Backport netty/netty#4844

Author: slandelle

netty-bp/resolver/src/main/java/io/netty/resolver/AddressResolverGroup.java

@@ -73,7 +73,9 @@ public AddressResolver<T> getResolver(final EventExecutor executor) {
                 executor.terminationFuture().addListener(new FutureListener<Object>() {
                     @Override
                     public void operationComplete(Future<Object> future) throws Exception {
-                        resolvers.remove(executor);
+                        synchronized (resolvers) {
+                            resolvers.remove(executor);
+                        }
                         newResolver.close();
                     }
                 });