Merge branch '3.1.x'

Closes gh-36092

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/embedded/UndertowWebServerFactoryCustomizerTests.java

@@ -103,6 +103,24 @@ class UndertowWebServerFactoryCustomizerTests {
 		assertThat(boundServerOption(UndertowOptions.MAX_HEADER_SIZE)).isNull();
 	}
 
+	@Test
+	void customMaxHttpRequestHeaderSize() {
+		bind("server.max-http-request-header-size=2048");
+		assertThat(boundServerOption(UndertowOptions.MAX_HEADER_SIZE)).isEqualTo(2048);
+	}
+
+	@Test
+	void customMaxHttpRequestHeaderSizeIgnoredIfNegative() {
+		bind("server.max-http-request-header-size=-1");
+		assertThat(boundServerOption(UndertowOptions.MAX_HEADER_SIZE)).isNull();
+	}
+
+	@Test
+	void customMaxHttpRequestHeaderSizeIgnoredIfZero() {
+		bind("server.max-http-request-header-size=0");
+		assertThat(boundServerOption(UndertowOptions.MAX_HEADER_SIZE)).isNull();
+	}
+
 	@Test
 	void customMaxHttpPostSize() {
 		bind("server.undertow.max-http-post-size=256");