fsck: warn about ".git" in trees

Having a ".git" entry inside a tree can cause confusing
results on checkout. At the top-level, you could not
checkout such a tree, as it would complain about overwriting
the real ".git" directory. In a subdirectory, you might
check it out, but performing operations in the subdirectory
would confusingly consider the in-tree ".git" directory as
the repository.

The regular git tools already make it hard to accidentally
add such an entry to a tree, and do not allow such entries
to enter the index at all. Teaching fsck about it provides
an additional safety check, and let's us avoid propagating
any such bogosity when transfer.fsckObjects is on.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fsck.c

@@ -144,6 +144,7 @@ static int fsck_tree(struct tree *item, int strict, fsck_error error_func)
 	int has_empty_name = 0;
 	int has_dot = 0;
 	int has_dotdot = 0;
+	int has_dotgit = 0;
 	int has_zero_pad = 0;
 	int has_bad_modes = 0;
 	int has_dup_entries = 0;
@@ -174,6 +175,8 @@ static int fsck_tree(struct tree *item, int strict, fsck_error error_func)
 			has_dot = 1;
 		if (!strcmp(name, ".."))
 			has_dotdot = 1;
+		if (!strcmp(name, ".git"))
+			has_dotgit = 1;
 		has_zero_pad |= *(char *)desc.buffer == '0';
 		update_tree_entry(&desc);
 
@@ -227,6 +230,8 @@ static int fsck_tree(struct tree *item, int strict, fsck_error error_func)
 		retval += error_func(&item->object, FSCK_WARN, "contains '.'");
 	if (has_dotdot)
 		retval += error_func(&item->object, FSCK_WARN, "contains '..'");
+	if (has_dotgit)
+		retval += error_func(&item->object, FSCK_WARN, "contains '.git'");
 	if (has_zero_pad)
 		retval += error_func(&item->object, FSCK_WARN, "contains zero-padded file modes");
 	if (has_bad_modes)

t/t1450-fsck.sh

@@ -253,4 +253,19 @@ test_expect_success 'fsck notices "." and ".." in trees' '
 	)
 '
 
+test_expect_success 'fsck notices ".git" in trees' '
+	(
+		git init dotgit &&
+		cd dotgit &&
+		blob=$(echo foo | git hash-object -w --stdin) &&
+		tab=$(printf "\\t") &&
+		git mktree <<-EOF &&
+		100644 blob $blob$tab.git
+		EOF
+		git fsck 2>out &&
+		cat out &&
+		grep "warning.*\\.git" out
+	)
+'
+
 test_done