fsck: avoid looking at NULL blob->object

Commit 159e7b080b (fsck: detect gitmodules files,
2018-05-02) taught fsck to look at the content of
.gitmodules files. If the object turns out not to be a blob
at all, we just complain and punt on checking the content.
And since this was such an obvious and trivial code path, I
didn't even bother to add a test.

Except it _does_ do one non-trivial thing, which is call the
report() function, which wants us to pass a pointer to a
"struct object". Which we don't have (we have only a "struct
object_id"). So we erroneously pass a NULL object to
report(), which gets dereferenced and causes a segfault.

It seems like we could refactor report() to just take the
object_id itself. But we pass the object pointer along to
a callback function, and indeed this ends up in
builtin/fsck.c's objreport() which does want to look at
other parts of the object (like the type).

So instead, let's just use lookup_unknown_object() to get
the real "struct object", and pass that.

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

1 files changed, 18 insertions, 0 deletions

diff --git a/t/t7415-submodule-names.sh b/t/t7415-submodule-names.sh
index 541bd81684..3c0f1a102a 100755
--- a/t/t7415-submodule-names.sh
+++ b/t/t7415-submodule-names.sh
@@ -148,4 +148,22 @@ test_expect_success 'fsck detects symlinked .gitmodules file' '
 	)
 '
 
+test_expect_success 'fsck detects non-blob .gitmodules' '
+	git init non-blob &&
+	(
+		cd non-blob &&
+
+		# As above, make the funny tree directly to avoid index
+		# restrictions.
+		mkdir subdir &&
+		cp ../.gitmodules subdir/file &&
+		git add subdir/file &&
+		git commit -m ok &&
+		git ls-tree HEAD | sed s/subdir/.gitmodules/ | git mktree &&
+
+		test_must_fail git fsck 2>output &&
+		grep gitmodulesBlob output
+	)
+'
+
 test_done