Do not send RST_STREAM when UnprocessedRequestException is raised

core/src/main/java/com/linecorp/armeria/client/Http2ResponseDecoder.java

@@ -92,13 +92,22 @@ private void onWrapperCompleted(HttpResponseWrapper resWrapper, int id, @Nullabl
         resWrapper.onSubscriptionCancelled(cause);
 
         if (cause != null) {
+            if (cause instanceof UnprocessedRequestException ||
+                cause instanceof ClosedStreamException) {
+                return;
+            }
+            final int streamId = idToStreamId(id);
+            final Http2Stream stream = conn.stream(streamId);
+            if (stream == null || !stream.isHeadersSent()) {
+                return;
+            }
             // Removing the response and decrementing `unfinishedResponses` isn't done immediately
             // here. Instead, we rely on `Http2ResponseDecoder#onStreamClosed` to decrement
             // `unfinishedResponses` after Netty decrements `numActiveStreams` in `DefaultHttp2Connection`
             // so that `unfinishedResponses` is never greater than `numActiveStreams`.
 
             // Reset the stream.
-            final int streamId = idToStreamId(id);
+
             final int lastStreamId = conn.local().lastStreamKnownByPeer();
             if (lastStreamId < 0 || // Did not receive a GOAWAY yet or
                 streamId <= lastStreamId) { // received a GOAWAY and the request's streamId <= lastStreamId

core/src/test/java/com/linecorp/armeria/client/HeaderListSizeExceptionTest.java

@@ -0,0 +1,64 @@
+/*
+ * Copyright 2025 LINE Corporation
+ *
+ * LINE Corporation licenses this file to you under the Apache License,
+ * version 2.0 (the "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at:
+ *
+ *   https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ */
+package com.linecorp.armeria.client;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+import java.time.Duration;
+import java.util.concurrent.CompletableFuture;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.RegisterExtension;
+
+import com.google.common.base.Strings;
+
+import com.linecorp.armeria.common.AggregatedHttpResponse;
+import com.linecorp.armeria.common.HttpMethod;
+import com.linecorp.armeria.common.HttpResponse;
+import com.linecorp.armeria.common.HttpStatus;
+import com.linecorp.armeria.common.RequestHeaders;
+import com.linecorp.armeria.server.ServerBuilder;
+import com.linecorp.armeria.testing.junit5.server.ServerExtension;
+
+import io.netty.handler.codec.http2.Http2Exception.HeaderListSizeException;
+
+class HeaderListSizeExceptionTest {
+
+    @RegisterExtension
+    static ServerExtension server = new ServerExtension() {
+        @Override
+        protected void configure(ServerBuilder sb) {
+            sb.service("/", (ctx, req) -> HttpResponse.delayed(
+                    () -> HttpResponse.of("OK"), Duration.ofMillis(100)));
+        }
+    };
+
+    @Test
+    void doNotSendRstStreamWhenHeaderListSizeExceptionIsRaised() throws InterruptedException {
+        final CompletableFuture<AggregatedHttpResponse> future = server.webClient().get("/").aggregate();
+        final String a = Strings.repeat("aa", 10000);
+        final RequestHeaders headers = RequestHeaders.of(HttpMethod.GET, "/", "foo", "bar",
+                                                         "baz", a);
+        assertThatThrownBy(() -> server.webClient().execute(headers).aggregate().join())
+                .hasCauseInstanceOf(UnprocessedRequestException.class)
+                .cause()
+                .hasCauseInstanceOf(HeaderListSizeException.class);
+        // If the client sends RST_STREAM with invalid stream ID, the server will send GOAWAY back thus
+        // the first request will be failed with ClosedSessionException.
+        assertThat(future.join().status()).isSameAs(HttpStatus.OK);
+    }
+}