SingleDirectoryDbLedgerStorage skip optimistic cache put sometimes (#4306)

* SingleDirectoryDbLedgerStorage skip optimistic cache put

When tryOptimisticRead returns 0, the lock is acquired
by another thread and we will fail the validation step.
We can therefore skip the eager cache insertion.

Author: michaeljmarshall

bookkeeper-server/src/main/java/org/apache/bookkeeper/bookie/storage/ldb/SingleDirectoryDbLedgerStorage.java

@@ -484,8 +484,12 @@ public long addEntry(ByteBuf entry) throws IOException, BookieException {
         long stamp = writeCacheRotationLock.tryOptimisticRead();
         boolean inserted = false;
 
-        inserted = writeCache.put(ledgerId, entryId, entry);
-        if (!writeCacheRotationLock.validate(stamp)) {
+        // If the stamp is 0, the lock was exclusively acquired, validation will fail, and we can skip this put.
+        if (stamp != 0) {
+            inserted = writeCache.put(ledgerId, entryId, entry);
+        }
+
+        if (stamp == 0 || !writeCacheRotationLock.validate(stamp)) {
             // The write cache was rotated while we were inserting. We need to acquire the proper read lock and repeat
             // the operation because we might have inserted in a write cache that was already being flushed and cleared,
             // without being sure about this last entry being flushed or not.