Include state of protocol negotiation in handshake exceptions.

These are logged.

enterprise/causal-clustering/src/main/java/org/neo4j/causalclustering/protocol/handshake/ClientHandshakeException.java

@@ -19,10 +19,25 @@
  */
 package org.neo4j.causalclustering.protocol.handshake;
 
+import java.util.List;
+import java.util.Optional;
+
+import javax.annotation.Nullable;
+
+import org.neo4j.causalclustering.protocol.Protocol;
+import org.neo4j.helpers.collection.Pair;
+
 public class ClientHandshakeException extends Exception
 {
     public ClientHandshakeException( String message )
     {
         super( message );
     }
+
+    public ClientHandshakeException( String message, @Nullable Protocol.ApplicationProtocol negotiatedApplicationProtocol,
+            List<Pair<String,Optional<Protocol.ModifierProtocol>>> negotiatedModifierProtocols )
+    {
+        super( message + " Negotiated application protocol: " + negotiatedApplicationProtocol +
+                " Negotiated modifier protocols: " + negotiatedModifierProtocols );
+    }
 }

enterprise/causal-clustering/src/main/java/org/neo4j/causalclustering/protocol/handshake/HandshakeClient.java

@@ -41,7 +41,7 @@ public class HandshakeClient implements ClientMessageHandler
     private ApplicationSupportedProtocols supportedApplicationProtocol;
     private ModifierProtocolRepository modifierProtocolRepository;
     private Collection<ModifierSupportedProtocols> supportedModifierProtocols;
-    private ApplicationProtocol applicationProtocol;
+    private ApplicationProtocol negotiatedApplicationProtocol;
     private List<Pair<String,Optional<ModifierProtocol>>> negotiatedModifierProtocols;
     private ProtocolStack protocolStack;
     private CompletableFuture<ProtocolStack> future = new CompletableFuture<>();
@@ -127,7 +127,7 @@ public void handle( ApplicationProtocolResponse applicationProtocolResponse )
         }
         else
         {
-            applicationProtocol = protocol.get();
+            negotiatedApplicationProtocol = protocol.get();
 
             sendSwitchOverRequestIfReady();
         }
@@ -153,22 +153,26 @@ public void handle( ModifierProtocolResponse modifierProtocolResponse )
 
     private void sendSwitchOverRequestIfReady()
     {
-        if ( applicationProtocol != null && negotiatedModifierProtocols.size() == supportedModifierProtocols.size() )
+        if ( negotiatedApplicationProtocol != null && negotiatedModifierProtocols.size() == supportedModifierProtocols.size() )
         {
             List<ModifierProtocol> agreedModifierProtocols = negotiatedModifierProtocols
                     .stream()
                     .map( Pair::other )
                     .flatMap( Streams::ofOptional )
                     .collect( Collectors.toList() );
 
-            protocolStack = new ProtocolStack( applicationProtocol, agreedModifierProtocols );
+            protocolStack = new ProtocolStack( negotiatedApplicationProtocol, agreedModifierProtocols );
             List<Pair<String,String>> switchOverModifierProtocols =
                     agreedModifierProtocols
                             .stream()
                             .map( protocol -> Pair.of( protocol.category(), protocol.implementation() ) )
                             .collect( Collectors.toList() );
 
-            channel.writeAndFlush( new SwitchOverRequest( applicationProtocol.category(), applicationProtocol.implementation(), switchOverModifierProtocols ) );
+            channel.writeAndFlush(
+                    new SwitchOverRequest(
+                            negotiatedApplicationProtocol.category(),
+                            negotiatedApplicationProtocol.implementation(),
+                            switchOverModifierProtocols ) );
         }
     }
 
@@ -202,6 +206,6 @@ boolean failIfNotDone( String message )
 
     private void decline( String message )
     {
-        future.completeExceptionally( new ClientHandshakeException( message ) );
+        future.completeExceptionally( new ClientHandshakeException( message, negotiatedApplicationProtocol, negotiatedModifierProtocols ) );
     }
 }

enterprise/causal-clustering/src/main/java/org/neo4j/causalclustering/protocol/handshake/HandshakeServer.java

@@ -200,7 +200,7 @@ private Set<Integer> supportedVersionsFor( ApplicationProtocolRequest request )
     private void decline( String message )
     {
         channel.dispose();
-        protocolStackFuture.completeExceptionally( new ServerHandshakeException( message ) );
+        protocolStackFuture.completeExceptionally( new ServerHandshakeException( message, protocolStackBuilder ) );
     }
 
     CompletableFuture<ProtocolStack> protocolStackFuture()

enterprise/causal-clustering/src/main/java/org/neo4j/causalclustering/protocol/handshake/ProtocolStack.java

@@ -105,5 +105,11 @@ ProtocolStack build()
         {
             return new ProtocolStack( applicationProtocol, modifierProtocols );
         }
+
+        @Override
+        public String toString()
+        {
+            return "Builder{" + "applicationProtocol=" + applicationProtocol + ", modifierProtocols=" + modifierProtocols + '}';
+        }
     }
 }

enterprise/causal-clustering/src/main/java/org/neo4j/causalclustering/protocol/handshake/ServerHandshakeException.java

@@ -21,8 +21,8 @@
 
 public class ServerHandshakeException extends Exception
 {
-    public ServerHandshakeException( String message )
+    public ServerHandshakeException( String message, ProtocolStack.Builder protocolStackBuilder )
     {
-        super( message );
+        super( message + " Negotiated protocols: " + protocolStackBuilder );
     }
 }