fsck: complain when .gitmodules is a symlink

We've recently forbidden .gitmodules to be a symlink in
verify_path(). And it's an easy way to circumvent our fsck
checks for .gitmodules content. So let's complain when we
see it.

Signed-off-by: Jeff King <peff@peff.net>

fsck.c

@@ -63,6 +63,7 @@ static struct oidset gitmodules_done = OIDSET_INIT;
 	FUNC(GITMODULES_BLOB, ERROR) \
 	FUNC(GITMODULES_PARSE, ERROR) \
 	FUNC(GITMODULES_NAME, ERROR) \
+	FUNC(GITMODULES_SYMLINK, ERROR) \
 	/* warnings */ \
 	FUNC(BAD_FILEMODE, WARN) \
 	FUNC(EMPTY_NAME, WARN) \
@@ -576,8 +577,14 @@ static int fsck_tree(struct tree *item, struct fsck_options *options)
 		has_dotgit |= is_hfs_dotgit(name) || is_ntfs_dotgit(name);
 		has_zero_pad |= *(char *)desc.buffer == '0';
 
-		if (is_hfs_dotgitmodules(name) || is_ntfs_dotgitmodules(name))
+		if (is_hfs_dotgitmodules(name) || is_ntfs_dotgitmodules(name)) {
+			if (!S_ISLNK(mode))
 				oidset_insert(&gitmodules_found, oid);
+			else
+				retval += report(options, &item->object,
+						 FSCK_MSG_GITMODULES_SYMLINK,
+						 ".gitmodules is a symbolic link");
+		}
 
 		if (update_tree_entry_gently(&desc)) {
 			retval += report(options, &item->object, FSCK_MSG_BAD_TREE, "cannot be parsed as a tree");

t/t7415-submodule-names.sh

@@ -122,4 +122,33 @@ test_expect_success 'transfer.fsckObjects handles odd pack (index)' '
 	test_must_fail git -C dst.git index-pack --strict --stdin <odd.pack
 '
 
+test_expect_success 'fsck detects symlinked .gitmodules file' '
+	git init symlink &&
+	(
+		cd symlink &&
+
+		# Make the tree directly to avoid index restrictions.
+		#
+		# Because symlinks store the target as a blob, choose
+		# a pathname that could be parsed as a .gitmodules file
+		# to trick naive non-symlink-aware checking.
+		tricky="[foo]bar=true" &&
+		content=$(git hash-object -w ../.gitmodules) &&
+		target=$(printf "$tricky" | git hash-object -w --stdin) &&
+		tree=$(
+			{
+				printf "100644 blob $content\t$tricky\n" &&
+				printf "120000 blob $target\t.gitmodules\n"
+			} | git mktree
+		) &&
+		commit=$(git commit-tree $tree) &&
+
+		# Check not only that we fail, but that it is due to the
+		# symlink detector; this grep string comes from the config
+		# variable name and will not be translated.
+		test_must_fail git fsck 2>output &&
+		grep gitmodulesSymlink output
+	)
+'
+
 test_done