fix detail task may failed by log file expired (#4475)

Author: ungreat

server/odc-service/src/main/java/com/oceanbase/odc/service/flow/FlowTaskInstanceService.java

@@ -34,6 +34,7 @@
 import java.util.function.Predicate;
 import java.util.stream.Collectors;
 
+import javax.annotation.Nullable;
 import javax.validation.constraints.NotNull;
 
 import org.apache.commons.collections4.CollectionUtils;
@@ -783,7 +784,10 @@ private void setDownloadUrlsIfNecessary(Long taskId, List<? extends FlowTaskResu
             return;
         }
         for (FlowTaskResult result : results) {
-            TaskDownloadUrls urls = databaseChangeOssUrlCache.get(taskId);
+            TaskDownloadUrls urls = getTaskDownloadUrls(taskId);
+            if (urls == null) {
+                return;
+            }
             if (result instanceof AbstractFlowTaskResult) {
                 ((AbstractFlowTaskResult) result)
                         .setFullLogDownloadUrl(urls.getLogDownloadUrl());
@@ -798,4 +802,14 @@ private void setDownloadUrlsIfNecessary(Long taskId, List<? extends FlowTaskResu
         }
     }
 
+    @Nullable
+    private TaskDownloadUrls getTaskDownloadUrls(Long taskId) {
+        try {
+            return databaseChangeOssUrlCache.get(taskId);
+        } catch (NotFoundException e) {
+            log.warn("Failed to retrieve task download urls from cloud object storage, taskId={}", taskId, e);
+            return null;
+        }
+    }
+
 }