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, 2 insertions, 1 deletions

diff --git a/fsck.c b/fsck.c
index 9339f31513..35af51553d 100644
--- a/fsck.c
+++ b/fsck.c
@@ -1032,7 +1032,8 @@ int fsck_finish(struct fsck_options *options)
 
 		blob = lookup_blob(oid);
 		if (!blob) {
-			ret |= report(options, &blob->object,
+			struct object *obj = lookup_unknown_object(oid->hash);
+			ret |= report(options, obj,
 				      FSCK_MSG_GITMODULES_BLOB,
 				      "non-blob found at .gitmodules");
 			continue;