Test fsck a bit harder

git-fsck, of all tools, has very few tests.  This adds some more:
* a corrupted object;
* a branch pointing to a non-commit;
* a tag pointing to a nonexistent object;
* and a tag pointing to an object of a type other than what the tag
  itself claims.

Only the first two are caught.  At least the third probably should,
too, but currently slips through.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1450-fsck.sh

@@ -28,4 +28,71 @@ test_expect_success 'loose objects borrowed from alternate are not missing' '
 	)
 '
 
+# Corruption tests follow.  Make sure to remove all traces of the
+# specific corruption you test afterwards, lest a later test trip over
+# it.
+
+test_expect_success 'object with bad sha1' '
+	sha=$(echo blob | git hash-object -w --stdin) &&
+	echo $sha &&
+	old=$(echo $sha | sed "s+^..+&/+") &&
+	new=$(dirname $old)/ffffffffffffffffffffffffffffffffffffff &&
+	sha="$(dirname $new)$(basename $new)"
+	mv .git/objects/$old .git/objects/$new &&
+	git update-index --add --cacheinfo 100644 $sha foo &&
+	tree=$(git write-tree) &&
+	cmt=$(echo bogus | git commit-tree $tree) &&
+	git update-ref refs/heads/bogus $cmt &&
+	(git fsck 2>out; true) &&
+	grep "$sha.*corrupt" out &&
+	rm -f .git/objects/$new &&
+	git update-ref -d refs/heads/bogus &&
+	git read-tree -u --reset HEAD
+'
+
+test_expect_success 'branch pointing to non-commit' '
+	git rev-parse HEAD^{tree} > .git/refs/heads/invalid &&
+	git fsck 2>out &&
+	grep "not a commit" out &&
+	git update-ref -d refs/heads/invalid
+'
+
+cat > invalid-tag <<EOF
+object ffffffffffffffffffffffffffffffffffffffff
+type commit
+tag invalid
+tagger T A Gger <tagger@example.com> 1234567890 -0000
+
+This is an invalid tag.
+EOF
+
+test_expect_failure 'tag pointing to nonexistent' '
+	tag=$(git hash-object -w --stdin < invalid-tag) &&
+	echo $tag > .git/refs/tags/invalid &&
+	git fsck --tags 2>out &&
+	cat out &&
+	grep "could not load tagged object" out &&
+	rm .git/refs/tags/invalid
+'
+
+cat > wrong-tag <<EOF
+object $(echo blob | git hash-object -w --stdin)
+type commit
+tag wrong
+tagger T A Gger <tagger@example.com> 1234567890 -0000
+
+This is an invalid tag.
+EOF
+
+test_expect_failure 'tag pointing to something else than its type' '
+	tag=$(git hash-object -w --stdin < wrong-tag) &&
+	echo $tag > .git/refs/tags/wrong &&
+	git fsck --tags 2>out &&
+	cat out &&
+	grep "some sane error message" out &&
+	rm .git/refs/tags/wrong
+'
+
+
+
 test_done