HBASE-20628 SegmentScanner does over-comparing when one flushing

Signed-off-by: eshcar <eshcar@oath.com>
Signed-off-by: anoopsjohn <anoopsamjohn@gmail.com>

hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/CompositeImmutableSegment.java

@@ -279,4 +279,13 @@ public class CompositeImmutableSegment extends ImmutableSegment {
     }
     return sb.toString();
   }
+
+  @Override
+  List<KeyValueScanner> getSnapshotScanners() {
+    List<KeyValueScanner> list = new ArrayList<>(this.segments.size());
+    for (ImmutableSegment segment: this.segments) {
+      list.add(new SnapshotSegmentScanner(segment));
+    }
+    return list;
+  }
 }

hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/ImmutableSegment.java

@@ -84,4 +84,8 @@ public abstract class ImmutableSegment extends Segment {
     res += "Num uniques "+getNumUniqueKeys()+"; ";
     return res;
   }
+
+  List<KeyValueScanner> getSnapshotScanners() {
+    return Collections.singletonList(new SnapshotSegmentScanner(this));
+  }
 }

hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/MemStoreSnapshot.java

@@ -40,7 +40,7 @@ public class MemStoreSnapshot implements Closeable {
     this.cellsCount = snapshot.getCellsCount();
     this.memStoreSize = snapshot.getMemStoreSize();
     this.timeRangeTracker = snapshot.getTimeRangeTracker();
-    this.scanners = snapshot.getScanners(Long.MAX_VALUE);
+    this.scanners = snapshot.getSnapshotScanners();
     this.tagsPresent = snapshot.isTagsPresent();
   }
 
@@ -95,5 +95,4 @@ public class MemStoreSnapshot implements Closeable {
       }
     }
   }
-
 }

hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/SnapshotSegmentScanner.java

@@ -0,0 +1,101 @@
+/*
+ *
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.hadoop.hbase.regionserver;
+
+import java.util.Iterator;
+
+import org.apache.hadoop.hbase.Cell;
+import org.apache.yetus.audience.InterfaceAudience;
+
+
+/**
+ * A basic SegmentScanner used against an ImmutableScanner snapshot
+ * Used flushing where we do a single pass, no reverse scanning or
+ * inserts happening. Its a dumbed-down Scanner that can go fast.
+ * Like {@link org.apache.hadoop.hbase.util.CollectionBackedScanner}
+ * (but making it know about Segments was onerous).
+ */
+@InterfaceAudience.Private
+public class SnapshotSegmentScanner extends NonReversedNonLazyKeyValueScanner {
+  private final ImmutableSegment segment;
+  private Iterator<Cell> iter;
+  private Cell current;
+
+  public SnapshotSegmentScanner(ImmutableSegment segment) {
+    this.segment = segment;
+    this.segment.incScannerCount();
+    this.iter = createIterator(this.segment);
+    if (this.iter.hasNext()){
+      this.current = this.iter.next();
+    }
+  }
+
+  private static Iterator<Cell> createIterator(Segment segment) {
+    return segment.getCellSet().iterator();
+  }
+
+  @Override
+  public Cell peek() {
+    return current;
+  }
+
+  @Override
+  public Cell next() {
+    Cell oldCurrent = current;
+    if(iter.hasNext()){
+      current = iter.next();
+    } else {
+      current = null;
+    }
+    return oldCurrent;
+  }
+
+  @Override
+  public boolean seek(Cell seekCell) {
+    // restart iterator
+    this.iter = createIterator(this.segment);
+    return reseek(seekCell);
+  }
+
+  @Override
+  public boolean reseek(Cell seekCell) {
+    while (this.iter.hasNext()){
+      Cell next = this.iter.next();
+      int ret = this.segment.getComparator().compare(next, seekCell);
+      if (ret >= 0) {
+        this.current = next;
+        return true;
+      }
+    }
+    return false;
+  }
+
+  /**
+   * @see KeyValueScanner#getScannerOrder()
+   */
+  @Override
+  public long getScannerOrder() {
+    return 0;
+  }
+
+  @Override
+  public void close() {
+    this.segment.decScannerCount();
+  }
+}