Fix handling of codec exceptions without cause

Driver tries to extract cause of every `CodecException` received in
`#exceptionCaught(Throwable)` netty callbacks. It did not handle
cases when the cause is null, which resulted in an attempt to
exceptionally complete a `CompletableFuture` with null.

This commit fixes the problem by making handlers extract the exception
cause only if it is defined and propagate the original error otherwise.

Author: lutovich

driver/src/main/java/org/neo4j/driver/internal/async/HandshakeHandler.java

@@ -165,18 +165,23 @@ private static Throwable protocolNoSupportedByDriverError( int suggestedProtocol
 
     private static Throwable transformError( Throwable error )
     {
-        Throwable cause = error instanceof DecoderException ? error.getCause() : error;
-        if ( cause instanceof ServiceUnavailableException )
+        if ( error instanceof DecoderException && error.getCause() != null )
         {
-            return cause;
+            // unwrap the DecoderException if it has a cause
+            error = error.getCause();
         }
-        else if ( cause instanceof SSLHandshakeException )
+
+        if ( error instanceof ServiceUnavailableException )
+        {
+            return error;
+        }
+        else if ( error instanceof SSLHandshakeException )
         {
-            return new SecurityException( "Failed to establish secured connection with the server", cause );
+            return new SecurityException( "Failed to establish secured connection with the server", error );
         }
         else
         {
-            return new ServiceUnavailableException( "Failed to establish connection with the server", cause );
+            return new ServiceUnavailableException( "Failed to establish connection with the server", error );
         }
     }
 }

driver/src/main/java/org/neo4j/driver/internal/async/inbound/ChannelErrorHandler.java

@@ -103,9 +103,9 @@ private void fail( ChannelHandlerContext ctx, Throwable error )
 
     private static Throwable transformError( Throwable error )
     {
-        if ( error instanceof CodecException )
+        if ( error instanceof CodecException && error.getCause() != null )
         {
-            // unwrap exception from message encoder/decoder
+            // unwrap the CodecException if it has a cause
             error = error.getCause();
         }
 

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

@@ -117,6 +117,18 @@ public void shouldHandleCodecException()
         assertFalse( channel.isOpen() );
     }
 
+    @Test
+    public void shouldHandleCodecExceptionWithoutCause()
+    {
+        CodecException codecException = new CodecException( "Unable to encode or decode message" );
+        channel.pipeline().fireExceptionCaught( codecException );
+
+        Throwable error = messageDispatcher.currentError();
+
+        assertEquals( codecException, error );
+        assertFalse( channel.isOpen() );
+    }
+
     @Test
     public void shouldHandleIOException()
     {

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

@@ -187,6 +187,31 @@ public void shouldUnwrapDecoderException()
         assertNull( await( channel.closeFuture() ) );
     }
 
+    @Test
+    public void shouldHandleDecoderExceptionWithoutCause()
+    {
+        ChannelPromise handshakeCompletedPromise = channel.newPromise();
+        HandshakeHandler handler = newHandler( handshakeCompletedPromise );
+        channel.pipeline().addLast( handler );
+
+        DecoderException decoderException = new DecoderException( "Unable to decode a message" );
+        channel.pipeline().fireExceptionCaught( decoderException );
+
+        try
+        {
+            // promise should fail
+            await( handshakeCompletedPromise );
+            fail( "Exception expected" );
+        }
+        catch ( ServiceUnavailableException e )
+        {
+            assertEquals( decoderException, e.getCause() );
+        }
+
+        // channel should be closed
+        assertNull( await( channel.closeFuture() ) );
+    }
+
     @Test
     public void shouldTranslateSSLHandshakeException()
     {