Fix compressed HEAD requests handling in JDK client

Prior to this commit, the `JdkClientHttpRequestFactory` would support
decompressing gziped/deflate encoded response bodies but would fail if
the response has no body but has a "Content-Encoding" response header.
This happens as a response to HEAD requests.

This commit ensures that only responses with actual message bodies are
decompressed.

Fixes gh-35966

spring-web/src/main/java/org/springframework/http/client/JdkClientHttpRequest.java

@@ -19,6 +19,7 @@ package org.springframework.http.client;
 import java.io.FilterInputStream;
 import java.io.IOException;
 import java.io.InputStream;
+import java.io.PushbackInputStream;
 import java.io.UncheckedIOException;
 import java.net.URI;
 import java.net.http.HttpClient;
@@ -60,6 +61,7 @@ import org.springframework.util.StringUtils;
  *
  * @author Marten Deinum
  * @author Arjen Poutsma
+ * @author Brian Clozel
  * @since 6.1
  */
 class JdkClientHttpRequest extends AbstractStreamingClientHttpRequest {
@@ -325,30 +327,61 @@ class JdkClientHttpRequest extends AbstractStreamingClientHttpRequest {
 	 */
 	private static final class DecompressingBodyHandler implements BodyHandler<InputStream> {
 
+
 		@Override
 		public BodySubscriber<InputStream> apply(ResponseInfo responseInfo) {
-			String contentEncoding = responseInfo.headers().firstValue(HttpHeaders.CONTENT_ENCODING).orElse("");
-			if (contentEncoding.equalsIgnoreCase("gzip")) {
-				return BodySubscribers.mapping(
+
+			String contentEncoding = responseInfo.headers()
+					.firstValue(HttpHeaders.CONTENT_ENCODING)
+					.orElse("")
+					.toLowerCase(Locale.ROOT);
+
+			return switch (contentEncoding) {
+				case "gzip", "deflate" -> BodySubscribers.mapping(
 						BodySubscribers.ofInputStream(),
-						(InputStream is) -> {
-							try {
-								return new GZIPInputStream(is);
-							}
-							catch (IOException ex) {
-								throw new UncheckedIOException(ex);
-							}
-						});
+						(InputStream is) -> decompressStream(is, contentEncoding));
+				default -> BodySubscribers.ofInputStream();
+			};
+		}
+
+		private static InputStream decompressStream(InputStream original, String contentEncoding) {
+			PushbackInputStream wrapped = new PushbackInputStream(original);
+			try {
+				if (hasResponseBody(wrapped)) {
+					if (contentEncoding.equals("gzip")) {
+						return new GZIPInputStream(wrapped);
+					}
+					else if (contentEncoding.equals("deflate")) {
+						return new InflaterInputStream(wrapped);
+					}
+				}
+				else {
+					return wrapped;
+				}
 			}
-			else if (contentEncoding.equalsIgnoreCase("deflate")) {
-				return BodySubscribers.mapping(
-						BodySubscribers.ofInputStream(),
-						InflaterInputStream::new);
+			catch (IOException ex) {
+				throw new UncheckedIOException(ex);
 			}
-			else {
-				return BodySubscribers.ofInputStream();
+			return wrapped;
+		}
+
+		private static boolean hasResponseBody(PushbackInputStream inputStream) {
+			try {
+				int b = inputStream.read();
+				if (b == -1) {
+					return false;
+				}
+				else {
+					inputStream.unread(b);
+					return true;
+				}
+
+			}
+			catch (IOException exc) {
+				return false;
 			}
 		}
 	}
 
+
 }

spring-web/src/test/java/org/springframework/http/client/AbstractMockWebServerTests.java

@@ -112,7 +112,7 @@ public abstract class AbstractMockWebServerTests {
 					String headerName = request.getTarget().replace("/header/","");
 					return new MockResponse.Builder().body(headerName + ":" + request.getHeaders().get(headerName)).code(200).build();
 				}
-				else if(request.getTarget().startsWith("/compress/") && request.getBody() != null) {
+				else if(request.getMethod().equals("POST") && request.getTarget().startsWith("/compress/") && request.getBody() != null) {
 					String encoding = request.getTarget().replace("/compress/","");
 					String requestBody = request.getBody().utf8();
 					ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
@@ -139,6 +139,13 @@ public abstract class AbstractMockWebServerTests {
 					builder.setHeader(HttpHeaders.CONTENT_LENGTH, buffer.size());
 					return builder.build();
 				}
+				else if (request.getMethod().equals("HEAD") && request.getTarget().startsWith("/headforcompress/")) {
+					String encoding = request.getTarget().replace("/headforcompress/","");
+					MockResponse.Builder builder = new MockResponse.Builder().code(200)
+							.setHeader(HttpHeaders.CONTENT_LENGTH, 500)
+							.setHeader(HttpHeaders.CONTENT_ENCODING, encoding);
+					return builder.build();
+				}
 				return new MockResponse.Builder().code(404).build();
 			}
 			catch (Throwable ex) {

spring-web/src/test/java/org/springframework/http/client/JdkClientHttpRequestFactoryTests.java

@@ -26,6 +26,8 @@ import org.junit.jupiter.api.BeforeAll;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.condition.EnabledForJreRange;
 import org.junit.jupiter.api.condition.JRE;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
 
 import org.springframework.http.HttpMethod;
 import org.springframework.http.HttpStatus;
@@ -159,6 +161,23 @@ class JdkClientHttpRequestFactoryTests extends AbstractHttpRequestFactoryTests {
 		}
 	}
 
+	@ParameterizedTest
+	@ValueSource(strings = {"gzip", "deflate"})
+	void gzipCompressionWithHeadRequest(String compression) throws IOException {
+		URI uri = URI.create(baseUrl + "/headforcompress/" + compression);
+		JdkClientHttpRequestFactory requestFactory = (JdkClientHttpRequestFactory) this.factory;
+		requestFactory.enableCompression(true);
+		ClientHttpRequest request = requestFactory.createRequest(uri, HttpMethod.HEAD);
+		try (ClientHttpResponse response = request.execute()) {
+			assertThat(response.getStatusCode()).as("Invalid response status").isEqualTo(HttpStatus.OK);
+			assertThat(response.getHeaders().getFirst("Content-Encoding"))
+					.as("Content Encoding should be removed").isNull();
+			assertThat(response.getHeaders().getFirst("Content-Length"))
+					.as("Content-Length should be removed").isNull();
+			assertThat(response.getBody()).as("Invalid response body").isEmpty();
+		}
+	}
+
 	@Test // gh-34971
 	@EnabledForJreRange(min = JRE.JAVA_19) // behavior fixed in Java 19
 	void requestContentLengthHeaderWhenNoBody() throws Exception {