Return 503 in public.php and OCS API when upgrade is due

To prevent unexpected behavior, public.php and the OCS API calls will
return 503 Service Unavailable when an upgrade is due.

ocs/v1.php

@@ -23,6 +23,14 @@
 
 require_once '../lib/base.php';
 
+if (\OCP\Util::needUpgrade()) {
+	// since the behavior of apps or remotes are unpredictable during
+	// an upgrade, return a 503 directly
+	OC_Response::setStatus(OC_Response::STATUS_SERVICE_UNAVAILABLE);
+	OC_Template::printErrorPage('Service unavailable');
+	exit;
+}
+
 use Symfony\Component\Routing\Exception\ResourceNotFoundException;
 use Symfony\Component\Routing\Exception\MethodNotAllowedException;
 

public.php

@@ -3,6 +3,14 @@
 try {
 
 	require_once 'lib/base.php';
+	if (\OCP\Util::needUpgrade()) {
+		// since the behavior of apps or remotes are unpredictable during
+		// an upgrade, return a 503 directly
+		OC_Response::setStatus(OC_Response::STATUS_SERVICE_UNAVAILABLE);
+		OC_Template::printErrorPage('Service unavailable');
+		exit;
+	}
+
 	OC::checkMaintenanceMode();
 	OC::checkSingleUserMode();
 	$pathInfo = OC_Request::getPathInfo();