Introduce Page Cursor Counters interface to split

cursor counters and tracers to isolate code that can only access counters
and actually do cursor events tracing.

community/io/src/main/java/org/neo4j/io/pagecache/tracing/cursor/PageCursorCounters.java

@@ -0,0 +1,52 @@
+/*
+ * Copyright (c) 2002-2017 "Neo Technology,"
+ * Network Engine for Objects in Lund AB [http://neotechnology.com]
+ *
+ * This file is part of Neo4j.
+ *
+ * Neo4j is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+package org.neo4j.io.pagecache.tracing.cursor;
+
+/**
+ * Expose information from page cache about particular page cursor events.
+ *
+ * Exposed counters are relevant only for particular cursor and do not represent global page cache wide picture.
+ *
+ * @see PageCursorCounters
+ * @see PageCursorTracer
+ */
+public interface PageCursorCounters
+{
+    /**
+     * @return The number of page faults observed thus far.
+     */
+    long faults();
+
+    /**
+     * @return The number of page pins observed thus far.
+     */
+    long pins();
+
+    /**
+     * @return The number of page unpins observed thus far.
+     */
+    long unpins();
+
+    /**
+     * @return The sum total of bytes read in through page faults thus far.
+     */
+    long bytesRead();
+
+}

community/io/src/main/java/org/neo4j/io/pagecache/tracing/cursor/PageCursorTracer.java

@@ -23,8 +23,16 @@
 import org.neo4j.io.pagecache.tracing.PageCacheTracer;
 import org.neo4j.io.pagecache.tracing.PinEvent;
 
-//:TODO javadoc
-public interface PageCursorTracer
+/**
+ * Event tracer for page cursors.
+ *
+ * Performs event tracing related to particular page cursors and expose simple counters around them.
+ * Since events of each particular page cursor are part of whole page cache events, each particular page cursor
+ * tracer will eventually report them to global page cache counters/tracers.
+ *
+ * @see PageCursorTracer
+ */
+public interface PageCursorTracer extends PageCursorCounters
 {
 
     PageCursorTracer NULL = new PageCursorTracer()
@@ -72,29 +80,8 @@ public void reportEvents()
         }
     };
 
-    /**
-     * @return The number of page faults observed thus far.
-     */
-    long faults();
-
-    /**
-     * @return The number of page pins observed thus far.
-     */
-    long pins();
-
-    /**
-     * @return The number of page unpins observed thus far.
-     */
-    long unpins();
-
-    /**
-     * @return The sum total of bytes read in through page faults thus far.
-     */
-    long bytesRead();
-
     PinEvent beginPin( boolean writeLock, long filePageId, PageSwapper swapper );
 
-    // TODO:: methods that glue current tracer to page cache tracer can be avoided?
     void init( PageCacheTracer tracer );
 
     void reportEvents();

community/kernel/src/main/java/org/neo4j/kernel/impl/api/KernelStatement.java

@@ -24,7 +24,7 @@
 
 import org.neo4j.graphdb.NotInTransactionException;
 import org.neo4j.graphdb.TransactionTerminatedException;
-import org.neo4j.io.pagecache.tracing.cursor.PageCursorTracer;
+import org.neo4j.io.pagecache.tracing.cursor.PageCursorCounters;
 import org.neo4j.kernel.api.DataWriteOperations;
 import org.neo4j.kernel.api.query.ExecutingQuery;
 import org.neo4j.kernel.api.ProcedureCallOperations;
@@ -53,14 +53,14 @@
  * <ol>
  * <li>Construct {@link KernelStatement} when {@link KernelTransactionImplementation} is constructed</li>
  * <li>For every transaction...</li>
- * <li>Call {@link #initialize(StatementLocks, StatementOperationParts, PageCursorTracer)} which makes this instance
+ * <li>Call {@link #initialize(StatementLocks, StatementOperationParts, PageCursorCounters)} which makes this instance
  * full available and ready to use. Call when the {@link KernelTransactionImplementation} is initialized.</li>
  * <li>Alternate {@link #acquire()} / {@link #close()} when acquiring / closing a statement for the transaction...
  * Temporarily asymmetric number of calls to {@link #acquire()} / {@link #close()} is supported, although in
  * the end an equal number of calls must have been issued.</li>
  * <li>To be safe call {@link #forceClose()} at the end of a transaction to force a close of the statement,
  * even if there are more than one current call to {@link #acquire()}. This instance is now again ready
- * to be {@link #initialize(StatementLocks, StatementOperationParts, PageCursorTracer)  initialized} and used for the transaction
+ * to be {@link #initialize(StatementLocks, StatementOperationParts, PageCursorCounters)}  initialized} and used for the transaction
  * instance again, when it's initialized.</li>
  * </ol>
  */
@@ -72,7 +72,7 @@ public class KernelStatement implements TxStateHolder, Statement
     private final KernelTransactionImplementation transaction;
     private final OperationsFacade facade;
     private StatementLocks statementLocks;
-    private PageCursorTracer pageCursorTracer;
+    private PageCursorCounters pageCursorCounters;
     private int referenceCount;
     private volatile ExecutingQueryList executingQueryList;
     private final LockTracer systemLockTracer;
@@ -186,10 +186,10 @@ void assertOpen()
     }
 
     public void initialize( StatementLocks statementLocks, StatementOperationParts operationParts,
-            PageCursorTracer pageCursorTracer )
+            PageCursorCounters pageCursorCounters )
     {
         this.statementLocks = statementLocks;
-        this.pageCursorTracer = pageCursorTracer;
+        this.pageCursorCounters = pageCursorCounters;
         facade.initialize( operationParts );
     }
 
@@ -204,9 +204,9 @@ public LockTracer lockTracer()
         return tracer == null ? systemLockTracer : systemLockTracer.combine( tracer );
     }
 
-    public PageCursorTracer getPageCursorTracer()
+    public PageCursorCounters getPageCursorCounters()
     {
-        return pageCursorTracer;
+        return pageCursorCounters;
     }
 
     public final void acquire()