RESTEasy Reactive: writers can add headers too
Originally I had to write https://github.com/quarkusio/quarkus/pull/13802/commits/77105396c3208a76e13283f9d2dc3e7b956e757a in order to fix an exception such as "response already written" where we tried to set the headers and status code after a writer had started writing on the response (wonder why this doesn't happen every time), but it makes the TCK fail:

```
2020-12-14T17:11:53.1704407Z [ERROR]   JAXRSProvidersClient0098.writeHeaderEntityUsingWriterTest:332->JAXRSCommonClient.invoke:342 [WebValidatorBase] Unable to find the following header in the server's response: READABLEWRITEABLE: <READABLEWRITEABLE>JAXRS</READABLEWRITEABLE>
2020-12-14T17:11:53.1706491Z [WebValidatorBase] Response headers recieved from server:
2020-12-14T17:11:53.1707380Z 	ResponseHeader -> Content-Type: text/xml;charset=UTF-8
2020-12-14T17:11:53.1708157Z 	ResponseHeader -> content-length: 0
```

Because writers can add headers.
So we need to find a way to delay writing headers until we detect someone trying to write on the response.

RESTEasy does this by wrapping the `OutputStream` passed to clients and only write headers once/if they start using it.

Not sure how that affects the optimised writers path.
diff --git a/extensions/resteasy-reactive/quarkus-resteasy-reactive-servlet/runtime/src/main/java/io/quarkus/resteasy/reactive/server/servlet/runtime/ServletRequestContext.java b/extensions/resteasy-reactive/quarkus-resteasy-reactive-servlet/runtime/src/main/java/io/quarkus/resteasy/reactive/server/servlet/runtime/ServletRequestContext.java
index 992c3d293b4..babee7adfb7 100644
--- a/extensions/resteasy-reactive/quarkus-resteasy-reactive-servlet/runtime/src/main/java/io/quarkus/resteasy/reactive/server/servlet/runtime/ServletRequestContext.java
+++ b/extensions/resteasy-reactive/quarkus-resteasy-reactive-servlet/runtime/src/main/java/io/quarkus/resteasy/reactive/server/servlet/runtime/ServletRequestContext.java
@@ -40,13 +40,15 @@
 import io.quarkus.resteasy.reactive.server.runtime.ResteasyReactiveSecurityContext;
 import io.quarkus.security.identity.SecurityIdentity;
 import io.quarkus.vertx.http.runtime.security.QuarkusHttpUser;
+import io.undertow.server.HttpServerExchange;
+import io.undertow.server.ResponseCommitListener;
 import io.vertx.core.http.HttpServerRequest;
 import io.vertx.core.http.HttpServerResponse;
 import io.vertx.core.net.impl.ConnectionBase;
 import io.vertx.ext.web.RoutingContext;
 
 public class ServletRequestContext extends ResteasyReactiveRequestContext
-        implements ServerHttpRequest, ServerHttpResponse {
+        implements ServerHttpRequest, ServerHttpResponse, ResponseCommitListener {
 
     private static final LazyValue<Event<SecurityIdentity>> SECURITY_IDENTITY_EVENT = new LazyValue<>(
             ServletRequestContext::createEvent);
@@ -57,15 +59,17 @@ public class ServletRequestContext extends ResteasyReactiveRequestContext
     ServletWriteListener writeListener;
     byte[] asyncWriteData;
     Consumer<Throwable> asyncWriteHandler;
+    protected Consumer<ResteasyReactiveRequestContext> preCommitTask;
 
     public ServletRequestContext(Deployment deployment, ProvidersImpl providers,
             HttpServletRequest request, HttpServletResponse response,
             ThreadSetupAction requestContext, ServerRestHandler[] handlerChain, ServerRestHandler[] abortHandlerChain,
-            RoutingContext context) {
+            RoutingContext context, HttpServerExchange exchange) {
         super(deployment, providers, requestContext, handlerChain, abortHandlerChain);
         this.request = request;
         this.response = response;
         this.context = context;
+        exchange.addResponseCommitListener(this);
     }
 
     protected boolean isRequestScopeManagementRequired() {
@@ -453,6 +457,18 @@ public OutputStream createResponseOutputStream() {
         }
     }
 
+    @Override
+    public void setPreCommitListener(Consumer<ResteasyReactiveRequestContext> task) {
+        preCommitTask = task;
+    }
+
+    @Override
+    public void beforeCommit(HttpServerExchange exchange) {
+        if (preCommitTask != null) {
+            preCommitTask.accept(this);
+        }
+    }
+
     class ServletWriteListener implements WriteListener {
 
         private final ServletOutputStream outputStream;
diff --git a/extensions/resteasy-reactive/quarkus-resteasy-reactive-servlet/runtime/src/main/java/io/quarkus/resteasy/reactive/server/servlet/runtime/ServletRequestContextFactory.java b/extensions/resteasy-reactive/quarkus-resteasy-reactive-servlet/runtime/src/main/java/io/quarkus/resteasy/reactive/server/servlet/runtime/ServletRequestContextFactory.java
index 7b1656ad3c9..2cc20536b9e 100644
--- a/extensions/resteasy-reactive/quarkus-resteasy-reactive-servlet/runtime/src/main/java/io/quarkus/resteasy/reactive/server/servlet/runtime/ServletRequestContextFactory.java
+++ b/extensions/resteasy-reactive/quarkus-resteasy-reactive-servlet/runtime/src/main/java/io/quarkus/resteasy/reactive/server/servlet/runtime/ServletRequestContextFactory.java
@@ -24,6 +24,6 @@ public ResteasyReactiveRequestContext createContext(Deployment deployment, Provi
         io.undertow.servlet.handlers.ServletRequestContext src = (io.undertow.servlet.handlers.ServletRequestContext) context;
         return new ServletRequestContext(deployment, providers, (HttpServletRequest) src.getServletRequest(),
                 (HttpServletResponse) src.getServletResponse(), requestContext, handlerChain, abortHandlerChain,
-                (RoutingContext) ((VertxHttpExchange) src.getExchange().getDelegate()).getContext());
+                (RoutingContext) ((VertxHttpExchange) src.getExchange().getDelegate()).getContext(), src.getExchange());
     }
 }
diff --git a/independent-projects/resteasy-reactive/server/runtime/src/main/java/org/jboss/resteasy/reactive/server/core/ServerSerialisers.java b/independent-projects/resteasy-reactive/server/runtime/src/main/java/org/jboss/resteasy/reactive/server/core/ServerSerialisers.java
index eb25baabcb4..e816c4a7fd6 100644
--- a/independent-projects/resteasy-reactive/server/runtime/src/main/java/org/jboss/resteasy/reactive/server/core/ServerSerialisers.java
+++ b/independent-projects/resteasy-reactive/server/runtime/src/main/java/org/jboss/resteasy/reactive/server/core/ServerSerialisers.java
@@ -17,6 +17,7 @@
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentMap;
+import java.util.function.Consumer;
 import java.util.function.Function;
 import javax.ws.rs.RuntimeType;
 import javax.ws.rs.WebApplicationException;
@@ -57,6 +58,13 @@
 
 public class ServerSerialisers extends Serialisers {
 
+    private static final Consumer<ResteasyReactiveRequestContext> HEADER_FUNCTION = new Consumer<ResteasyReactiveRequestContext>() {
+        @Override
+        public void accept(ResteasyReactiveRequestContext context) {
+            ServerSerialisers.encodeResponseHeaders(context);
+        }
+    };
+
     public static BuiltinReader[] BUILTIN_READERS = new BuiltinReader[] {
             new BuiltinReader(String.class, ServerStringMessageBodyHandler.class,
                     MediaType.WILDCARD),
@@ -164,49 +172,62 @@ public static boolean invokeWriter(ResteasyReactiveRequestContext context, Objec
 
         WriterInterceptor[] writerInterceptors = context.getWriterInterceptors();
         boolean outputStreamSet = context.getOutputStream() != null;
-        if (writer instanceof ServerMessageBodyWriter && writerInterceptors == null && !outputStreamSet) {
-            ServerMessageBodyWriter<Object> quarkusRestWriter = (ServerMessageBodyWriter<Object>) writer;
-            RuntimeResource target = context.getTarget();
-            ServerSerialisers.encodeResponseHeaders(context);
-            Type genericType;
-            if (context.hasGenericReturnType()) { // make sure that when a Response with a GenericEntity was returned, we use it
-                genericType = context.getGenericReturnType();
-            } else {
-                genericType = target == null ? null : target.getReturnType();
-            }
-            Class<?> entityClass = entity.getClass();
-            if (quarkusRestWriter.isWriteable(
-                    entityClass,
-                    genericType,
-                    target == null ? null : target.getLazyMethod(),
-                    context.getResponseMediaType())) {
-                if (mediaType != null) {
-                    context.setResponseContentType(mediaType);
+        context.serverResponse().setPreCommitListener(HEADER_FUNCTION);
+        try {
+            if (writer instanceof ServerMessageBodyWriter && writerInterceptors == null && !outputStreamSet) {
+                ServerMessageBodyWriter<Object> quarkusRestWriter = (ServerMessageBodyWriter<Object>) writer;
+                RuntimeResource target = context.getTarget();
+                Type genericType;
+                if (context.hasGenericReturnType()) { // make sure that when a Response with a GenericEntity was returned, we use it
+                    genericType = context.getGenericReturnType();
+                } else {
+                    genericType = target == null ? null : target.getReturnType();
                 }
-                quarkusRestWriter.writeResponse(entity, genericType, context);
-                return true;
-            } else {
-                return false;
-            }
-        } else {
-            if (writer.isWriteable(entity.getClass(), context.getGenericReturnType(), context.getAllAnnotations(),
-                    context.getResponseMediaType())) {
-                Response response = context.getResponse().get();
-                if (mediaType != null) {
-                    context.setResponseContentType(mediaType);
+                Class<?> entityClass = entity.getClass();
+                if (quarkusRestWriter.isWriteable(
+                        entityClass,
+                        genericType,
+                        target == null ? null : target.getLazyMethod(),
+                        context.getResponseMediaType())) {
+                    if (mediaType != null) {
+                        context.setResponseContentType(mediaType);
+                    }
+                    quarkusRestWriter.writeResponse(entity, genericType, context);
+                    return true;
+                } else {
+                    return false;
                 }
-                if (writerInterceptors == null) {
-                    writer.writeTo(entity, entity.getClass(), context.getGenericReturnType(),
-                            context.getAllAnnotations(), response.getMediaType(), response.getHeaders(),
-                            context.getOrCreateOutputStream());
-                    ServerSerialisers.encodeResponseHeaders(context);
-                    context.getOrCreateOutputStream().close();
+            } else {
+                if (writer.isWriteable(entity.getClass(), context.getGenericReturnType(), context.getAllAnnotations(),
+                        context.getResponseMediaType())) {
+                    Response response = context.getResponse().get();
+                    if (mediaType != null) {
+                        context.setResponseContentType(mediaType);
+                    }
+                    if (writerInterceptors == null) {
+                        writer.writeTo(entity, entity.getClass(), context.getGenericReturnType(),
+                                context.getAllAnnotations(), response.getMediaType(), response.getHeaders(),
+                                context.getOrCreateOutputStream());
+                        context.getOrCreateOutputStream().close();
+                    } else {
+                        runWriterInterceptors(context, entity, writer, response, writerInterceptors, serialisers);
+                    }
+                    return true;
                 } else {
-                    runWriterInterceptors(context, entity, writer, response, writerInterceptors, serialisers);
+                    return false;
                 }
-                return true;
+            }
+        } catch (Throwable e) {
+            //clear the pre-commit listener, as if this error is unrecoverable
+            //the error handling will want to write out its own response
+            //and the pre commit listener will interfere with that
+            context.serverResponse().setPreCommitListener(null);
+            if (e instanceof RuntimeException) {
+                throw (RuntimeException) e;
+            } else if (e instanceof IOException) {
+                throw (IOException) e;
             } else {
-                return false;
+                throw new RuntimeException(e);
             }
         }
     }
diff --git a/independent-projects/resteasy-reactive/server/runtime/src/main/java/org/jboss/resteasy/reactive/server/spi/ServerHttpResponse.java b/independent-projects/resteasy-reactive/server/runtime/src/main/java/org/jboss/resteasy/reactive/server/spi/ServerHttpResponse.java
index fa9d12e8d98..39761308baf 100644
--- a/independent-projects/resteasy-reactive/server/runtime/src/main/java/org/jboss/resteasy/reactive/server/spi/ServerHttpResponse.java
+++ b/independent-projects/resteasy-reactive/server/runtime/src/main/java/org/jboss/resteasy/reactive/server/spi/ServerHttpResponse.java
@@ -4,6 +4,7 @@
 import java.util.Map;
 import java.util.concurrent.CompletionStage;
 import java.util.function.Consumer;
+import org.jboss.resteasy.reactive.server.core.ResteasyReactiveRequestContext;
 
 public interface ServerHttpResponse {
 
@@ -34,4 +35,6 @@ public interface ServerHttpResponse {
     CompletionStage<Void> write(byte[] data);
 
     OutputStream createResponseOutputStream();
+
+    void setPreCommitListener(Consumer<ResteasyReactiveRequestContext> task);
 }
diff --git a/independent-projects/resteasy-reactive/server/vertx/src/main/java/org/jboss/resteasy/reactive/server/vertx/VertxResteasyReactiveRequestContext.java b/independent-projects/resteasy-reactive/server/vertx/src/main/java/org/jboss/resteasy/reactive/server/vertx/VertxResteasyReactiveRequestContext.java
index 869f75cc009..e8af88c0d04 100644
--- a/independent-projects/resteasy-reactive/server/vertx/src/main/java/org/jboss/resteasy/reactive/server/vertx/VertxResteasyReactiveRequestContext.java
+++ b/independent-projects/resteasy-reactive/server/vertx/src/main/java/org/jboss/resteasy/reactive/server/vertx/VertxResteasyReactiveRequestContext.java
@@ -29,11 +29,12 @@
 import org.jboss.resteasy.reactive.spi.ThreadSetupAction;
 
 public class VertxResteasyReactiveRequestContext extends ResteasyReactiveRequestContext
-        implements ServerHttpRequest, ServerHttpResponse {
+        implements ServerHttpRequest, ServerHttpResponse, Handler<Void> {
 
     protected final RoutingContext context;
     protected final HttpServerRequest request;
     protected final HttpServerResponse response;
+    protected Consumer<ResteasyReactiveRequestContext> preCommitTask;
 
     public VertxResteasyReactiveRequestContext(Deployment deployment, ProvidersImpl providers,
             RoutingContext context,
@@ -42,6 +43,7 @@ public VertxResteasyReactiveRequestContext(Deployment deployment, ProvidersImpl
         this.context = context;
         this.request = context.request();
         this.response = context.response();
+        context.addHeadersEndHandler(this);
     }
 
     public RoutingContext getContext() {
@@ -313,4 +315,16 @@ public void handle(AsyncResult<Void> event) {
     public OutputStream createResponseOutputStream() {
         return new ResteasyReactiveOutputStream(this);
     }
+
+    @Override
+    public void setPreCommitListener(Consumer<ResteasyReactiveRequestContext> task) {
+        preCommitTask = task;
+    }
+
+    @Override
+    public void handle(Void event) {
+        if (preCommitTask != null) {
+            preCommitTask.accept(this);
+        }
+    }
 }
['independent-projects/resteasy-reactive/server/runtime/src/main/java/org/jboss/resteasy/reactive/server/spi/ServerHttpResponse.java', 'independent-projects/resteasy-reactive/server/vertx/src/main/java/org/jboss/resteasy/reactive/server/vertx/VertxResteasyReactiveRequestContext.java', 'independent-projects/resteasy-reactive/server/runtime/src/main/java/org/jboss/resteasy/reactive/server/core/ServerSerialisers.java', 'extensions/resteasy-reactive/quarkus-resteasy-reactive-servlet/runtime/src/main/java/io/quarkus/resteasy/reactive/server/servlet/runtime/ServletRequestContext.java', 'extensions/resteasy-reactive/quarkus-resteasy-reactive-servlet/runtime/src/main/java/io/quarkus/resteasy/reactive/server/servlet/runtime/ServletRequestContextFactory.java']