Added permanent store feature

tests/OfflineTests/ObjectRetrieverTest.php

@@ -26,4 +26,30 @@ class ObjectRetrieverTest extends \PHPUnit\Framework\TestCase {
         ]);
     }
 
+    public function testPermanentStoreWithExistingDirectory() {
+        $retriever = new ObjectRetriever([
+            'permanent_store_directory' => sys_get_temp_dir(),
+            'permanent_store_scope' => '*',
+        ]);
+        $this->assertInstanceOf(ObjectRetriever::class, $retriever);
+    }
+
+    public function testPermanentStoreWithMissingDirectory() {
+        $this->expectException(InvalidSDKConfigurationException::class);
+        new ObjectRetriever([
+            'permanent_store_directory' => sys_get_temp_dir().'/does-not-exist-'.uniqid(),
+            'permanent_store_scope' => '*',
+        ]);
+    }
+
+    public function testPermanentStoreWithSameDirectoryAsFallbackStore() {
+        $this->expectException(InvalidSDKConfigurationException::class);
+        new ObjectRetriever([
+            'fallback_store_directory' => sys_get_temp_dir(),
+            'fallback_store_scope' => '*',
+            'permanent_store_directory' => sys_get_temp_dir(),
+            'permanent_store_scope' => '*',
+        ]);
+    }
+
 }