Merge branch 'rj/avoid-switching-to-already-used-branch'

A few subcommands have been taught to stop users from working on a
branch that is being used in another worktree linked to the same
repository.

* rj/avoid-switching-to-already-used-branch:
  switch: reject if the branch is already checked out elsewhere (test)
  rebase: refuse to switch to a branch already checked out elsewhere (test)
  branch: fix die_if_checked_out() when ignore_current_worktree
  worktree: introduce is_shared_symref()

branch.c

@@ -821,12 +821,16 @@ void remove_branch_state(struct repository *r, int verbose)
 void die_if_checked_out(const char *branch, int ignore_current_worktree)
 {
 	struct worktree **worktrees = get_worktrees();
-	const struct worktree *wt;
 
-	wt = find_shared_symref(worktrees, "HEAD", branch);
-	if (wt && (!ignore_current_worktree || !wt->is_current)) {
+	for (int i = 0; worktrees[i]; i++) {
+		if (worktrees[i]->is_current && ignore_current_worktree)
+			continue;
+
+		if (is_shared_symref(worktrees[i], "HEAD", branch)) {
 			skip_prefix(branch, "refs/heads/", &branch);
-		die(_("'%s' is already checked out at '%s'"), branch, wt->path);
+			die(_("'%s' is already checked out at '%s'"),
+				branch, worktrees[i]->path);
+		}
 	}
 
 	free_worktrees(worktrees);

t/t2060-switch.sh

@@ -146,4 +146,33 @@ test_expect_success 'tracking info copied with autoSetupMerge=inherit' '
 	test_cmp_config "" --default "" branch.main2.merge
 '
 
+test_expect_success 'switch back when temporarily detached and checked out elsewhere ' '
+	test_when_finished "
+		git worktree remove wt1 ||:
+		git worktree remove wt2 ||:
+		git checkout - ||:
+		git branch -D shared ||:
+	" &&
+	git checkout -b shared &&
+	test_commit shared-first &&
+	HASH1=$(git rev-parse --verify HEAD) &&
+	test_commit shared-second &&
+	test_commit shared-third &&
+	HASH2=$(git rev-parse --verify HEAD) &&
+	git worktree add wt1 -f shared &&
+	git -C wt1 bisect start &&
+	git -C wt1 bisect good $HASH1 &&
+	git -C wt1 bisect bad $HASH2 &&
+	git worktree add wt2 -f shared &&
+	git -C wt2 bisect start &&
+	git -C wt2 bisect good $HASH1 &&
+	git -C wt2 bisect bad $HASH2 &&
+	# we test in both worktrees to ensure that works
+	# as expected with "first" and "next" worktrees
+	test_must_fail git -C wt1 switch shared &&
+	git -C wt1 switch --ignore-other-worktrees shared &&
+	test_must_fail git -C wt2 switch shared &&
+	git -C wt2 switch --ignore-other-worktrees shared
+'
+
 test_done

t/t3400-rebase.sh

@@ -388,6 +388,20 @@ test_expect_success 'switch to branch checked out here' '
 	git rebase main main
 '
 
+test_expect_success 'switch to branch checked out elsewhere fails' '
+	test_when_finished "
+		git worktree remove wt1 &&
+		git worktree remove wt2 &&
+		git branch -d shared
+	" &&
+	git worktree add wt1 -b shared &&
+	git worktree add wt2 -f shared &&
+	# we test in both worktrees to ensure that works
+	# as expected with "first" and "next" worktrees
+	test_must_fail git -C wt1 rebase shared shared &&
+	test_must_fail git -C wt2 rebase shared shared
+'
+
 test_expect_success 'switch to branch not checked out' '
 	git checkout main &&
 	git branch other &&

worktree.c

@@ -404,44 +404,43 @@ int is_worktree_being_bisected(const struct worktree *wt,
  * bisect). New commands that do similar things should update this
  * function as well.
  */
-const struct worktree *find_shared_symref(struct worktree **worktrees,
-					  const char *symref,
+int is_shared_symref(const struct worktree *wt, const char *symref,
 		     const char *target)
 {
-	const struct worktree *existing = NULL;
-	int i = 0;
-
-	for (i = 0; worktrees[i]; i++) {
-		struct worktree *wt = worktrees[i];
 	const char *symref_target;
 	struct ref_store *refs;
 	int flags;
 
 	if (wt->is_bare)
-			continue;
+		return 0;
 
 	if (wt->is_detached && !strcmp(symref, "HEAD")) {
-			if (is_worktree_being_rebased(wt, target)) {
-				existing = wt;
-				break;
-			}
-			if (is_worktree_being_bisected(wt, target)) {
-				existing = wt;
-				break;
-			}
+		if (is_worktree_being_rebased(wt, target))
+			return 1;
+		if (is_worktree_being_bisected(wt, target))
+			return 1;
 	}
 
 	refs = get_worktree_ref_store(wt);
 	symref_target = refs_resolve_ref_unsafe(refs, symref, 0,
 						NULL, &flags);
 	if ((flags & REF_ISSYMREF) &&
-		    symref_target && !strcmp(symref_target, target)) {
-			existing = wt;
-			break;
-		}
+	    symref_target && !strcmp(symref_target, target))
+		return 1;
+
+	return 0;
 }
 
-	return existing;
+const struct worktree *find_shared_symref(struct worktree **worktrees,
+					  const char *symref,
+					  const char *target)
+{
+
+	for (int i = 0; worktrees[i]; i++)
+		if (is_shared_symref(worktrees[i], symref, target))
+			return worktrees[i];
+
+	return NULL;
 }
 
 int submodule_uses_worktrees(const char *path)

worktree.h

@@ -148,6 +148,12 @@ const struct worktree *find_shared_symref(struct worktree **worktrees,
 					  const char *symref,
 					  const char *target);
 
+/*
+ * Returns true if a symref points to a ref in a worktree.
+ */
+int is_shared_symref(const struct worktree *wt,
+		     const char *symref, const char *target);
+
 /*
  * Similar to head_ref() for all HEADs _except_ one from the current
  * worktree, which is covered by head_ref().