fix(repo): sync a branch without switching to it

tea repo sync ran git switch on the synced branch, so syncing main while
on experiment left the user on main. A branch that is not checked out now
moves with update-ref, guarded by its old value; the checked-out branch
merges with --ff-only and needs a clean working tree; a missing branch is
created and set to track the remote. Diverged branches stay untouched.

Sync reports "✓ Synced main from origin/main (fast-forward a..b)" on a
terminal, "already up to date" when nothing changed, and the JSON result
gains the previous commit. Outside a clone, or without a remote for the
repository, the errors now name repo sync instead of checkout.

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

internal/app/app_test.go

@@ -514,7 +514,6 @@ const (
 	repo list
 	repo migrate
 	repo set-default
-	repo sync
 	repo view
 	search issue
 	search pr

internal/app/repos_test.go

@@ -0,0 +1,18 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package app
+
+import (
+	"bytes"
+	"testing"
+)
+
+// repoTTYDeps returns serverDeps for a 100-column terminal without color or links, as people read tea's output.
+func repoTTYDeps(t *testing.T, serverURL string) (Dependencies, *bytes.Buffer, *bytes.Buffer) {
+	t.Helper()
+	deps, out, diagnostics := serverDeps(t, serverURL)
+	deps.Getenv = serverEnv(serverURL, map[string]string{"TERM": "dumb"})
+	deps.Streams.OutTTY, deps.Streams.Width = true, 100
+	return deps, out, diagnostics
+}

internal/app/repository_sync.go

@@ -5,7 +5,6 @@ package app
 
 import (
 	"errors"
-	"fmt"
 	"strings"
 
 	"gitea.dev/tea/internal/clierr"
@@ -17,7 +16,21 @@ import (
 
 func (a *application) repositorySyncCommand() *cobra.Command {
 	var branch string
-	command := &cobra.Command{Use: "sync [repository]", Short: "Fast-forward a local branch from the selected repository remote", Args: usageArgs(cobra.MaximumNArgs(1)), RunE: func(cmd *cobra.Command, args []string) error {
+	command := &cobra.Command{
+		Use:   "sync [<repository>]",
+		Short: "Sync a local branch with its remote",
+		Long: `Fast-forward a local branch to the branch of the same name on the repository's Git remote.
+
+The branch defaults to the repository's default branch. It is created when it is missing, and it is
+never checked out: the branch you are on stays checked out. When the branch is checked out, the
+working tree must be clean. Branches that have diverged from the remote are left unchanged.`,
+		Example: `# Fast-forward the default branch from the repository's remote
+$ tea repo sync
+
+# Fast-forward another branch
+$ tea repo sync --branch release/1.0`,
+		Args: usageArgs(cobra.MaximumNArgs(1)),
+		RunE: func(cmd *cobra.Command, args []string) error {
 			if cmd.Flags().Changed("branch") && strings.TrimSpace(branch) == "" {
 				return clierr.Wrap(clierr.Usage, errors.New("--branch cannot be empty"))
 			}
@@ -28,11 +41,17 @@ func (a *application) repositorySyncCommand() *cobra.Command {
 			if err != nil {
 				return err
 			}
+			if inside, _ := a.gitProbe().Output(cmd.Context(), a.options.cwd, false, "rev-parse", "--is-inside-work-tree"); inside != "true" {
+				return clierr.Wrap(clierr.Usage, errors.New("repo sync must run inside a Git clone of "+target.Repo))
+			}
 			snapshot, err := a.deps.Settings.Load()
 			if err != nil {
 				return err
 			}
 			remote, err := a.checkoutRemote(cmd.Context(), target, snapshot)
+			if errors.Is(err, errNoMatchingRemote) {
+				return clierr.Wrap(clierr.Usage, errors.New("no Git remote of this clone points to "+target.Repo+"; add one with git remote add, or pass --remote"))
+			}
 			if err != nil {
 				return clierr.Wrap(clierr.Usage, err)
 			}
@@ -61,9 +80,29 @@ func (a *application) repositorySyncCommand() *cobra.Command {
 			if a.options.json != "" {
 				return a.writeJSON(cmd, result)
 			}
-		_, err = fmt.Fprintf(cmd.OutOrStdout(), "Synced %s (%s) from %s\n", result.Branch, result.Commit, result.Remote)
-		return err
-	}}
-	command.Flags().StringVarP(&branch, "branch", "b", "", "Local and remote branch (default: repository default branch)")
+			a.reportSync(cmd, result, remote+"/"+result.Branch)
+			return nil
+		},
+	}
+	command.Flags().StringVarP(&branch, "branch", "b", "", "Branch to sync (default: the repository's default branch)")
+	_ = command.RegisterFlagCompletionFunc("branch", a.completeBranches())
+	command.ValidArgsFunction = a.completeRepos()
 	return command
 }
+
+// reportSync tells how a sync from source, such as origin/main, changed the branch.
+func (a *application) reportSync(cmd *cobra.Command, result git.SyncResult, source string) {
+	switch result.Previous {
+	case result.Commit:
+		a.done(cmd, "%s is already up to date with %s", result.Branch, source)
+	case "":
+		a.done(cmd, "Created %s from %s (%s)", result.Branch, source, shortSHA(result.Commit))
+	default:
+		a.done(cmd, "Synced %s from %s (fast-forward %s..%s)", result.Branch, source, shortSHA(result.Previous), shortSHA(result.Commit))
+	}
+}
+
+// shortSHA abbreviates a commit ID to seven characters, as Gitea and Git show them.
+func shortSHA(sha string) string {
+	return sha[:min(len(sha), 7)]
+}

internal/app/repository_sync_test.go

@@ -0,0 +1,75 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package app
+
+import (
+	"context"
+	"errors"
+	"net/http"
+	"strings"
+	"testing"
+
+	"gitea.dev/tea/internal/clierr"
+	"github.com/stretchr/testify/require"
+)
+
+// syncGit answers the Git commands of a local repo sync in a clone whose origin is url, with
+// experiment checked out and main one commit behind origin.
+type syncGit struct {
+	url, inside string
+	calls       []string
+}
+
+func (g *syncGit) Output(_ context.Context, _ string, _ bool, args ...string) (string, error) {
+	for len(args) > 2 && args[0] == "-c" {
+		args = args[2:]
+	}
+	call := strings.Join(args, " ")
+	g.calls = append(g.calls, call)
+	switch call {
+	case "rev-parse --is-inside-work-tree":
+		if g.inside == "" {
+			return "", errors.New("not a git repository")
+		}
+		return g.inside, nil
+	case "remote get-url origin":
+		return g.url, nil
+	case "symbolic-ref --quiet HEAD":
+		return "refs/heads/experiment", nil
+	case "rev-parse --verify --quiet refs/heads/main^{commit}":
+		return "1111111aaaa", nil
+	case "rev-parse --verify FETCH_HEAD^{commit}":
+		return "2222222bbbb", nil
+	}
+	return "", nil
+}
+
+func TestRepoSyncFastForwardsWithoutSwitchingBranches(t *testing.T) {
+	server := newAuthenticatedServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		t.Errorf("unexpected request %s %s", r.Method, r.URL.Path)
+	}))
+	defer server.Close()
+	deps, out, diagnostics := repoTTYDeps(t, server.URL)
+	runner := &syncGit{url: server.URL + "/alice/project.git", inside: "true"}
+	deps.Git = runner
+	require.NoError(t, Execute(context.Background(), deps, []string{"repo", "sync", "-R", "alice/project", "--remote", "origin", "-b", "main"}))
+	require.Empty(t, out.String())
+	require.Equal(t, "✓ Synced main from origin/main (fast-forward 1111111..2222222)\n", diagnostics.String())
+	require.Contains(t, runner.calls, "update-ref -m tea repo sync refs/heads/main 2222222bbbb 1111111aaaa")
+	for _, call := range runner.calls {
+		require.False(t, strings.HasPrefix(call, "switch") || strings.HasPrefix(call, "checkout") || strings.HasPrefix(call, "merge "), call)
+	}
+}
+
+func TestRepoSyncOutsideACloneExplainsWhatItNeeds(t *testing.T) {
+	server := newAuthenticatedServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		t.Errorf("unexpected request %s %s", r.Method, r.URL.Path)
+	}))
+	defer server.Close()
+	deps, _, _ := serverDeps(t, server.URL)
+	deps.Git = &syncGit{}
+	err := Execute(context.Background(), deps, []string{"repo", "sync", "-R", "alice/project", "-b", "main"})
+	require.Equal(t, clierr.Usage, clierr.ExitCode(err), "%v", err)
+	require.ErrorContains(t, err, "repo sync must run inside a Git clone of alice/project")
+}

internal/git/sync.go

@@ -9,21 +9,27 @@ import (
 	"fmt"
 )
 
-// SyncOptions selects one existing local branch and its source remote branch.
+// SyncOptions selects the local branch to fast-forward and the remote whose branch of the same name it follows.
 type SyncOptions struct {
 	Directory, Remote, Branch string
 	CredentialArgs            []string
 	Prompt                    bool
 }
 
-// SyncResult reports a completed local fast-forward.
+// SyncResult reports a synced branch. Previous is the branch's commit before the sync, empty when
+// the sync created the branch; it equals Commit when there was nothing to sync. For a sync on the
+// server, Remote names the repository the branch was synced from.
 type SyncResult struct {
 	Branch   string `json:"branch"`
 	Commit   string `json:"commit"`
+	Previous string `json:"previous"`
 	Remote   string `json:"remote"`
 }
 
-// SyncBranch fetches and fast-forwards an existing branch without resetting commits.
+// 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.
 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 == "" {
@@ -35,20 +41,27 @@ func SyncBranch(ctx context.Context, runner Runner, opts SyncOptions) (SyncResul
 	if err := requireWorkTree(run, "sync"); err != nil {
 		return SyncResult{}, err
 	}
+	ref := "refs/heads/" + opts.Branch
+	head, err := optional(run("symbolic-ref", "--quiet", "HEAD"))
+	if err != nil {
+		return SyncResult{}, err
+	}
+	checkedOut := head == ref
+	if checkedOut {
 		status, err := run("status", "--porcelain=v1", "--untracked-files=normal")
 		if err != nil {
 			return SyncResult{}, err
 		}
 		if status != "" {
-		return SyncResult{}, errors.New("working tree has changes; commit or stash them before syncing")
+			return SyncResult{}, fmt.Errorf("working tree has changes; commit or stash them before syncing %s", opts.Branch)
 		}
-	localRef := "refs/heads/" + opts.Branch + "^{commit}"
-	previous, err := run("rev-parse", "--verify", localRef)
+	}
+	previous, err := optional(run("rev-parse", "--verify", "--quiet", ref+"^{commit}"))
 	if err != nil {
-		return SyncResult{}, fmt.Errorf("local branch is unavailable; create it before syncing: %w", err)
+		return SyncResult{}, err
 	}
 	fetch := append([]string{}, opts.CredentialArgs...)
-	fetch = append(fetch, "fetch", "--no-tags", "--", opts.Remote, "refs/heads/"+opts.Branch)
+	fetch = append(fetch, "fetch", "--no-tags", "--", opts.Remote, ref)
 	if _, err := run(fetch...); err != nil {
 		return SyncResult{}, err
 	}
@@ -56,39 +69,57 @@ func SyncBranch(ctx context.Context, runner Runner, opts SyncOptions) (SyncResul
 	if err != nil {
 		return SyncResult{}, err
 	}
-	current, err := run("rev-parse", "--verify", localRef)
-	if err != nil {
-		return SyncResult{}, err
+	result := SyncResult{Branch: opts.Branch, Commit: commit, Previous: previous, Remote: opts.Remote}
+	if previous == commit {
+		return result, nil
 	}
-	if current != previous {
-		return SyncResult{}, errors.New("local branch changed while fetching; retry sync")
-	}
-	if previous != commit {
+	if previous != "" {
 		if _, err := run("merge-base", "--is-ancestor", previous, commit); err != nil {
 			if failure, ok := errors.AsType[*CommandError](err); !ok || failure.ExitCode != 1 {
 				return SyncResult{}, err
 			}
 			if _, err := run("merge-base", "--is-ancestor", commit, previous); err != nil {
-				return SyncResult{}, fmt.Errorf("branch cannot be fast-forwarded; resolve divergence before syncing: %w", err)
+				return SyncResult{}, fmt.Errorf("%s and %s/%s have diverged; merge or rebase before syncing: %w", opts.Branch, opts.Remote, opts.Branch, err)
 			}
-			// The local branch already contains the remote history.
-			commit = previous
+			result.Commit = previous
+			return result, nil
 		}
 	}
-	if _, err := run("switch", opts.Branch); err != nil {
+	if !checkedOut {
+		// The old value makes update-ref fail if the branch moved, or appeared, while fetching.
+		if _, err := run("update-ref", "-m", "tea repo sync", ref, commit, previous); err != nil {
+			return SyncResult{}, fmt.Errorf("%s changed while fetching; retry sync: %w", opts.Branch, err)
+		}
+		if previous == "" {
+			for _, setting := range [][2]string{{"remote", opts.Remote}, {"merge", ref}} {
+				if _, err := run("config", "branch."+opts.Branch+"."+setting[0], setting[1]); err != nil {
+					return SyncResult{}, fmt.Errorf("created %s, but could not set it to track %s: %w", opts.Branch, opts.Remote, err)
+				}
+			}
+		}
+		return result, nil
+	}
+	current, err := run("rev-parse", "--verify", "HEAD^{commit}")
+	if err != nil {
 		return SyncResult{}, err
 	}
-	if previous != commit {
+	if current != previous {
+		return SyncResult{}, fmt.Errorf("%s changed while fetching; retry sync", opts.Branch)
+	}
 	if _, err := run("merge", "--ff-only", "--no-edit", commit); err != nil {
-			return SyncResult{}, fmt.Errorf("branch was selected but sync failed: %w", err)
+		return SyncResult{}, fmt.Errorf("could not fast-forward %s: %w", opts.Branch, err)
 	}
-	}
-	actual, err := run("rev-parse", "--verify", "HEAD^{commit}")
-	if err != nil {
-		return SyncResult{}, fmt.Errorf("sync finished, but HEAD could not be verified: %w", err)
-	}
-	if actual != commit {
+	if actual, err := run("rev-parse", "--verify", "HEAD^{commit}"); err != nil || actual != commit {
 		return SyncResult{}, errors.New("HEAD changed during sync; inspect the working tree before retrying")
 	}
-	return SyncResult{Branch: opts.Branch, Commit: commit, Remote: opts.Remote}, nil
+	return result, nil
+}
+
+// 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) {
+	if failure, ok := errors.AsType[*CommandError](err); ok && failure.ExitCode == 1 {
+		return "", nil
+	}
+	return output, err
 }

internal/git/sync_test.go

@@ -5,128 +5,165 @@ package git
 
 import (
 	"context"
-	"strings"
+	"os"
+	"os/exec"
+	"path/filepath"
 	"testing"
 
 	"github.com/stretchr/testify/require"
 )
 
-func TestSyncValidatesBeforeChangingBranches(t *testing.T) {
-	for _, test := range []struct {
-		dirty, diverged bool
-		message         string
+// syncFixture is a clone of a bare remote whose main branch is one commit ahead of the clone's.
+type syncFixture struct {
+	t             *testing.T
+	runner        System
+	work          string
+	behind, ahead string
+}
+
+func newSyncFixture(t *testing.T) syncFixture {
+	t.Helper()
+	if _, err := exec.LookPath("git"); err != nil {
+		t.Skip("Git is not installed")
+	}
+	root := t.TempDir()
+	env := append(os.Environ(), "GIT_CONFIG_GLOBAL="+filepath.Join(root, "gitconfig"), "GIT_CONFIG_NOSYSTEM=1",
+		"GIT_AUTHOR_NAME=tea", "GIT_AUTHOR_EMAIL=tea@example.invalid", "GIT_COMMITTER_NAME=tea", "GIT_COMMITTER_EMAIL=tea@example.invalid")
+	f := syncFixture{t: t, runner: System{Env: env}, work: filepath.Join(root, "work")}
+	remote, pusher := filepath.Join(root, "remote.git"), filepath.Join(root, "pusher")
+	f.git(root, "init", "--bare", "-b", "main", remote)
+	f.git(root, "clone", remote, pusher)
+	f.git(pusher, "commit", "--allow-empty", "-m", "first")
+	f.git(pusher, "push", "origin", "HEAD:main")
+	f.git(root, "clone", remote, f.work)
+	f.git(f.work, "switch", "-c", "experiment")
+	f.git(pusher, "commit", "--allow-empty", "-m", "second")
+	f.git(pusher, "push", "origin", "HEAD:main")
+	f.behind, f.ahead = f.git(f.work, "rev-parse", "main"), f.git(pusher, "rev-parse", "HEAD")
+	return f
+}
+
+func (f syncFixture) git(dir string, args ...string) string {
+	f.t.Helper()
+	output, err := f.runner.Output(context.Background(), dir, false, args...)
+	require.NoError(f.t, err, args)
+	return output
+}
+
+func (f syncFixture) sync(branch string) (SyncResult, error) {
+	return SyncBranch(context.Background(), f.runner, SyncOptions{Directory: f.work, Remote: "origin", Branch: branch})
+}
+
+func TestSyncBranchKeepsTheCheckedOutBranch(t *testing.T) {
+	for _, tc := range []struct {
+		name, checkout string
+		prepare        func(f syncFixture)
+		want           func(f syncFixture) SyncResult
+		tracks         bool
 	}{
-		{true, false, "working tree has changes"},
-		{false, true, "cannot be fast-forwarded"},
+		{
+			name: "another branch is checked out", checkout: "experiment",
+			want: func(f syncFixture) SyncResult { return SyncResult{"main", f.ahead, f.behind, "origin"} },
+		},
+		{
+			name: "another branch with local changes", checkout: "experiment",
+			prepare: func(f syncFixture) {
+				require.NoError(t, os.WriteFile(filepath.Join(f.work, "draft.txt"), []byte("draft"), 0o600))
+			},
+			want: func(f syncFixture) SyncResult { return SyncResult{"main", f.ahead, f.behind, "origin"} },
+		},
+		{
+			name: "the branch is checked out", checkout: "main",
+			want: func(f syncFixture) SyncResult { return SyncResult{"main", f.ahead, f.behind, "origin"} },
+		},
+		{
+			name: "the branch is missing", checkout: "experiment",
+			prepare: func(f syncFixture) { f.git(f.work, "branch", "-D", "main") },
+			want:    func(f syncFixture) SyncResult { return SyncResult{"main", f.ahead, "", "origin"} },
+			tracks:  true,
+		},
+		{
+			name: "the branch is up to date", checkout: "experiment",
+			prepare: func(f syncFixture) {
+				f.git(f.work, "fetch", "origin")
+				f.git(f.work, "branch", "-f", "main", f.ahead)
+			},
+			want: func(f syncFixture) SyncResult { return SyncResult{"main", f.ahead, f.ahead, "origin"} },
+		},
+		{
+			name: "the branch is ahead", checkout: "main",
+			prepare: func(f syncFixture) {
+				f.git(f.work, "fetch", "origin")
+				f.git(f.work, "reset", "--hard", f.ahead)
+				f.git(f.work, "commit", "--allow-empty", "-m", "local")
+			},
+			want: func(f syncFixture) SyncResult {
+				local := f.git(f.work, "rev-parse", "main")
+				return SyncResult{"main", local, local, "origin"}
+			},
+		},
 	} {
-		runner := &checkoutRunner{dirty: test.dirty, diverged: test.diverged, localCommit: "previous"}
-		_, err := SyncBranch(context.Background(), runner, SyncOptions{Directory: "/work", Remote: "origin", Branch: "main"})
-		require.ErrorContains(t, err, test.message)
-		for _, call := range runner.calls {
-			require.False(t, strings.HasPrefix(call, "switch "))
-			require.False(t, strings.HasPrefix(call, "merge "))
-			if test.dirty {
-				require.False(t, strings.HasPrefix(call, "fetch "))
+		t.Run(tc.name, func(t *testing.T) {
+			f := newSyncFixture(t)
+			if tc.checkout == "main" {
+				f.git(f.work, "switch", "main")
 			}
+			if tc.prepare != nil {
+				tc.prepare(f)
 			}
-	}
-	runner := &checkoutRunner{localCommit: "previous"}
-	result, err := SyncBranch(context.Background(), runner, SyncOptions{Directory: "/work", Remote: "origin", Branch: "main", CredentialArgs: []string{"-c", "credential.helper=tea"}})
+			want := tc.want(f)
+			result, err := f.sync("main")
 			require.NoError(t, err)
-	require.Equal(t, SyncResult{Branch: "main", Commit: "abc123", Remote: "origin"}, result)
-	require.Contains(t, runner.calls, "-c credential.helper=tea fetch --no-tags -- origin refs/heads/main")
-	require.Equal(t, []string{"switch main", "merge --ff-only --no-edit abc123", "rev-parse --verify HEAD^{commit}"}, runner.calls[len(runner.calls)-3:])
+			require.Equal(t, tc.checkout, f.git(f.work, "branch", "--show-current"))
+			require.Equal(t, want.Commit, f.git(f.work, "rev-parse", "main"))
+			require.Equal(t, want, result)
+			if tc.tracks {
+				require.Equal(t, "origin", f.git(f.work, "config", "branch.main.remote"))
+				require.Equal(t, "refs/heads/main", f.git(f.work, "config", "branch.main.merge"))
+			}
+		})
+	}
 }
 
-type syncFailureRunner struct {
-	checkoutRunner
-	missing, changed bool
-	reads            int
-}
-
-func (r *syncFailureRunner) Output(ctx context.Context, dir string, prompt bool, args ...string) (string, error) {
-	if len(args) == 3 && args[0] == "rev-parse" && strings.HasPrefix(args[2], "refs/heads/") {
-		r.reads++
-		if r.missing {
-			return "", &CommandError{ExitCode: 1}
-		}
-		if r.changed && r.reads == 2 {
-			return "other-commit", nil
-		}
-	}
-	return r.checkoutRunner.Output(ctx, dir, prompt, args...)
-}
-
-func TestSyncRejectsMissingOrChangedBranch(t *testing.T) {
-	for _, test := range []struct {
-		missing, changed bool
-		message          string
+func TestSyncBranchRefusesWhatItCannotFastForward(t *testing.T) {
+	for _, tc := range []struct {
+		name, message string
+		prepare       func(f syncFixture)
 	}{
-		{true, false, "local branch is unavailable"},
-		{false, true, "local branch changed while fetching"},
+		{
+			name: "local changes on the checked-out branch", message: "working tree has changes",
+			prepare: func(f syncFixture) {
+				f.git(f.work, "switch", "main")
+				require.NoError(t, os.WriteFile(filepath.Join(f.work, "draft.txt"), []byte("draft"), 0o600))
+			},
+		},
+		{
+			name: "diverged history", message: "main and origin/main have diverged",
+			prepare: func(f syncFixture) {
+				f.git(f.work, "switch", "main")
+				f.git(f.work, "commit", "--allow-empty", "-m", "local")
+				f.git(f.work, "switch", "experiment")
+			},
+		},
 	} {
-		runner := &syncFailureRunner{missing: test.missing, changed: test.changed}
-		_, err := SyncBranch(context.Background(), runner, SyncOptions{Directory: "/work", Remote: "origin", Branch: "main"})
-		require.ErrorContains(t, err, test.message)
-		for _, call := range runner.calls {
-			require.False(t, strings.HasPrefix(call, "switch "))
-			require.False(t, strings.HasPrefix(call, "merge "))
-			if test.missing {
-				require.False(t, strings.HasPrefix(call, "fetch "))
-			}
-		}
+		t.Run(tc.name, func(t *testing.T) {
+			f := newSyncFixture(t)
+			tc.prepare(f)
+			branch, before := f.git(f.work, "branch", "--show-current"), f.git(f.work, "rev-parse", "main")
+			_, err := f.sync("main")
+			require.ErrorContains(t, err, tc.message)
+			require.Equal(t, branch, f.git(f.work, "branch", "--show-current"))
+			require.Equal(t, before, f.git(f.work, "rev-parse", "main"))
+		})
 	}
 }
 
-type aheadSyncRunner struct{ checkoutRunner }
-
-func (r *aheadSyncRunner) Output(ctx context.Context, dir string, prompt bool, args ...string) (string, error) {
-	if len(args) == 3 && args[2] == "HEAD^{commit}" {
-		r.calls = append(r.calls, strings.Join(args, " "))
-		return r.localCommit, nil
-	}
-	if len(args) == 4 && args[0] == "merge-base" && args[2] == "previous" {
-		r.calls = append(r.calls, strings.Join(args, " "))
-		return "", &CommandError{ExitCode: 1}
-	}
-	return r.checkoutRunner.Output(ctx, dir, prompt, args...)
-}
-
-func TestSyncPreservesBranchAheadOfRemote(t *testing.T) {
-	runner := &aheadSyncRunner{localCommit: "previous"}
-	result, err := SyncBranch(context.Background(), runner, SyncOptions{Directory: "/work", Remote: "origin", Branch: "main"})
-	require.NoError(t, err)
-	require.Equal(t, "previous", result.Commit)
-	require.Contains(t, runner.calls, "merge-base --is-ancestor abc123 previous")
-	require.Equal(t, "switch main", runner.calls[len(runner.calls)-2])
-	for _, call := range runner.calls {
-		require.False(t, strings.HasPrefix(call, "merge "))
-	}
-}
-
-func TestSyncEqualHistoryDoesNotMerge(t *testing.T) {
-	runner := &checkoutRunner{}
-	result, err := SyncBranch(context.Background(), runner, SyncOptions{Directory: "/work", Remote: "origin", Branch: "main"})
-	require.NoError(t, err)
-	require.Equal(t, "abc123", result.Commit)
-	for _, call := range runner.calls {
-		require.False(t, strings.HasPrefix(call, "merge-base "))
-		require.False(t, strings.HasPrefix(call, "merge "))
-	}
-}
-
-type changedHeadRunner struct{ checkoutRunner }
-
-func (r *changedHeadRunner) Output(ctx context.Context, dir string, prompt bool, args ...string) (string, error) {
-	if len(args) == 3 && args[2] == "HEAD^{commit}" {
-		return "unexpected", nil
-	}
-	return r.checkoutRunner.Output(ctx, dir, prompt, args...)
-}
-
-func TestSyncRejectsUnexpectedFinalHead(t *testing.T) {
-	runner := &changedHeadRunner{}
-	result, err := SyncBranch(context.Background(), runner, SyncOptions{Directory: "/work", Remote: "origin", Branch: "main"})
-	require.ErrorContains(t, err, "HEAD changed during sync")
-	require.Equal(t, SyncResult{}, result)
+func TestSyncBranchValidatesItsInput(t *testing.T) {
+	f := newSyncFixture(t)
+	_, err := SyncBranch(context.Background(), f.runner, SyncOptions{Directory: f.work, Remote: "origin"})
+	require.ErrorContains(t, err, "requires a remote and branch")
+	_, err = f.sync("bad..name")
+	require.Error(t, err)
+	_, err = SyncBranch(context.Background(), f.runner, SyncOptions{Directory: t.TempDir(), Remote: "origin", Branch: "main"})
+	require.Error(t, err)
 }