MDL-68696 sessions: Fixed redis session handler for readonly

In 39770792ca read-only sessions were allowed.
In the redis case, as called from the mobile application,
this can lead to returning 'false' for session rather than ''.

lib/classes/session/redis.php

@@ -259,8 +259,10 @@ class redis extends handler {
                 $this->lock_session($id);
             }
             $sessiondata = $this->connection->get($id);
-            if ($sessiondata === false && $this->requires_write_lock()) {
-                $this->unlock_session($id);
+            if ($sessiondata === false) {
+                if ($this->requires_write_lock()) {
+                    $this->unlock_session($id);
+                }
                 return '';
             }
             $this->connection->expire($id, $this->timeout);

lib/tests/session_redis_test.php

@@ -91,6 +91,14 @@ class core_session_redis_testcase extends advanced_testcase {
         $this->redis->close();
     }
 
+    public function test_normal_session_read_only() {
+        $sess = new \core\session\redis();
+        $sess->set_requires_write_lock(false);
+        $sess->init();
+        $this->assertSame('', $sess->handler_read('sess1'));
+        $this->assertTrue($sess->handler_close());
+    }
+
     public function test_normal_session_start_stop_works() {
         $sess = new \core\session\redis();
         $sess->init();