ref: check whether the target of the symref is a ref

Ideally, we want to the users use "git symbolic-ref" to create symrefs
instead of writing raw contents into the filesystem. However, "git
symbolic-ref" is strict with the refname but not strict with the
referent. For example, we can make the "referent" located at the
"$(gitdir)/logs/aaa" and manually write the content into this where we
can still successfully parse this symref by using "git rev-parse".

  $ git init repo && cd repo && git commit --allow-empty -mx
  $ git symbolic-ref refs/heads/test logs/aaa
  $ echo $(git rev-parse HEAD) > .git/logs/aaa
  $ git rev-parse test

We may need to add some restrictions for "referent" parameter when using
"git symbolic-ref" to create symrefs because ideally all the
nonpseudo-refs should be located under the "refs" directory and we may
tighten this in the future.

In order to tell the user we may tighten the above situation, create
a new fsck message "symrefTargetIsNotARef" to notify the user that this
may become an error in the future.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: shejialuo <shejialuo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/fsck-msgids.txt

@@ -183,6 +183,15 @@
 	git@vger.kernel.org mailing list if you see this error, as
 	we need to know what tools created such a file.
 
+`symrefTargetIsNotARef`::
+	(INFO) The target of a symbolic reference points neither to
+	a root reference nor to a reference starting with "refs/".
+	Although we allow create a symref pointing to the referent which
+	is outside the "ref" by using `git symbolic-ref`, we may tighten
+	the rule in the future. Report to the git@vger.kernel.org
+	mailing list if you see this error, as we need to know what tools
+	created such a file.
+
 `trailingRefContent`::
 	(INFO) A loose ref has trailing content. As valid implementations
 	of Git never created such a loose ref file, it may become an

fsck.h

@@ -87,6 +87,7 @@ enum fsck_msg_type {
 	FUNC(BAD_TAG_NAME, INFO) \
 	FUNC(MISSING_TAGGER_ENTRY, INFO) \
 	FUNC(REF_MISSING_NEWLINE, INFO) \
+	FUNC(SYMREF_TARGET_IS_NOT_A_REF, INFO) \
 	FUNC(TRAILING_REF_CONTENT, INFO) \
 	/* ignored (elevated when requested) */ \
 	FUNC(EXTRA_HEADER_ENTRY, IGNORE)

refs/files-backend.c

@@ -3513,6 +3513,7 @@ static int files_fsck_symref_target(struct fsck_options *o,
 				    struct fsck_ref_report *report,
 				    struct strbuf *referent)
 {
+	int is_referent_root;
 	char orig_last_byte;
 	size_t orig_len;
 	int ret = 0;
@@ -3521,8 +3522,17 @@ static int files_fsck_symref_target(struct fsck_options *o,
 	orig_last_byte = referent->buf[orig_len - 1];
 	strbuf_rtrim(referent);
 
-	if (!is_root_ref(referent->buf) &&
+	is_referent_root = is_root_ref(referent->buf);
-	    check_refname_format(referent->buf, 0)) {
+	if (!is_referent_root &&
+	    !starts_with(referent->buf, "refs/") &&
+	    !starts_with(referent->buf, "worktrees/")) {
+		ret = fsck_report_ref(o, report,
+				      FSCK_MSG_SYMREF_TARGET_IS_NOT_A_REF,
+				      "points to non-ref target '%s'", referent->buf);
+
+	}
+
+	if (!is_referent_root && check_refname_format(referent->buf, 0)) {
 		ret = fsck_report_ref(o, report,
 				      FSCK_MSG_BAD_REFERENT_NAME,
 				      "points to invalid refname '%s'", referent->buf);

t/t0602-reffiles-fsck.sh

@@ -366,6 +366,35 @@ test_expect_success 'textual symref content should be checked (aggregate)' '
 	test_cmp expect sorted_err
 '
 
+test_expect_success 'the target of the textual symref should be checked' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	branch_dir_prefix=.git/refs/heads &&
+	tag_dir_prefix=.git/refs/tags &&
+	cd repo &&
+	test_commit default &&
+	mkdir -p "$branch_dir_prefix/a/b" &&
+
+	for good_referent in "refs/heads/branch" "HEAD" "refs/tags/tag"
+	do
+		printf "ref: %s\n" $good_referent >$branch_dir_prefix/branch-good &&
+		git refs verify 2>err &&
+		rm $branch_dir_prefix/branch-good &&
+		test_must_be_empty err || return 1
+	done &&
+
+	for nonref_referent in "refs-back/heads/branch" "refs-back/tags/tag" "reflogs/refs/heads/branch"
+	do
+		printf "ref: %s\n" $nonref_referent >$branch_dir_prefix/branch-bad-1 &&
+		git refs verify 2>err &&
+		cat >expect <<-EOF &&
+		warning: refs/heads/branch-bad-1: symrefTargetIsNotARef: points to non-ref target '\''$nonref_referent'\''
+		EOF
+		rm $branch_dir_prefix/branch-bad-1 &&
+		test_cmp expect err || return 1
+	done
+'
+
 test_expect_success 'ref content checks should work with worktrees' '
 	test_when_finished "rm -rf repo" &&
 	git init repo &&