From e59722037c650228617fb94d7c0e4bd733b6d97b Mon Sep 17 00:00:00 2001
From: ktqco <75426052+ktqco@users.noreply.github.com>
Date: Fri, 18 Jun 2021 10:45:32 +0100
Subject: [PATCH] Accept smaller `server_max_window_bits` than requested
 (#11394)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Motivation:

Netty will fail a handshake for the Per-Message Deflate WebSocket
extension if the server response contains a smaller
`server_max_window_bits` value than the client offered.

However, this is allowed by RFC 7692:

> A server accepts an extension negotiation offer with this parameter
> by including the “server_max_window_bits” extension parameter in the
> extension negotiation response to send back to the client with the
> same or smaller value as the offer.

Modifications:

- Allow the server to respond with a smaller value than offered.
- Change the unit tests to test for this.

Result:

The client will not fail when the server indicates it is using a
smaller window size than offered by the client.
---
 .../compression/PerMessageDeflateClientExtensionHandshaker.java | 2 +-
 .../PerMessageDeflateClientExtensionHandshakerTest.java         | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/codec-http/src/main/java/io/netty/handler/codec/http/websocketx/extensions/compression/PerMessageDeflateClientExtensionHandshaker.java b/codec-http/src/main/java/io/netty/handler/codec/http/websocketx/extensions/compression/PerMessageDeflateClientExtensionHandshaker.java
index 84c0a8d746..db364f5190 100644
--- a/codec-http/src/main/java/io/netty/handler/codec/http/websocketx/extensions/compression/PerMessageDeflateClientExtensionHandshaker.java
+++ b/codec-http/src/main/java/io/netty/handler/codec/http/websocketx/extensions/compression/PerMessageDeflateClientExtensionHandshaker.java
@@ -183,7 +183,7 @@ public final class PerMessageDeflateClientExtensionHandshaker implements WebSock
         }
 
         if ((requestedServerNoContext && !serverNoContext) ||
-                requestedServerWindowSize != serverWindowSize) {
+                requestedServerWindowSize < serverWindowSize) {
             succeed = false;
         }
 
diff --git a/codec-http/src/test/java/io/netty/handler/codec/http/websocketx/extensions/compression/PerMessageDeflateClientExtensionHandshakerTest.java b/codec-http/src/test/java/io/netty/handler/codec/http/websocketx/extensions/compression/PerMessageDeflateClientExtensionHandshakerTest.java
index d44244ed6e..3cc7492219 100644
--- a/codec-http/src/test/java/io/netty/handler/codec/http/websocketx/extensions/compression/PerMessageDeflateClientExtensionHandshakerTest.java
+++ b/codec-http/src/test/java/io/netty/handler/codec/http/websocketx/extensions/compression/PerMessageDeflateClientExtensionHandshakerTest.java
@@ -90,7 +90,7 @@ public class PerMessageDeflateClientExtensionHandshakerTest {
 
         parameters = new HashMap<>();
         parameters.put(CLIENT_MAX_WINDOW, "12");
-        parameters.put(SERVER_MAX_WINDOW, "10");
+        parameters.put(SERVER_MAX_WINDOW, "8");
         parameters.put(CLIENT_NO_CONTEXT, null);
         parameters.put(SERVER_NO_CONTEXT, null);