Support HTTP trailers forwarding

src/main/java/io/vertx/httpproxy/impl/HttpUtils.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2011-2020 Contributors to the Eclipse Foundation
+ * Copyright (c) 2011-2026 Contributors to the Eclipse Foundation
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Public License 2.0 which is available at
@@ -12,6 +12,7 @@
 
 import io.vertx.core.MultiMap;
 import io.vertx.core.http.HttpHeaders;
+import io.vertx.core.http.HttpServerResponse;
 
 import java.time.Instant;
 import java.util.List;
@@ -52,4 +53,9 @@ static Instant dateHeader(MultiMap headers) {
       return ParseUtils.parseHeaderDate(dateHeader);
     }
   }
+
+  public static boolean trailersSupported(HttpServerResponse proxiedResponse) {
+    return proxiedResponse.streamId() >= 0 // HTTP/2 and HTTP/3
+      || proxiedResponse.isChunked(); // Required for HTTP/1.1
+  }
 }

src/main/java/io/vertx/httpproxy/impl/ProxiedResponse.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2011-2025 Contributors to the Eclipse Foundation
+ * Copyright (c) 2011-2026 Contributors to the Eclipse Foundation
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Public License 2.0 which is available at
@@ -21,7 +21,6 @@
 import io.vertx.core.streams.Pipe;
 import io.vertx.core.streams.ReadStream;
 import io.vertx.httpproxy.Body;
-import io.vertx.httpproxy.MediaType;
 import io.vertx.httpproxy.ProxyRequest;
 import io.vertx.httpproxy.ProxyResponse;
 
@@ -266,15 +265,25 @@ public ProxyResponse release() {
 
   private Future<Void> sendResponse(ReadStream<Buffer> body) {
     Pipe<Buffer> pipe = body.pipe();
-    pipe.endOnSuccess(true);
+    pipe.endOnSuccess(false);
     pipe.endOnFailure(false);
     return pipe
       .to(proxiedResponse)
+      .compose(v -> {
+        // Only forward trailers if using the original backend response stream
+        if (body.equals(response)) {
+          MultiMap trailers = response.trailers();
+          if (!trailers.isEmpty() && HttpUtils.trailersSupported(proxiedResponse)) {
+            proxiedResponse.trailers().addAll(trailers);
+          }
+        }
+        return proxiedResponse.end();
+      })
       .andThen(ar -> {
-      if (ar.failed()) {
-        request.request.reset();
-        proxiedResponse.reset();
-      }
-    });
+        if (ar.failed()) {
+          request.request.reset();
+          proxiedResponse.reset();
+        }
+      });
   }
 }

src/test/java/io/vertx/tests/ProxyTest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2011-2025 Contributors to the Eclipse Foundation
+ * Copyright (c) 2011-2026 Contributors to the Eclipse Foundation
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Public License 2.0 which is available at
@@ -231,4 +231,63 @@ public Future<ProxyResponse> handleProxyRequest(ProxyContext context) {
       )
       .onComplete(ctx.asyncAssertSuccess(buffer -> ctx.assertEquals("HOLA", buffer.toString())));
   }
+
+  @Test
+  public void testTrailersForwarding(TestContext ctx) {
+    testTrailers(ctx, null);
+  }
+
+  @Test
+  public void testTrailersNotForwardedWhenBodyIsModified(TestContext ctx) {
+    testTrailers(ctx, new ProxyInterceptor() {
+      @Override
+      public Future<Void> handleProxyResponse(ProxyContext context) {
+        ProxyResponse proxyResponse = context.response();
+        proxyResponse.setBody(Body.body(io.vertx.core.buffer.Buffer.buffer("modified body")));
+        return context.sendResponse();
+      }
+    });
+  }
+
+  @Test
+  public void testTrailersNotForwardedWhenBodyIsTransformed(TestContext ctx) {
+    ProxyInterceptor interceptor = ProxyInterceptor.builder()
+      .transformingResponseBody(BodyTransformers.text(String::toUpperCase, "UTF-8"))
+      .build();
+    testTrailers(ctx, interceptor);
+  }
+
+  private void testTrailers(TestContext ctx, ProxyInterceptor interceptor) {
+    SocketAddress backend = startHttpBackend(ctx, 8081, req -> {
+      req.response()
+        .setChunked(true)
+        .putHeader("content-type", "text/plain")
+        .write("response body");
+      req.response().trailers()
+        .add("X-Custom-Trailer", "trailer-value")
+        .add("X-Another-Trailer", "another-value");
+      req.response().end();
+    });
+    startProxy(proxy -> {
+      proxy.origin(backend);
+      if (interceptor != null) {
+        proxy.addInterceptor(interceptor);
+      }
+    });
+    client = vertx.createHttpClient();
+    client.request(HttpMethod.GET, 8080, "localhost", "/")
+      .compose(req -> req.send().compose(resp -> {
+        ctx.assertEquals(200, resp.statusCode());
+        return resp.body().map(body -> resp);
+      }))
+      .onComplete(ctx.asyncAssertSuccess(resp -> {
+        if (interceptor == null) {
+          ctx.assertEquals("trailer-value", resp.trailers().get("X-Custom-Trailer"));
+          ctx.assertEquals("another-value", resp.trailers().get("X-Another-Trailer"));
+        } else {
+          ctx.assertNull(resp.trailers().get("X-Custom-Trailer"));
+          ctx.assertNull(resp.trailers().get("X-Another-Trailer"));
+        }
+      }));
+  }
 }