refs: reject ref updates while GIT_QUARANTINE_PATH is set

As documented in git-receive-pack(1), updating a ref from
within the pre-receive hook is dangerous and can corrupt
your repo. This patch forbids ref updates entirely during
the hook to make it harder for adventurous hook writers to
shoot themselves in the foot.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-receive-pack.txt

@@ -239,7 +239,8 @@ This has a few user-visible effects and caveats:
   3. The `pre-receive` hook MUST NOT update any refs to point to
      quarantined objects. Other programs accessing the repository will
      not be able to see the objects (and if the pre-receive hook fails,
-     those refs would become corrupted).
+     those refs would become corrupted). For safety, any ref updates
+     from within `pre-receive` are automatically rejected.
 
 
 SEE ALSO

refs.c

@@ -1465,6 +1465,12 @@ int ref_transaction_commit(struct ref_transaction *transaction,
 {
 	struct ref_store *refs = get_ref_store(NULL);
 
+	if (getenv(GIT_QUARANTINE_ENVIRONMENT)) {
+		strbuf_addstr(err,
+			      _("ref updates forbidden inside quarantine environment"));
+		return -1;
+	}
+
 	return refs->be->transaction_commit(refs, transaction, err);
 }
 

t/t5547-push-quarantine.sh

@@ -33,4 +33,15 @@ test_expect_success 'rejected objects are removed' '
 	test_cmp expect actual
 '
 
+test_expect_success 'updating a ref from quarantine is forbidden' '
+	git init --bare update.git &&
+	write_script update.git/hooks/pre-receive <<-\EOF &&
+	read old new refname
+	git update-ref refs/heads/unrelated $new
+	exit 1
+	EOF
+	test_must_fail git push update.git HEAD &&
+	git -C update.git fsck
+'
+
 test_done