HBASE-19473 Miscellaneous changes to ClientScanner

- Remove superfluous logging code guard
- Simplify some of the code
- Use ArrayDeque instead of LinkedList for queue implementation

hbase-client/src/main/java/org/apache/hadoop/hbase/client/ClientScanner.java

@@ -25,29 +25,30 @@ import org.apache.hbase.thirdparty.com.google.common.annotations.VisibleForTesti
 
 import java.io.IOException;
 import java.io.InterruptedIOException;
-import java.util.LinkedList;
+import java.util.ArrayDeque;
 import java.util.Queue;
 import java.util.concurrent.ExecutorService;
 
 import org.apache.commons.lang3.mutable.MutableBoolean;
 import org.apache.hadoop.conf.Configuration;
+import org.apache.hadoop.hbase.client.ScannerCallable.MoreResults;
 import org.apache.hadoop.hbase.DoNotRetryIOException;
+import org.apache.hadoop.hbase.exceptions.OutOfOrderScannerNextException;
+import org.apache.hadoop.hbase.exceptions.ScannerResetException;
 import org.apache.hadoop.hbase.HBaseConfiguration;
 import org.apache.hadoop.hbase.HConstants;
 import org.apache.hadoop.hbase.HRegionInfo;
+import org.apache.hadoop.hbase.ipc.RpcControllerFactory;
 import org.apache.hadoop.hbase.NotServingRegionException;
+import org.apache.hadoop.hbase.regionserver.RegionServerStoppedException;
 import org.apache.hadoop.hbase.TableName;
 import org.apache.hadoop.hbase.UnknownScannerException;
+import org.apache.hadoop.hbase.util.Bytes;
 import org.apache.yetus.audience.InterfaceAudience;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
-import org.apache.hadoop.hbase.client.ScannerCallable.MoreResults;
-import org.apache.hadoop.hbase.exceptions.OutOfOrderScannerNextException;
-import org.apache.hadoop.hbase.exceptions.ScannerResetException;
-import org.apache.hadoop.hbase.ipc.RpcControllerFactory;
-import org.apache.hadoop.hbase.regionserver.RegionServerStoppedException;
+
 import org.apache.hadoop.hbase.shaded.protobuf.ProtobufUtil;
-import org.apache.hadoop.hbase.util.Bytes;
 
 /**
  * Implements the scanner interface for the HBase client. If there are multiple regions in a table,
@@ -294,25 +295,30 @@ public abstract class ClientScanner extends AbstractClientScanner {
   }
 
   protected void initSyncCache() {
-    cache = new LinkedList<>();
+    cache = new ArrayDeque<>();
   }
 
   protected Result nextWithSyncCache() throws IOException {
-    // If the scanner is closed and there's nothing left in the cache, next is a no-op.
-    if (cache.isEmpty() && this.closed) {
+    Result result = cache.poll();
+    if (result != null) {
+      return result;
+    }
+    // If there is nothing left in the cache and the scanner is closed,
+    // return a no-op
+    if (this.closed) {
       return null;
     }
-    if (cache.isEmpty()) {
-      loadCache();
-    }
 
-    if (cache.size() > 0) {
-      return cache.poll();
-    }
+    loadCache();
+
+    // try again to load from cache
+    result = cache.poll();
 
     // if we exhausted this scanner before calling close, write out the scan metrics
+    if (result == null) {
       writeScanMetrics();
-    return null;
+    }
+    return result;
   }
 
   @VisibleForTesting
@@ -410,12 +416,10 @@ public abstract class ClientScanner extends AbstractClientScanner {
     long remainingResultSize = maxScannerResultSize;
     int countdown = this.caching;
     // This is possible if we just stopped at the boundary of a region in the previous call.
-    if (callable == null) {
-      if (!moveToNextRegion()) {
+    if (callable == null && !moveToNextRegion()) {
       closed = true;
       return;
     }
-    }
     // This flag is set when we want to skip the result returned. We do
     // this when we reset scanner because it split under us.
     MutableBoolean retryAfterOutOfOrderException = new MutableBoolean(true);
@@ -463,7 +467,6 @@ public abstract class ClientScanner extends AbstractClientScanner {
           scanResultCache.addAndGet(values, callable.isHeartbeatMessage());
       int numberOfCompleteRows =
           scanResultCache.numberOfCompleteRows() - numberOfCompleteRowsBefore;
-      if (resultsToAddToCache.length > 0) {
       for (Result rs : resultsToAddToCache) {
         cache.add(rs);
         long estimatedHeapSizeOfResult = calcEstimatedSize(rs);
@@ -472,7 +475,6 @@ public abstract class ClientScanner extends AbstractClientScanner {
         addEstimatedSize(estimatedHeapSizeOfResult);
         this.lastResult = rs;
       }
-      }
 
       if (scan.getLimit() > 0) {
         int newLimit = scan.getLimit() - numberOfCompleteRows;
@@ -491,10 +493,8 @@ public abstract class ClientScanner extends AbstractClientScanner {
           // processing of the scan is taking a long time server side. Rather than continue to
           // loop until a limit (e.g. size or caching) is reached, break out early to avoid causing
           // unnecesary delays to the caller
-          if (LOG.isTraceEnabled()) {
-            LOG.trace("Heartbeat message received and cache contains Results." +
-                " Breaking out of scan loop");
-          }
+          LOG.trace("Heartbeat message received and cache contains Results. " +
+            "Breaking out of scan loop");
           // we know that the region has not been exhausted yet so just break without calling
           // closeScannerIfExhausted
           break;
@@ -560,9 +560,7 @@ public abstract class ClientScanner extends AbstractClientScanner {
         // We used to catch this error, interpret, and rethrow. However, we
         // have since decided that it's not nice for a scanner's close to
         // throw exceptions. Chances are it was just due to lease time out.
-        if (LOG.isDebugEnabled()) {
         LOG.debug("scanner failed to close", e);
-        }
       } catch (IOException e) {
         /* An exception other than UnknownScanner is unexpected. */
         LOG.warn("scanner failed to close.", e);
@@ -574,19 +572,20 @@ public abstract class ClientScanner extends AbstractClientScanner {
 
   @Override
   public boolean renewLease() {
-    if (callable != null) {
+    if (callable == null) {
+      return false;
+    }
     // do not return any rows, do not advance the scanner
     callable.setRenew(true);
     try {
       this.caller.callWithoutRetries(callable, this.scannerTimeout);
+      return true;
     } catch (Exception e) {
+      LOG.debug("scanner failed to renew lease", e);
       return false;
     } finally {
       callable.setRenew(false);
     }
-      return true;
-    }
-    return false;
   }
 
   protected void initCache() {