Preserve Accept header order in Quarkus REST

Fixes: #51078

Author: geoand

independent-projects/resteasy-reactive/common/runtime/src/main/java/org/jboss/resteasy/reactive/common/headers/HeaderUtil.java

@@ -287,7 +287,24 @@ public static List<MediaType> getAcceptableMediaTypes(MultivaluedMap<String, ? e
                 list.add(MediaTypeHelper.valueOf(accept.trim()));
             }
         }
+        // Sort by weight (q) while preserving header-provided order on total ties
         MediaTypeHelper.sortByWeight(list);
+
+        // The returned acceptable media types must not include the quality factor parameter ("q").
+        // Strip any present "q" from the parameters while keeping other parameters (e.g., charset).
+        for (int i = 0; i < list.size(); i++) {
+            MediaType mt = list.get(i);
+            Map<String, String> params = mt.getParameters();
+            if (params != null && params.containsKey("q")) {
+                if (params.size() == 1) {
+                    list.set(i, new MediaType(mt.getType(), mt.getSubtype()));
+                } else {
+                    Map<String, String> copy = new java.util.LinkedHashMap<>(params);
+                    copy.remove("q");
+                    list.set(i, new MediaType(mt.getType(), mt.getSubtype(), copy));
+                }
+            }
+        }
         return list;
     }
 

independent-projects/resteasy-reactive/common/runtime/src/main/java/org/jboss/resteasy/reactive/common/util/MediaTypeHelper.java

@@ -127,25 +127,6 @@ public int compare(MediaType mediaType2, MediaType mediaType) {
             if (!isWildcardCompositeSubtype(mediaType.getSubtype()) && isWildcardCompositeSubtype(mediaType2.getSubtype()))
                 return 1;
 
-            int numNonQ = 0;
-            if (mediaType.getParameters() != null) {
-                numNonQ = mediaType.getParameters().size();
-                if (wasQ)
-                    numNonQ--;
-            }
-
-            int numNonQ2 = 0;
-            if (mediaType2.getParameters() != null) {
-                numNonQ2 = mediaType2.getParameters().size();
-                if (wasQ2)
-                    numNonQ2--;
-            }
-
-            if (numNonQ < numNonQ2)
-                return -1;
-            if (numNonQ > numNonQ2)
-                return 1;
-
             return 0;
         }
     }

independent-projects/resteasy-reactive/server/vertx/src/test/java/org/jboss/resteasy/reactive/server/vertx/test/resource/basic/AcceptHeaderOrderTest.java

@@ -0,0 +1,47 @@
+package org.jboss.resteasy.reactive.server.vertx.test.resource.basic;
+
+import static io.restassured.RestAssured.given;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.stream.Collectors;
+
+import jakarta.ws.rs.GET;
+import jakarta.ws.rs.Path;
+import jakarta.ws.rs.core.Context;
+import jakarta.ws.rs.core.HttpHeaders;
+import jakarta.ws.rs.core.MediaType;
+
+import org.jboss.resteasy.reactive.server.vertx.test.framework.ResteasyReactiveUnitTest;
+import org.jboss.shrinkwrap.api.ShrinkWrap;
+import org.jboss.shrinkwrap.api.spec.JavaArchive;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.RegisterExtension;
+
+public class AcceptHeaderOrderTest {
+
+    @RegisterExtension
+    static ResteasyReactiveUnitTest test = new ResteasyReactiveUnitTest()
+            .setArchiveProducer(() -> ShrinkWrap.create(JavaArchive.class).addClasses(Resource.class));
+
+    @Path("/headers")
+    public static class Resource {
+        @GET
+        @Path("/amt")
+        public String amt(@Context HttpHeaders hs) {
+            return hs.getAcceptableMediaTypes().stream().map(MediaType::toString).collect(Collectors.joining(","));
+        }
+    }
+
+    @Test
+    public void preservesOrderOnTies() {
+        String response = given()
+                .header("Accept",
+                        "application/json,text/html; charset=UTF-8,text/plain; charset=UTF-8,*/*;q=0.8")
+                .get("/headers/amt")
+                .then()
+                .statusCode(200)
+                .extract()
+                .asString();
+        assertEquals("application/json,text/html;charset=UTF-8,text/plain;charset=UTF-8,*/*", response);
+    }
+}