[GitHub] [incubator-pegasus] acelyc111 commented on a diff in pull request #1019: feat(java-client): Add hasNext() for PegasusScannerInterface

[GitBox <gi...@apache.org>]

acelyc111 commented on code in PR #1019:
URL: https://github.com/apache/incubator-pegasus/pull/1019#discussion_r909186933


##########
java-client/src/main/java/org/apache/pegasus/client/PegasusScanner.java:
##########
@@ -88,9 +88,27 @@ public PegasusScanner(
     _needCheckHash = needCheckHash;
     _incomplete = false;
     _fullScan = fullScan;
+    _nextItem = null;
+  }
+
+  public boolean hasNext() throws PException {
+    synchronized (_promisesLock) {
+      if (_nextItem != null) {
+        return true;
+      }
+    }
+    _nextItem = next();

Review Comment:
   It's not thread safe in some cases since `_nextItem` will be overwritten.
   
   ```
   time  thread1         thread2
     t1  hasNext         hasNext
     t2  _nextItem = a
     t3                  _nextItem = b
     t4  next -> b
     t5                  next -> nullptr
   ```



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