Added couple unit tests

 * to verify that created event loop threads are the same as default
   Netty event loop threads

 * to verify that it's possible to execute blocking operations in
   `ForkJoinPool.commonPool()` when chaining async stages

Author: lutovich

driver/src/test/java/org/neo4j/driver/internal/async/EventLoopGroupFactoryTest.java

@@ -43,10 +43,7 @@ public class EventLoopGroupFactoryTest
     @After
     public void tearDown()
     {
-        if ( eventLoopGroup != null )
-        {
-            eventLoopGroup.shutdownGracefully().syncUninterruptibly();
-        }
+        shutdown( eventLoopGroup );
     }
 
     @Test
@@ -91,4 +88,46 @@ public void shouldAssertNotInEventLoopThread() throws Exception
             assertThat( e.getCause(), is( blockingOperationInEventLoopError() ) );
         }
     }
+
+    /**
+     * Test verifies that our event loop group uses same kind of thread as Netty does by default.
+     * It's needed because default Netty setup has good performance.
+     */
+    @Test
+    public void shouldUseSameThreadClassAsNioEventLoopGroupDoesByDefault() throws Exception
+    {
+        NioEventLoopGroup nioEventLoopGroup = new NioEventLoopGroup( 1 );
+        eventLoopGroup = EventLoopGroupFactory.newEventLoopGroup( 1 );
+        try
+        {
+            Thread defaultThread = getThread( nioEventLoopGroup );
+            Thread driverThread = getThread( eventLoopGroup );
+
+            assertEquals( defaultThread.getClass(), driverThread.getClass().getSuperclass() );
+            assertEquals( defaultThread.getPriority(), driverThread.getPriority() );
+        }
+        finally
+        {
+            shutdown( nioEventLoopGroup );
+        }
+    }
+
+    private static Thread getThread( EventLoopGroup eventLoopGroup ) throws Exception
+    {
+        return eventLoopGroup.next().submit( Thread::currentThread ).get( 10, SECONDS );
+    }
+
+    private static void shutdown( EventLoopGroup group )
+    {
+        if ( group != null )
+        {
+            try
+            {
+                group.shutdownGracefully().syncUninterruptibly();
+            }
+            catch ( Throwable ignore )
+            {
+            }
+        }
+    }
 }

driver/src/test/java/org/neo4j/driver/v1/integration/SessionAsyncIT.java

@@ -1088,6 +1088,22 @@ public void shouldFailToExecuteBlockingRunChainedWithAsyncRun()
         }
     }
 
+    @Test
+    public void shouldAllowBlockingOperationInCommonPoolWhenChaining()
+    {
+        CompletionStage<Node> nodeStage = session.runAsync( "RETURN 42 AS value" )
+                .thenCompose( StatementResultCursor::singleAsync )
+                // move execution to ForkJoinPool.commonPool()
+                .thenApplyAsync( record -> session.run( "CREATE (n:Node {value: $value}) RETURN n", record ) )
+                .thenApply( StatementResult::single )
+                .thenApply( record -> record.get( 0 ).asNode() );
+
+        Node node = await( nodeStage );
+
+        assertEquals( 42, node.get( "value" ).asInt() );
+        assertEquals( 1, countNodesByLabel( "Node" ) );
+    }
+
     private Future<List<CompletionStage<Record>>> runNestedQueries( StatementResultCursor inputCursor )
     {
         CompletableFuture<List<CompletionStage<Record>>> resultFuture = new CompletableFuture<>();

driver/src/test/java/org/neo4j/driver/v1/integration/TransactionAsyncIT.java

@@ -1258,6 +1258,28 @@ public void shouldFailToExecuteBlockingRunChainedWithAsyncTransaction()
         }
     }
 
+    @Test
+    public void shouldAllowUsingBlockingApiInCommonPoolWhenChaining()
+    {
+        CompletionStage<Transaction> txStage = session.beginTransactionAsync()
+                // move execution to ForkJoinPool.commonPool()
+                .thenApplyAsync( tx ->
+                {
+                    tx.run( "UNWIND [1,1,2] AS x CREATE (:Node {id: x})" );
+                    tx.run( "CREATE (:Node {id: 42})" );
+                    tx.success();
+                    tx.close();
+                    return tx;
+                } );
+
+        Transaction tx = await( txStage );
+
+        assertFalse( tx.isOpen() );
+        assertEquals( 2, countNodes( 1 ) );
+        assertEquals( 1, countNodes( 2 ) );
+        assertEquals( 1, countNodes( 42 ) );
+    }
+
     private int countNodes( Object id )
     {
         StatementResult result = session.run( "MATCH (n:Node {id: $id}) RETURN count(n)", parameters( "id", id ) );