Respect existing content-length for HTTP HEAD

Closes gh-23484

spring-web/src/test/java/org/springframework/http/server/reactive/HttpHeadResponseDecoratorTests.java

@@ -0,0 +1,73 @@
+/*
+ * Copyright 2002-2019 the original author or authors.
+ *
+ * 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
+ *
+ *      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 org.springframework.http.server.reactive;
+
+import java.nio.charset.StandardCharsets;
+
+import io.netty.buffer.PooledByteBufAllocator;
+import org.junit.After;
+import org.junit.Test;
+import reactor.core.publisher.Flux;
+
+import org.springframework.core.io.buffer.DataBuffer;
+import org.springframework.core.io.buffer.LeakAwareDataBufferFactory;
+import org.springframework.core.io.buffer.NettyDataBufferFactory;
+import org.springframework.mock.http.server.reactive.test.MockServerHttpResponse;
+
+import static org.junit.Assert.assertEquals;
+
+/**
+ * Unit tests for {@link HttpHeadResponseDecorator}.
+ * @author Rossen Stoyanchev
+ */
+public class HttpHeadResponseDecoratorTests {
+
+	private final LeakAwareDataBufferFactory bufferFactory =
+			new LeakAwareDataBufferFactory(new NettyDataBufferFactory(PooledByteBufAllocator.DEFAULT));
+
+	private final ServerHttpResponse response =
+			new HttpHeadResponseDecorator(new MockServerHttpResponse(this.bufferFactory));
+
+
+	@After
+	public void tearDown() {
+		this.bufferFactory.checkForLeaks();
+	}
+
+
+	@Test
+	public void write() {
+		Flux<DataBuffer> body = Flux.just(toDataBuffer("data1"), toDataBuffer("data2"));
+		response.writeWith(body).block();
+		assertEquals(10, response.getHeaders().getContentLength());
+	}
+
+	@Test // gh-23484
+	public void writeWithGivenContentLength() {
+		int length = 15;
+		this.response.getHeaders().setContentLength(length);
+		this.response.writeWith(Flux.empty()).block();
+		assertEquals(length, this.response.getHeaders().getContentLength());
+	}
+
+
+	private DataBuffer toDataBuffer(String s) {
+		DataBuffer buffer = this.bufferFactory.allocateBuffer();
+		buffer.write(s.getBytes(StandardCharsets.UTF_8));
+		return buffer;
+	}
+
+}