init: make --separate-git-dir work from within linked worktree

The intention of `git init --separate-work-dir=<path>` is to move the
.git/ directory to a location outside of the main worktree. When used
within a linked worktree, however, rather than moving the .git/
directory as intended, it instead incorrectly moves the worktree's
.git/worktrees/<id> directory to <path>, thus disconnecting the linked
worktree from its parent repository and breaking the worktree in the
process since its local .git file no longer points at a location at
which it can find the object database. Fix this broken behavior.

An intentional side-effect of this change is that it also closes a
loophole not caught by ccf236a23a (init: disallow --separate-git-dir
with bare repository, 2020-08-09) in which the check to prevent
--separate-git-dir being used in conjunction with a bare repository was
unable to detect the invalid combination when invoked from within a
linked worktree. Therefore, add a test to verify that this loophole is
closed, as well.

Reported-by: Henré Botha <henrebotha@gmail.com>
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/init-db.c

@@ -642,6 +642,30 @@ int cmd_init_db(int argc, const char **argv, const char *prefix)
 	if (!git_dir)
 		git_dir = DEFAULT_GIT_DIR_ENVIRONMENT;
 
+	/*
+	 * When --separate-git-dir is used inside a linked worktree, take
+	 * care to ensure that the common .git/ directory is relocated, not
+	 * the worktree-specific .git/worktrees/<id>/ directory.
+	 */
+	if (real_git_dir) {
+		int err;
+		const char *p;
+		struct strbuf sb = STRBUF_INIT;
+
+		p = read_gitfile_gently(git_dir, &err);
+		if (p && get_common_dir(&sb, p)) {
+			struct strbuf mainwt = STRBUF_INIT;
+
+			strbuf_addbuf(&mainwt, &sb);
+			strbuf_strip_suffix(&mainwt, "/.git");
+			if (chdir(mainwt.buf) < 0)
+				die_errno(_("cannot chdir to %s"), mainwt.buf);
+			strbuf_release(&mainwt);
+			git_dir = strbuf_detach(&sb, NULL);
+		}
+		strbuf_release(&sb);
+	}
+
 	if (is_bare_repository_cfg < 0)
 		is_bare_repository_cfg = guess_repository_type(git_dir);
 

t/t0001-init.sh

@@ -329,6 +329,15 @@ test_expect_success 'implicit bare & --separate-git-dir incompatible' '
 	test_i18ngrep "incompatible" err
 '
 
+test_expect_success 'bare & --separate-git-dir incompatible within worktree' '
+	test_when_finished "rm -rf bare.git linkwt seprepo" &&
+	test_commit gumby &&
+	git clone --bare . bare.git &&
+	git -C bare.git worktree add --detach ../linkwt &&
+	test_must_fail git -C linkwt init --separate-git-dir seprepo 2>err &&
+	test_i18ngrep "incompatible" err
+'
+
 test_lazy_prereq GETCWD_IGNORES_PERMS '
 	base=GETCWD_TEST_BASE_DIR &&
 	mkdir -p $base/dir &&
@@ -405,15 +414,23 @@ test_expect_success SYMLINKS 're-init to move gitdir symlink' '
 	test_path_is_dir realgitdir/refs
 '
 
-test_expect_success 're-init to move gitdir with linked worktrees' '
+sep_git_dir_worktree ()  {
 	test_when_finished "rm -rf mainwt linkwt seprepo" &&
 	git init mainwt &&
 	test_commit -C mainwt gumby &&
 	git -C mainwt worktree add --detach ../linkwt &&
-	git -C mainwt init --separate-git-dir ../seprepo &&
+	git -C "$1" init --separate-git-dir ../seprepo &&
 	git -C mainwt rev-parse --git-common-dir >expect &&
 	git -C linkwt rev-parse --git-common-dir >actual &&
 	test_cmp expect actual
+}
+
+test_expect_success 're-init to move gitdir with linked worktrees' '
+	sep_git_dir_worktree mainwt
+'
+
+test_expect_success 're-init to move gitdir within linked worktree' '
+	sep_git_dir_worktree linkwt
 '
 
 test_expect_success MINGW '.git hidden' '