More reliable wait for the StubServer

Previously code had a `Thread.sleep(500)` which sometimes caused flaky tests.
This commit makes it actually try to establish socket connections with
300ms delay.

Author: lutovich

driver/src/main/java/org/neo4j/driver/internal/net/SocketClient.java

@@ -130,7 +130,7 @@ public void start()
         {
             throw new ServiceUnavailableException( format(
                     "Unable to connect to %s, ensure the database is running and that there is a " +
-                    "working network connection to it.", address ) );
+                    "working network connection to it.", address ), e );
         }
         catch ( IOException e )
         {

driver/src/test/java/org/neo4j/driver/v1/util/StubServer.java

@@ -20,6 +20,9 @@
 
 import java.io.File;
 import java.io.IOException;
+import java.net.InetSocketAddress;
+import java.net.SocketAddress;
+import java.nio.channels.SocketChannel;
 import java.util.ArrayList;
 import java.util.List;
 
@@ -33,6 +36,8 @@
 
 public class StubServer
 {
+    private static final int SOCKET_CONNECT_ATTEMPTS = 20;
+
     public static final Config INSECURE_CONFIG = Config.build()
             .withEncryptionLevel( Config.EncryptionLevel.NONE ).toConfig();
 
@@ -50,7 +55,7 @@ private StubServer( String script, int port ) throws IOException, InterruptedExc
         command.addAll( asList( Integer.toString( port ), script ) );
         ProcessBuilder server = new ProcessBuilder().inheritIO().command( command );
         process = server.start();
-        sleep( 500 );  // might take a moment for the socket to start listening
+        waitForSocket( port );
     }
 
     public static StubServer start( String resource, int port ) throws IOException, InterruptedException
@@ -100,4 +105,22 @@ private static boolean boltKitAvailable()
             return false;
         }
     }
+
+    private static void waitForSocket( int port ) throws InterruptedException
+    {
+        SocketAddress address = new InetSocketAddress( "localhost", port );
+        for ( int i = 0; i < SOCKET_CONNECT_ATTEMPTS; i++ )
+        {
+            try
+            {
+                SocketChannel.open( address );
+                return;
+            }
+            catch ( Exception e )
+            {
+                sleep( 300 );
+            }
+        }
+        throw new AssertionError( "Can't connect to " + address );
+    }
 }