worktree: update is_bare heuristics

When "git branch -D <name>" is run, Git usually first checks if that
branch is currently checked out. But this check is not performed if the
Git directory of that repository is not at "<repo>/.git", which is the
case if that repository is a submodule that has its Git directory stored
as "super/.git/modules/<repo>", for example. This results in the branch
being deleted even though it is checked out.

This is because get_main_worktree() in worktree.c sets is_bare on a
worktree only using the heuristic that a repo is bare if the worktree's
path does not end in "/.git", and not bare otherwise. This is_bare code
was introduced in 92718b7438 ("worktree: add details to the worktree
struct", 2015-10-08), following a pre-core.bare heuristic. This patch
does 2 things:

 - Teach get_main_worktree() to use is_bare_repository() instead,
   introduced in 7d1864ce67 ("Introduce is_bare_repository() and
   core.bare configuration variable", 2007-01-07) and updated in
   e90fdc39b6 ("Clean up work-tree handling", 2007-08-01). This solves
   the "git branch -D <name>" problem described above. However...

 - If a repository has core.bare=1 but the "git" command is being run
   from one of its secondary worktrees, is_bare_repository() returns
   false (which is fine, since there is a worktree available). However,
   treating the main worktree as non-bare when it is bare causes issues:
   for example, failure to delete a branch from a secondary worktree
   that is referred to by a main worktree's HEAD, even if that main
   worktree is bare.

   In order to avoid that, also check core.bare when setting is_bare. If
   core.bare=1, trust it, and otherwise, use is_bare_repository().

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3200-branch.sh

@@ -264,6 +264,30 @@ test_expect_success 'git branch --list -d t should fail' '
 	test_must_fail git rev-parse refs/heads/t
 '
 
+test_expect_success 'deleting checked-out branch from repo that is a submodule' '
+	test_when_finished "rm -rf repo1 repo2" &&
+
+	git init repo1 &&
+	git init repo1/sub &&
+	test_commit -C repo1/sub x &&
+	git -C repo1 submodule add ./sub &&
+	git -C repo1 commit -m "adding sub" &&
+
+	git clone --recurse-submodules repo1 repo2 &&
+	git -C repo2/sub checkout -b work &&
+	test_must_fail git -C repo2/sub branch -D work
+'
+
+test_expect_success 'bare main worktree has HEAD at branch deleted by secondary worktree' '
+	test_when_finished "rm -rf nonbare base secondary" &&
+
+	git init nonbare &&
+	test_commit -C nonbare x &&
+	git clone --bare nonbare bare &&
+	git -C bare worktree add --detach ../secondary master &&
+	git -C secondary branch -D master
+'
+
 test_expect_success 'git branch --list -v with --abbrev' '
 	test_when_finished "git branch -D t" &&
 	git branch t &&

worktree.c

@@ -49,18 +49,24 @@ static struct worktree *get_main_worktree(void)
 	struct worktree *worktree = NULL;
 	struct strbuf path = STRBUF_INIT;
 	struct strbuf worktree_path = STRBUF_INIT;
-	int is_bare = 0;
 
 	strbuf_add_absolute_path(&worktree_path, get_git_common_dir());
-	is_bare = !strbuf_strip_suffix(&worktree_path, "/.git");
-	if (is_bare)
+	if (!strbuf_strip_suffix(&worktree_path, "/.git"))
 		strbuf_strip_suffix(&worktree_path, "/.");
 
 	strbuf_addf(&path, "%s/HEAD", get_git_common_dir());
 
 	worktree = xcalloc(1, sizeof(*worktree));
 	worktree->path = strbuf_detach(&worktree_path, NULL);
-	worktree->is_bare = is_bare;
+	/*
+	 * NEEDSWORK: If this function is called from a secondary worktree and
+	 * config.worktree is present, is_bare_repository_cfg will reflect the
+	 * contents of config.worktree, not the contents of the main worktree.
+	 * This means that worktree->is_bare may be set to 0 even if the main
+	 * worktree is configured to be bare.
+	 */
+	worktree->is_bare = (is_bare_repository_cfg == 1) ||
+		is_bare_repository();
 	add_head_info(worktree);
 
 	strbuf_release(&path);