Fix Spark Job remote debug hang regression

Author: wezhang

Utils/hdinsight-node-common/src/com/microsoft/azure/hdinsight/spark/common/SparkBatchJob.java

@@ -387,12 +387,18 @@ public Observable<SimpleImmutableEntry<String, Long>> getDriverLog(@NotNull Stri
                         return Observable.empty();
                     }
 
-                    return Observable.just(new SimpleImmutableEntry<>(JobUtils.getInformationFromYarnLogDom(
+                    String logGot = JobUtils.getInformationFromYarnLogDom(
                             getSubmission().getCredentialsProvider(),
                             getCurrentLogUrl(),
                             type,
                             offset,
-                            size), offset));
+                            size);
+
+                    if (StringUtils.isEmpty(logGot)) {
+                        return Observable.empty();
+                    }
+
+                    return Observable.just(new SimpleImmutableEntry<>(logGot, offset));
                 });
     }
 

Utils/hdinsight-node-common/src/com/microsoft/azure/hdinsight/spark/run/SparkJobLogInputStream.java

@@ -64,7 +64,7 @@ public ISparkBatchJob attachJob(@NotNull ISparkBatchJob sparkJob) {
     private synchronized Optional<SimpleImmutableEntry<String, Long>> fetchLog(long logOffset, int fetchSize) {
         return getAttachedJob()
                 .map(job -> job.getDriverLog(getLogType(), logOffset, fetchSize)
-                               .toBlocking().singleOrDefault(new SimpleImmutableEntry<>("", logOffset)));
+                               .toBlocking().singleOrDefault(null));
     }
 
     void setSparkBatchJob(@Nullable ISparkBatchJob sparkBatchJob) {