[GitHub] [pulsar] tjiuming commented on pull request #16909: fix NPE

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

tjiuming commented on PR #16909:
URL: https://github.com/apache/pulsar/pull/16909#issuecomment-1201983074

   > I also met this problem before. Could you explain why it causes NPE here?
   
   I believe there existing null element in the `entries`, but I don't think it caused by multi-threads adding. 
   ```
       private void asyncReadEntry0(ReadHandle lh, PositionImpl position, final ReadEntryCallback callback,
               final Object ctx) {
           if (log.isDebugEnabled()) {
               log.debug("[{}] Reading entry ledger {}: {}", ml.getName(), lh.getId(), position.getEntryId());
           }
           EntryImpl entry = entries.get(position);
           if (entry != null) {
               EntryImpl cachedEntry = EntryImpl.create(entry);
               entry.release();
               manager.mlFactoryMBean.recordCacheHit(cachedEntry.getLength());
               callback.readEntryComplete(cachedEntry, ctx);
           } else {
               lh.readAsync(position.getEntryId(), position.getEntryId()).thenAcceptAsync(
                       ledgerEntries -> {
                           try {
                               Iterator<LedgerEntry> iterator = ledgerEntries.iterator();
                               if (iterator.hasNext()) {
                                   LedgerEntry ledgerEntry = iterator.next();
                                   EntryImpl returnEntry = RangeEntryCacheManagerImpl.create(ledgerEntry, interceptor);
   
                                   manager.mlFactoryMBean.recordCacheMiss(1, returnEntry.getLength());
                                   ml.getMbean().addReadEntriesSample(1, returnEntry.getLength());
                                   callback.readEntryComplete(returnEntry, ctx);
                               } else {
                                   // got an empty sequence
                                   callback.readEntryFailed(new ManagedLedgerException("Could not read given position"),
                                                            ctx);
                               }
                           } finally {
                               ledgerEntries.close();
                           }
                       }, ml.getExecutor().chooseThread(ml.getName())).exceptionally(exception -> {
                           ml.invalidateLedgerHandle(lh);
                           callback.readEntryFailed(createManagedLedgerException(exception), ctx);
                           return null;
               });
           }
       }
   ```
   `ml.getExecutor().chooseThread(ml.getName())` a ManagedLedger always use the same SingleThreadExecutor.


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