UNDERTOW-2019: Blocking exchanges can set HEAD response length

core/src/main/java/io/undertow/io/UndertowOutputStream.java

@@ -26,6 +26,7 @@
 import io.undertow.UndertowMessages;
 import io.undertow.server.HttpServerExchange;
 import io.undertow.util.Headers;
+import io.undertow.util.Methods;
 import org.xnio.Buffers;
 import io.undertow.connector.ByteBufferPool;
 import io.undertow.connector.PooledByteBuffer;
@@ -327,7 +328,9 @@ public void close() throws IOException {
         if (anyAreSet(state, FLAG_CLOSED)) return;
         try {
             state |= FLAG_CLOSED;
-            if (anyAreClear(state, FLAG_WRITE_STARTED) && channel == null) {
+            if (anyAreClear(state, FLAG_WRITE_STARTED)
+                    && channel == null
+                    && !isHeadRequestWithContentLength(exchange)) {
                 if (buffer == null) {
                     exchange.getResponseHeaders().put(Headers.CONTENT_LENGTH, "0");
                 } else {
@@ -356,6 +359,12 @@ public void close() throws IOException {
         }
     }
 
+    // Head request handlers may set the content-length response header in lieu of writing bytes
+    private static boolean isHeadRequestWithContentLength(HttpServerExchange exchange) {
+        return Methods.HEAD.equals(exchange.getRequestMethod())
+                && exchange.getResponseHeaders().contains(Headers.CONTENT_LENGTH);
+    }
+
     private ByteBuffer buffer() {
         ByteBuffer buffer = this.buffer;
         if (buffer != null) {

core/src/test/java/io/undertow/server/handlers/HeadBlockingExchangeTestCase.java

@@ -0,0 +1,63 @@
+/*
+ * JBoss, Home of Professional Open Source.
+ * Copyright 2022 Red Hat, Inc., and individual contributors
+ * as indicated by the @author tags.
+ *
+ * Licensed 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
+ *
+ *     http://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 io.undertow.server.handlers;
+
+import io.undertow.testutils.DefaultServer;
+import io.undertow.testutils.HttpClientUtils;
+import io.undertow.testutils.TestHttpClient;
+import io.undertow.util.Headers;
+import io.undertow.util.StatusCodes;
+import org.apache.http.HttpResponse;
+import org.apache.http.client.methods.HttpHead;
+import org.junit.Assert;
+import org.junit.BeforeClass;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+import java.io.IOException;
+
+/**
+ * Test that {@code HEAD} requests can be used with a blocking exchange, setting response
+ * content-length without unnecessarily writing bytes.
+ *
+ * @author Carter Kozak
+ */
+@RunWith(DefaultServer.class)
+public class HeadBlockingExchangeTestCase {
+
+    @BeforeClass
+    public static void setup() {
+        DefaultServer.setRootHandler(new BlockingHandler(
+                exchange -> exchange.getResponseHeaders().put(Headers.CONTENT_LENGTH, "" + 100)));
+    }
+
+    @Test
+    public void sendHttpHead() throws IOException {
+        HttpHead head = new HttpHead(DefaultServer.getDefaultServerURL());
+        TestHttpClient client = new TestHttpClient();
+        try {
+            HttpResponse result = client.execute(head);
+            Assert.assertEquals(StatusCodes.OK, result.getStatusLine().getStatusCode());
+            Assert.assertEquals("", HttpClientUtils.readResponse(result));
+            Assert.assertEquals("100", result.getFirstHeader("Content-Length").getValue());
+        } finally {
+            client.getConnectionManager().shutdown();
+        }
+    }
+}