Allow customization of disallowed JdkClientHttpRequest headers

By default, the JDK HttpClient's HttpRequest does not allow Connection,
Content-Length, Expect, Host, or Upgrade headers to be set, but this can
be overriden with the `jdk.httpclient.allowRestrictedHeaders` system
property.

See https://bugs.openjdk.org/browse/JDK-8213696

Closes gh-30787

Author: spencergibb

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

@@ -25,14 +25,17 @@
 import java.net.http.HttpResponse;
 import java.nio.ByteBuffer;
 import java.time.Duration;
-import java.util.List;
+import java.util.Collections;
+import java.util.Set;
+import java.util.TreeSet;
 import java.util.concurrent.Executor;
 import java.util.concurrent.Flow;
 
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpMethod;
 import org.springframework.lang.Nullable;
 import org.springframework.util.StreamUtils;
+import org.springframework.util.StringUtils;
 
 /**
  * {@link ClientHttpRequest} implementation based the Java {@link HttpClient}.
@@ -48,8 +51,19 @@ class JdkClientHttpRequest extends AbstractStreamingClientHttpRequest {
 	 * The JDK HttpRequest doesn't allow all headers to be set. The named headers are taken from the default
 	 * implementation for HttpRequest.
 	 */
-	private static final List<String> DISALLOWED_HEADERS =
-			List.of("connection", "content-length", "expect", "host", "upgrade");
+	protected static final Set<String> DISALLOWED_HEADERS = getDisallowedHeaders();
+
+	private static Set<String> getDisallowedHeaders() {
+		TreeSet<String> headers = new TreeSet<>(String.CASE_INSENSITIVE_ORDER);
+		headers.addAll(Set.of("connection", "content-length", "expect", "host", "upgrade"));
+
+		String headersToAllow = System.getProperty("jdk.httpclient.allowRestrictedHeaders");
+		if (headersToAllow != null) {
+			Set<String> toAllow = StringUtils.commaDelimitedListToSet(headersToAllow);
+			headers.removeAll(toAllow);
+		}
+		return Collections.unmodifiableSet(headers);
+	}
 
 	private final HttpClient httpClient;
 

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

@@ -16,10 +16,17 @@
 
 package org.springframework.http.client;
 
+import java.net.URI;
+import java.net.http.HttpClient;
+import java.time.Duration;
+import java.util.concurrent.Executor;
+
 import org.junit.jupiter.api.Test;
 
 import org.springframework.http.HttpMethod;
 
+import static org.assertj.core.api.Assertions.assertThat;
+
 /**
  * @author Marten Deinum
  */
@@ -37,4 +44,26 @@ public void httpMethods() throws Exception {
 		assertHttpMethod("patch", HttpMethod.PATCH);
 	}
 
+	@Test
+	public void customizeDisallowedHeaders() {
+		String original = System.getProperty("jdk.httpclient.allowRestrictedHeaders");
+		System.setProperty("jdk.httpclient.allowRestrictedHeaders", "host");
+
+		assertThat(TestJdkClientHttpRequest.DISALLOWED_HEADERS).doesNotContain("host");
+
+		if (original != null) {
+			System.setProperty("jdk.httpclient.allowRestrictedHeaders", original);
+		}
+		else {
+			System.clearProperty("jdk.httpclient.allowRestrictedHeaders");
+		}
+	}
+
+	static class TestJdkClientHttpRequest extends JdkClientHttpRequest {
+
+		public TestJdkClientHttpRequest(HttpClient httpClient, URI uri, HttpMethod method, Executor executor, Duration readTimeout) {
+			super(httpClient, uri, method, executor, readTimeout);
+		}
+	}
+
 }