Issue #4349 - describe algorithm to negotiate position of internal exts

Signed-off-by: Lachlan Roberts <lachlan@webtide.com>

jetty-websocket/websocket-core/src/main/java/org/eclipse/jetty/websocket/core/client/ClientUpgradeRequest.java

@@ -298,10 +298,13 @@ public void upgrade(HttpResponse response, HttpConnectionOverHTTP httpConnection
             }
         }
 
-        // Put requested internal Extensions into the negotiatedExtension list.
+        // Insert client requested internal Extensions into the negotiatedExtension list from the server.
+        // The order of extensions in the response is assumed to be the same as what the client negotiated.
         int i = 0;
         for (ExtensionConfig reqConfig : requestedExtensions)
         {
+            // If this is an internal extension insert in negotiatedExtensions at position i.
+            // If this is not an internal extension we must advance i only if this extension was negotiated by the server.
             if (reqConfig.isInternalExtension())
                 negotiatedExtensions.add(i++, reqConfig);
             else if (i < negotiatedExtensions.size() && reqConfig.getName().equals(negotiatedExtensions.get(i).getName()))

jetty-websocket/websocket-core/src/test/java/org/eclipse/jetty/websocket/core/WebSocketNegotiationTest.java

@@ -369,7 +369,8 @@ public static Stream<Arguments> internalExtensionScenarios() throws Exception
             Arguments.of("ext1, ext2, @int1", "ext1, ext2, @int1"),
             Arguments.of("@int1, ext1, @int2, ext2, @int3", "@int1, ext1, @int2, ext2, @int3"),
             Arguments.of("ext1, ext1, ext1, @int1, ext2", "ext1, @int1, ext2"),
-            Arguments.of("ext1, @int1, ext1, ext1, ext2", "ext1, @int1, ext2")
+            Arguments.of("ext1, @int1, ext1, ext1, ext2", "ext1, @int1, ext2"),
+            Arguments.of("ext1, ext2, ext3, @int1", "ext1, ext2, ext3, @int1")
         );
     }