in the worktree %s; sync it from there", opts.Branch, path)
+		}
+	}
 	if checkedOut {
 		status, err := run("status", "--porcelain=v1", "--untracked-files=normal")
 		if err != nil {
@@ -115,6 +127,21 @@ func SyncBranch(ctx context.Context, runner Runner, opts SyncOptions) (SyncResul
 	return result, nil
 }
 
+// worktreeWithBranch returns the path of the worktree in git worktree list --porcelain output
+// that has ref checked out, or "" when none has.
+func worktreeWithBranch(porcelain, ref string) string {
+	path := ""
+	for line := range strings.Lines(porcelain) {
+		line = strings.TrimRight(line, "\r\n")
+		if value, ok := strings.CutPrefix(line, "worktree "); ok {
+			path = value
+		} else if line == "branch "+ref {
+			return path
+		}
+	}
+	return ""
+}
+
 // optional returns "" without an error for a Git command that exits with status 1 to report
 // that what it looked for does not exist, such as rev-parse --verify --quiet.
 func optional(output string, err error) (string, error) {

internal/git/sync_test.go

@@ -158,6 +158,24 @@ func TestSyncBranchRefusesWhatItCannotFastForward(t *testing.T) {
 	}
 }
 
+func TestSyncBranchRefusesABranchCheckedOutInAnotherWorktree(t *testing.T) {
+	f := newSyncFixture(t)
+	linked := filepath.Join(t.TempDir(), "linked")
+	f.git(f.work, "worktree", "add", linked, "main")
+	before := f.git(f.work, "rev-parse", "main")
+	_, err := f.sync("main")
+	require.ErrorContains(t, err, "main is checked out in the worktree")
+	require.Equal(t, before, f.git(f.work, "rev-parse", "main"))
+	require.Equal(t, "experiment", f.git(f.work, "branch", "--show-current"))
+}
+
+func TestWorktreeWithBranch(t *testing.T) {
+	porcelain := "worktree /src\nHEAD abc\nbranch refs/heads/experiment\n\nworktree /linked\nHEAD def\nbranch refs/heads/main\n\nworktree /detached\nHEAD 123\ndetached\n"
+	require.Equal(t, "/linked", worktreeWithBranch(porcelain, "refs/heads/main"))
+	require.Equal(t, "/src", worktreeWithBranch(porcelain, "refs/heads/experiment"))
+	require.Empty(t, worktreeWithBranch(porcelain, "refs/heads/other"))
+}
+
 func TestSyncBranchValidatesItsInput(t *testing.T) {
 	f := newSyncFixture(t)
 	_, err := SyncBranch(context.Background(), f.runner, SyncOptions{Directory: f.work, Remote: "origin"})