Fix late trailers on streaming HTTP responses

changes.md

@@ -72,6 +72,8 @@ Changes log
           `Response.toString()` now shows it.
         - Added request and response trailers to the HTTP connectors, supporting HTTP/1.1 chunked
           transfers and HTTP/2 trailing headers.
+        - HTTP/2 and HTTP/3 responses can add trailers after streaming the response head and body;
+          HTTP/1.1 still requires trailer names to be known before the response head is sent.
         - HTTP/2 clients receive a response as soon as its headers arrive, while the request entity
           continues to be written on a virtual thread.
         - Active client requests can be aborted; HTTP stream failures surface as I/O errors and a

org.restlet/src/main/java/org/restlet/engine/netty/NettyServerCall.java

@@ -552,7 +552,7 @@
      *
      * @param emptyBody True if no body follows.
      */
     private void writeHead(boolean emptyBody) {
         if (responseHeadWritten) {
             return;
         }
@@ -583,7 +583,8 @@
         if (noBodyStatus || "HEAD".equals(getMethod())) {
             hasTrailers = false;
         }
-        responseTrailersEnabled = hasTrailers;
+        responseTrailersEnabled =
+                hasTrailers || (kind.isStream() && !noBodyStatus && !"HEAD".equals(getMethod()));
 
         if (kind.isStream()) {
             // Connection-specific headers are forbidden in HTTP/2 and HTTP/3.

org.restlet/src/test/java/org/restlet/engine/connector/Http3TestCase.java

@@ -18,6 +18,7 @@
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.OutputStream;
+import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.security.cert.X509Certificate;
@@ -175,6 +176,32 @@ void getAndPutOverHttp3() throws Exception {
         assertEquals(Set.of("HTTP/3.0"), versions);
     }
 
+    @Test
+    void exchangesTrailersAddedAfterTheFirstBodyFlushOverHttp3() throws Exception {
+        Restlet streaming =
+                new Restlet() {
+                    @Override
+                    public void handle(Request request, Response response) {
+                        response.setEntity(
+                                new OutputRepresentation(MediaType.APPLICATION_OCTET_STREAM) {
+                                    @Override
+                                    public void write(OutputStream output) throws IOException {
+                                        output.write("first".getBytes(StandardCharsets.UTF_8));
+                                        output.flush();
+                                        response.getTrailers().add("X-Response-Trailer", "late");
+                                        output.write("second".getBytes(StandardCharsets.UTF_8));
+                                    }
+                                });
+                    }
+                };
+        startServer("HTTP1_1,HTTP3", streaming);
+        startClient("HTTP3");
+
+        Response response = client.handle(new Request(Method.GET, uri("/")));
+        assertEquals("firstsecond", response.getEntityAsText());
+        assertEquals("late", response.getTrailers().getFirstValue("X-Response-Trailer", true));
+    }
+
     @Test
     void tcpResponsesAdvertiseHttp3() throws Exception {
         startServer("HTTP2,HTTP1_1,HTTP3", echo);

org.restlet/src/test/java/org/restlet/engine/connector/HttpTrailersTestCase.java

@@ -10,6 +10,9 @@
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 
+import java.io.IOException;
+import java.io.OutputStream;
+import java.nio.charset.StandardCharsets;
 import java.util.concurrent.atomic.AtomicReference;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
@@ -25,6 +28,7 @@
 import org.restlet.data.Method;
 import org.restlet.data.Protocol;
 import org.restlet.engine.Engine;
+import org.restlet.representation.OutputRepresentation;
 import org.restlet.representation.StringRepresentation;
 
 class HttpTrailersTestCase {
@@ -107,4 +111,38 @@ void exchangesTrailersOverHttp11() throws Exception {
     void exchangesTrailersOverHttp2() throws Exception {
         roundTrip("HTTP2", "HTTP2");
     }
+
+    @Test
+    void exchangesHttp2TrailersAddedAfterTheFirstBodyFlush() throws Exception {
+        Restlet streaming =
+                new Restlet() {
+                    @Override
+                    public void handle(Request request, Response response) {
+                        response.setEntity(
+                                new OutputRepresentation(MediaType.APPLICATION_OCTET_STREAM) {
+                                    @Override
+                                    public void write(OutputStream output) throws IOException {
+                                        output.write("first".getBytes(StandardCharsets.UTF_8));
+                                        output.flush();
+                                        response.getTrailers().add(RESPONSE_TRAILER, "late");
+                                        output.write("second".getBytes(StandardCharsets.UTF_8));
+                                    }
+                                });
+                    }
+                };
+        Context serverContext = new Context();
+        serverContext.getParameters().add("http.transport.protocol", "HTTP2");
+        server = new Server(serverContext, Protocol.HTTP, 0, streaming);
+        server.start();
+        Context clientContext = new Context();
+        clientContext.getParameters().add("httpClientTransportMode", "HTTP2");
+        client = new Client(clientContext, Protocol.HTTP);
+        client.start();
+
+        Response response =
+                client.handle(
+                        new Request(Method.GET, "http://localhost:" + server.getActualPort()));
+        assertEquals("firstsecond", response.getEntityAsText());
+        assertEquals("late", response.getTrailers().getFirstValue(RESPONSE_TRAILER, true));
+    }
 }