Fix a few cases where a static initialiser references a subclass.

These situations can lead to classloader deadlocks, if the super-class and the sub-class ends up being loaded concurrently in two different threads.
Not all cases are fixed: some a locked in place by being public API, and another was left be due to unpredictable consequences for forward merging.

community/command-line/src/main/java/org/neo4j/commandline/admin/AdminCommandSection.java

@@ -26,14 +26,12 @@
 
 public abstract class AdminCommandSection
 {
-    private static final AdminCommandSection GENERAL = new GeneralSection();
-
     @Nonnull
     public abstract String printable();
 
     public static AdminCommandSection general()
     {
-        return GENERAL;
+        return new GeneralSection();
     }
 
     @Override

community/kernel/src/main/java/org/neo4j/helpers/FutureAdapter.java

@@ -76,7 +76,26 @@ public static <T> Present<T> present( T value )
         return new Present<>( value );
     }
 
-    public static final Future<Void> VOID = new Present<>( null );
+    public static final Future<Void> VOID = new FutureAdapter<Void>()
+    {
+        @Override
+        public boolean isDone()
+        {
+            return true;
+        }
+
+        @Override
+        public Void get()
+        {
+            return null;
+        }
+
+        @Override
+        public Void get( long timeout, TimeUnit unit )
+        {
+            return null;
+        }
+    };
 
     public static <T> Future<T> latchGuardedValue( final Supplier<T> supplier, final CountDownLatch guardedByLatch,
                                                    final String jobDescription )

enterprise/backup/src/test/java/org/neo4j/backup/BackupProtocolTest.java

@@ -24,6 +24,7 @@
 import org.neo4j.backup.BackupClient.BackupRequestType;
 import org.neo4j.com.RequestContext;
 import org.neo4j.com.Response;
+import org.neo4j.com.Responses;
 import org.neo4j.com.TargetCaller;
 import org.neo4j.com.monitor.RequestMonitor;
 import org.neo4j.com.storecopy.ResponseUnpacker;
@@ -73,7 +74,7 @@ public void shouldHandleNoForensicsSpecifiedInFullBackupRequest() throws Excepti
     private void shouldGatherForensicsInFullBackupRequest( boolean forensics ) throws Exception
     {
         // GIVEN
-        Response<Void> response = Response.EMPTY;
+        Response<Void> response = Responses.empty();
         StoreId storeId = response.getStoreId();
         String host = "localhost";
         int port = BackupServer.DEFAULT_PORT;
@@ -111,7 +112,7 @@ public Response<Void> fullBackup( StoreWriter writer, boolean forensics )
         {
             this.receivedForensics = forensics;
             writer.close();
-            return Response.EMPTY;
+            return Responses.empty();
         }
 
         @Override

enterprise/com/src/main/java/org/neo4j/com/Response.java

@@ -59,12 +59,6 @@ public void close()
         releaser.release();
     }
 
-    @SuppressWarnings( "unchecked" )
-    public static <T> Response<T> empty()
-    {
-        return (Response<T>) EMPTY;
-    }
-
     public abstract void accept( Handler handler ) throws Exception;
 
     /**
@@ -93,6 +87,4 @@ public interface Handler
         Visitor<CommittedTransactionRepresentation,Exception> transactions();
     }
 
-    public static final Response<Void> EMPTY = new TransactionObligationResponse<>( null, StoreId.DEFAULT,
-            -1, ResourceReleaser.NO_OP );
 }

enterprise/com/src/main/java/org/neo4j/com/Responses.java

@@ -0,0 +1,34 @@
+/*
+ * Copyright (c) 2002-2018 "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 Affero 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 Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+package org.neo4j.com;
+
+import org.neo4j.kernel.impl.store.StoreId;
+
+public class Responses
+{
+    private static final Response<Void> EMPTY =
+            new TransactionObligationResponse<>( null, StoreId.DEFAULT, -1, ResourceReleaser.NO_OP );;
+
+    @SuppressWarnings( "unchecked" )
+    public static <T> Response<T> empty()
+    {
+        return (Response<T>) EMPTY;
+    }
+}

enterprise/ha/src/main/java/org/neo4j/kernel/ha/com/slave/SlaveImpl.java

@@ -20,6 +20,7 @@
 package org.neo4j.kernel.ha.com.slave;
 
 import org.neo4j.com.Response;
+import org.neo4j.com.Responses;
 import org.neo4j.com.storecopy.TransactionObligationFulfiller;
 import org.neo4j.helpers.Exceptions;
 import org.neo4j.kernel.ha.com.master.Slave;
@@ -44,7 +45,7 @@ public Response<Void> pullUpdates( long upToAndIncludingTxId )
         {
             throw Exceptions.launderedException( e );
         }
-        return Response.EMPTY;
+        return Responses.empty();
     }
 
     @Override

enterprise/ha/src/test/java/org/neo4j/ha/upgrade/MasterClientTest.java

@@ -28,6 +28,7 @@
 import org.neo4j.com.RequestContext;
 import org.neo4j.com.ResourceReleaser;
 import org.neo4j.com.Response;
+import org.neo4j.com.Responses;
 import org.neo4j.com.Server;
 import org.neo4j.com.StoreIdTestFactory;
 import org.neo4j.com.TransactionStream;
@@ -155,7 +156,7 @@ public void endLockSessionDoesNotUnpackResponse() throws Throwable
         ResponseUnpacker responseUnpacker = mock( ResponseUnpacker.class );
         MasterImpl.SPI masterImplSPI = MasterImplTest.mockedSpi( storeId );
         when( masterImplSPI.packTransactionObligationResponse( any( RequestContext.class ), Matchers.anyObject() ) )
-                .thenReturn( Response.empty() );
+                .thenReturn( Responses.empty() );
         when( masterImplSPI.getTransactionChecksum( anyLong() ) ).thenReturn( txChecksum );
 
         newMasterServer( masterImplSPI );

enterprise/ha/src/test/java/org/neo4j/kernel/ha/SlaveUpdatePullerTest.java

@@ -36,6 +36,7 @@
 import org.neo4j.com.ComException;
 import org.neo4j.com.RequestContext;
 import org.neo4j.com.Response;
+import org.neo4j.com.Responses;
 import org.neo4j.kernel.AvailabilityGuard;
 import org.neo4j.kernel.configuration.Config;
 import org.neo4j.kernel.ha.UpdatePuller.Condition;
@@ -237,7 +238,7 @@ public void shouldCopeWithHardExceptionsLikeOutOfMemory() throws Exception
         OutOfMemoryError oom = new OutOfMemoryError();
         when( master.pullUpdates( any( RequestContext.class ) ) )
                 .thenThrow( oom )
-                .thenReturn( Response.EMPTY );
+                .thenReturn( Responses.empty() );
 
         // WHEN making the first pull
         updatePuller.pullUpdates();
@@ -265,7 +266,7 @@ public void shouldCapExcessiveComExceptionLogging() throws Exception
         logProvider.assertContainsThrowablesMatching( 0, repeat( new ComException(), SlaveUpdatePuller.LOG_CAP ) );
 
         // And we should be able to recover afterwards
-        updatePullStubbing.thenReturn( Response.EMPTY ).thenThrow( new ComException() );
+        updatePullStubbing.thenReturn( Responses.empty() ).thenThrow( new ComException() );
 
         updatePuller.pullUpdates(); // This one will succeed and unlock the circuit breaker
         updatePuller.pullUpdates(); // And then we log another exception
@@ -298,7 +299,7 @@ public void shouldCapExcessiveInvalidEpochExceptionLogging() throws Exception
                 repeat( new InvalidEpochException( 2, 1 ), SlaveUpdatePuller.LOG_CAP ) );
 
         // And we should be able to recover afterwards
-        updatePullStubbing.thenReturn( Response.EMPTY ).thenThrow( new InvalidEpochException( 2, 1 ) );
+        updatePullStubbing.thenReturn( Responses.empty() ).thenThrow( new InvalidEpochException( 2, 1 ) );
 
         updatePuller.pullUpdates(); // This one will succeed and unlock the circuit breaker
         updatePuller.pullUpdates(); // And then we log another exception