[I] Bookie translate to readonly mode due to NullPointerException while flushing mem table. [bookkeeper]

["zyj5340 (via GitHub)" <gi...@apache.org>]

zyj5340 opened a new issue, #4270:
URL: https://github.com/apache/bookkeeper/issues/4270

   **BUG REPORT**
   When flushing mem table, the EntryMemTable.flush() method throws an NullPointerException and it cause the Bookie translate into readonly mode.
   ***Describe the bug***
   While processing FileInfo.readAbsolute() method , it passed the  first check (fc == null) but got NullPointerException while doing fc.read(bb.start), we think the fc may be closed during two synchronized lock, and that cause the problem.
   
   ERROR LOG:
   ```
   2024-04-04 04:26:10,349Z [SortedLedgerStorage-0] ERROR org.apache.bookkeeper.bookie.SortedLedgerStorage - Exception thrown while flushing skip list cache.
   java.lang.NullPointerException: Cannot invoke "java.nio.channels.FileChannel.read(java.nio.ByteBuffer, long)" because "this.fc" is null
           at org.apache.bookkeeper.bookie.FileInfo.readAbsolute(FileInfo.java:426) ~[org.apache.bookkeeper-bookkeeper-server-4.14.3]
           at org.apache.bookkeeper.bookie.FileInfo.read(FileInfo.java:396) ~[org.apache.bookkeeper-bookkeeper-server-4.14.3]
           at org.apache.bookkeeper.bookie.LedgerEntryPage.readPage(LedgerEntryPage.java:196) ~[org.apache.bookkeeper-bookkeeper-server-4.14.3]
           at org.apache.bookkeeper.bookie.IndexPersistenceMgr.updatePage(IndexPersistenceMgr.java:650) ~[org.apache.bookkeeper-bookkeeper-server-4.14.3]
           at org.apache.bookkeeper.bookie.IndexInMemPageMgr.grabLedgerEntryPage(IndexInMemPageMgr.java:449) ~[org.apache.bookkeeper-bookkeeper-server-4.14.3]
           at org.apache.bookkeeper.bookie.IndexInMemPageMgr.getLedgerEntryPage(IndexInMemPageMgr.java:414) ~[org.apache.bookkeeper-bookkeeper-server-4.14.3]
           at org.apache.bookkeeper.bookie.IndexInMemPageMgr.putEntryOffset(IndexInMemPageMgr.java:573) ~[org.apache.bookkeeper-bookkeeper-server-4.14.3]
           at org.apache.bookkeeper.bookie.LedgerCacheImpl.putEntryOffset(LedgerCacheImpl.java:107) ~[org.apache.bookkeeper-bookkeeper-server-4.14.3]
           at org.apache.bookkeeper.bookie.InterleavedLedgerStorage.processEntry(InterleavedLedgerStorage.java:539) ~[org.apache.bookkeeper-bookkeeper-server-4.14.3]
           at org.apache.bookkeeper.bookie.SortedLedgerStorage.process(SortedLedgerStorage.java:288) ~[org.apache.bookkeeper-bookkeeper-server-4.14.3]
           at org.apache.bookkeeper.bookie.EntryMemTable.flushSnapshot(EntryMemTable.java:255) ~[org.apache.bookkeeper-bookkeeper-server-4.14.3]
           at org.apache.bookkeeper.bookie.EntryMemTable.flush(EntryMemTable.java:205) ~[org.apache.bookkeeper-bookkeeper-server-4.14.3]
          [root@stage-master-0001 ~]# timed out waiting for input: auto-logoutgerStorage.java:317) ~[org.apache.bookkeeper-bookkeeper-server-4.14.3]
   Connection closed.il.concurrent.Executors$RunnableAdapter.call(Unknown Source) ~[?:?]
           at java.util.concurrent.FutureTask.run(Unknown Source) ~[?:?]
   
   ```
   
   ***To Reproduce***
   It difficult to reproduce, happen occasionally.
   
   ***Additional context***
   We want to add a second check if the fc is null in second synchronized, and if the fc equals null ,we clear the ByteBuffer and return 0. However, we don't know whether this change will cause another problem or is there a better way to fix this bug. 
   ![image](https://github.com/apache/bookkeeper/assets/91134060/f21391ea-d23d-4017-b944-cc9cd4c38473)
   
   


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