Fix possible NPE when using HttpClientCodec (#9465)

Motivation:

It was possible to produce a NPE when we for examples received more responses as requests as we did not check if the queue did not contain a method before trying to compare method names.

Modifications:

- Add extra null check
- Add unit tet

Result:

Fixes https://github.com/netty/netty/issues/9459

codec-http/src/main/java/io/netty/handler/codec/http/HttpClientCodec.java

@@ -160,7 +160,7 @@ public final class HttpClientCodec extends CombinedChannelDuplexHandler<HttpResp
                 return;
             }
 
-            if (msg instanceof HttpRequest && !done) {
+            if (msg instanceof HttpRequest) {
                 queue.offer(((HttpRequest) msg).method());
             }
 
@@ -233,6 +233,9 @@ public final class HttpClientCodec extends CombinedChannelDuplexHandler<HttpResp
             // current response.
             HttpMethod method = queue.poll();
 
+            // If the remote peer did for example send multiple responses for one request (which is not allowed per
+            // spec but may still be possible) method will be null so guard against it.
+            if (method != null) {
                 char firstChar = method.name().charAt(0);
                 switch (firstChar) {
                     case 'H':
@@ -261,8 +264,8 @@ public final class HttpClientCodec extends CombinedChannelDuplexHandler<HttpResp
                         // Successful CONNECT request results in a response with empty body.
                         if (statusCode == 200) {
                             if (HttpMethod.CONNECT.equals(method)) {
-                        // Proxy connection established - Parse HTTP only if configured by parseHttpAfterConnectRequest,
+                                // Proxy connection established - Parse HTTP only if configured by
-                        // else pass through.
+                                // parseHttpAfterConnectRequest, else pass through.
                                 if (!parseHttpAfterConnectRequest) {
                                     done = true;
                                     queue.clear();
@@ -272,7 +275,7 @@ public final class HttpClientCodec extends CombinedChannelDuplexHandler<HttpResp
                         }
                         break;
                 }
-
+            }
             return super.isContentAlwaysEmpty(msg);
         }
 

codec-http/src/test/java/io/netty/handler/codec/http/HttpClientCodecTest.java

@@ -326,4 +326,28 @@ public class HttpClientCodecTest {
 
         assertThat(ch.readInbound(), is(nullValue()));
     }
+
+    @Test
+    public void testMultipleResponses() {
+        String response = "HTTP/1.1 200 OK\r\n" +
+                "Content-Length: 0\r\n\r\n";
+
+        HttpClientCodec codec = new HttpClientCodec(4096, 8192, 8192, true);
+        EmbeddedChannel ch = new EmbeddedChannel(codec, new HttpObjectAggregator(1024));
+
+        HttpRequest request = new DefaultFullHttpRequest(HttpVersion.HTTP_1_1, HttpMethod.GET, "http://localhost/");
+        assertTrue(ch.writeOutbound(request));
+
+        assertTrue(ch.writeInbound(Unpooled.copiedBuffer(response, CharsetUtil.UTF_8)));
+        assertTrue(ch.writeInbound(Unpooled.copiedBuffer(response, CharsetUtil.UTF_8)));
+        FullHttpResponse resp = ch.readInbound();
+        assertTrue(resp.decoderResult().isSuccess());
+        resp.release();
+
+        resp = ch.readInbound();
+        assertTrue(resp.decoderResult().isSuccess());
+        resp.release();
+        assertTrue(ch.finishAndReleaseAll());
+    }
+
 }