Merge pull request #3323, fix double-checked locking.

Author: biyuhao

dubbo-metadata-report/dubbo-metadata-report-api/src/main/java/org/apache/dubbo/metadata/integration/MetadataReportService.java

@@ -38,7 +38,7 @@ public class MetadataReportService {
 
     protected final Logger logger = LoggerFactory.getLogger(getClass());
 
-    private static MetadataReportService metadataReportService;
+    private static volatile MetadataReportService metadataReportService;
     private static Object lock = new Object();
 
     private MetadataReportFactory metadataReportFactory = ExtensionLoader.getExtensionLoader(MetadataReportFactory.class).getAdaptiveExtension();

dubbo-metadata-report/dubbo-metadata-report-api/src/main/java/org/apache/dubbo/metadata/support/AbstractMetadataReport.java

@@ -357,7 +357,7 @@ class MetadataReportRetry {
         protected final Logger logger = LoggerFactory.getLogger(getClass());
 
         final ScheduledExecutorService retryExecutor = Executors.newScheduledThreadPool(0, new NamedThreadFactory("DubboRegistryFailedRetryTimer", true));
-        ScheduledFuture retryScheduledFuture;
+        volatile ScheduledFuture retryScheduledFuture;
         AtomicInteger retryCounter = new AtomicInteger(0);
         // retry task schedule period
         long retryPeriod;