Support explicitly setting forward headers strategy to NONE

Prior to this commit, there was no distinction between explicitly setting
forward headers strategy to a value of NONE and not setting it at all.
This meant that in a cloud environment, a cloud provider was always checked
to see if it was active and using forward headers and there was no way to
prevent that.

This commit changes the default value of the property to null so that there
is a way to determine if the property was explicitly set to NONE.

Fixes gh-19333

Author: mbhave

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/ServerProperties.java

@@ -84,7 +84,7 @@ public class ServerProperties {
 	/**
 	 * Strategy for handling X-Forwarded-* headers.
 	 */
-	private ForwardHeadersStrategy forwardHeadersStrategy = ForwardHeadersStrategy.NONE;
+	private ForwardHeadersStrategy forwardHeadersStrategy;
 
 	/**
 	 * Value to use for the Server response header (if empty, no header is sent).

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/embedded/JettyWebServerFactoryCustomizer.java

@@ -102,7 +102,7 @@ private boolean isPositive(Integer value) {
 	}
 
 	private boolean getOrDeduceUseForwardHeaders() {
-		if (this.serverProperties.getForwardHeadersStrategy().equals(ServerProperties.ForwardHeadersStrategy.NONE)) {
+		if (this.serverProperties.getForwardHeadersStrategy() == null) {
 			CloudPlatform platform = CloudPlatform.getActive(this.environment);
 			return platform != null && platform.isUsingForwardHeaders();
 		}

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/embedded/NettyWebServerFactoryCustomizer.java

@@ -68,7 +68,7 @@ public void customize(NettyReactiveWebServerFactory factory) {
 	}
 
 	private boolean getOrDeduceUseForwardHeaders() {
-		if (this.serverProperties.getForwardHeadersStrategy().equals(ServerProperties.ForwardHeadersStrategy.NONE)) {
+		if (this.serverProperties.getForwardHeadersStrategy() == null) {
 			CloudPlatform platform = CloudPlatform.getActive(this.environment);
 			return platform != null && platform.isUsingForwardHeaders();
 		}

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/embedded/TomcatWebServerFactoryCustomizer.java

@@ -201,7 +201,7 @@ private void customizeRemoteIpValve(ConfigurableTomcatWebServerFactory factory)
 	}
 
 	private boolean getOrDeduceUseForwardHeaders() {
-		if (this.serverProperties.getForwardHeadersStrategy().equals(ServerProperties.ForwardHeadersStrategy.NONE)) {
+		if (this.serverProperties.getForwardHeadersStrategy() == null) {
 			CloudPlatform platform = CloudPlatform.getActive(this.environment);
 			return platform != null && platform.isUsingForwardHeaders();
 		}

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/embedded/UndertowWebServerFactoryCustomizer.java

@@ -123,7 +123,7 @@ private void mapAccessLogProperties(ConfigurableUndertowWebServerFactory factory
 	}
 
 	private boolean getOrDeduceUseForwardHeaders() {
-		if (this.serverProperties.getForwardHeadersStrategy().equals(ServerProperties.ForwardHeadersStrategy.NONE)) {
+		if (this.serverProperties.getForwardHeadersStrategy() == null) {
 			CloudPlatform platform = CloudPlatform.getActive(this.environment);
 			return platform != null && platform.isUsingForwardHeaders();
 		}

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

@@ -86,6 +86,23 @@ void defaultUseForwardHeaders() {
 		verify(factory).setUseForwardHeaders(false);
 	}
 
+	@Test
+	void forwardHeadersWhenStrategyIsNativeShouldConfigureValve() {
+		this.serverProperties.setForwardHeadersStrategy(ServerProperties.ForwardHeadersStrategy.NATIVE);
+		ConfigurableJettyWebServerFactory factory = mock(ConfigurableJettyWebServerFactory.class);
+		this.customizer.customize(factory);
+		verify(factory).setUseForwardHeaders(true);
+	}
+
+	@Test
+	void forwardHeadersWhenStrategyIsNoneShouldNotConfigureValve() {
+		this.environment.setProperty("DYNO", "-");
+		this.serverProperties.setForwardHeadersStrategy(ServerProperties.ForwardHeadersStrategy.NONE);
+		ConfigurableJettyWebServerFactory factory = mock(ConfigurableJettyWebServerFactory.class);
+		this.customizer.customize(factory);
+		verify(factory).setUseForwardHeaders(false);
+	}
+
 	@Test
 	void accessLogCanBeCustomized() throws IOException {
 		File logFile = File.createTempFile("jetty_log", ".log");

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

@@ -92,6 +92,23 @@ void setUseForwardHeaders() {
 		verify(factory).setUseForwardHeaders(true);
 	}
 
+	@Test
+	void forwardHeadersWhenStrategyIsNativeShouldConfigureValve() {
+		this.serverProperties.setForwardHeadersStrategy(ServerProperties.ForwardHeadersStrategy.NATIVE);
+		NettyReactiveWebServerFactory factory = mock(NettyReactiveWebServerFactory.class);
+		this.customizer.customize(factory);
+		verify(factory).setUseForwardHeaders(true);
+	}
+
+	@Test
+	void forwardHeadersWhenStrategyIsNoneShouldNotConfigureValve() {
+		this.environment.setProperty("DYNO", "-");
+		this.serverProperties.setForwardHeadersStrategy(ServerProperties.ForwardHeadersStrategy.NONE);
+		NettyReactiveWebServerFactory factory = mock(NettyReactiveWebServerFactory.class);
+		this.customizer.customize(factory);
+		verify(factory).setUseForwardHeaders(false);
+	}
+
 	@Test
 	void setServerConnectionTimeoutAsZero() {
 		setupServerConnectionTimeout(Duration.ZERO);

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

@@ -234,6 +234,26 @@ void deduceUseForwardHeaders() {
 		testRemoteIpValveConfigured();
 	}
 
+	@Test
+	void defaultUseForwardHeaders() {
+		TomcatServletWebServerFactory factory = customizeAndGetFactory();
+		assertThat(factory.getEngineValves()).hasSize(0);
+	}
+
+	@Test
+	void forwardHeadersWhenStrategyIsNativeShouldConfigureValve() {
+		this.serverProperties.setForwardHeadersStrategy(ServerProperties.ForwardHeadersStrategy.NATIVE);
+		testRemoteIpValveConfigured();
+	}
+
+	@Test
+	void forwardHeadersWhenStrategyIsNoneShouldNotConfigureValve() {
+		this.environment.setProperty("DYNO", "-");
+		this.serverProperties.setForwardHeadersStrategy(ServerProperties.ForwardHeadersStrategy.NONE);
+		TomcatServletWebServerFactory factory = customizeAndGetFactory();
+		assertThat(factory.getEngineValves()).hasSize(0);
+	}
+
 	@Test
 	void defaultRemoteIpValve() {
 		// Since 1.1.7 you need to specify at least the protocol

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

@@ -202,6 +202,23 @@ void setUseForwardHeaders() {
 		verify(factory).setUseForwardHeaders(true);
 	}
 
+	@Test
+	void forwardHeadersWhenStrategyIsNativeShouldConfigureValve() {
+		this.serverProperties.setForwardHeadersStrategy(ServerProperties.ForwardHeadersStrategy.NATIVE);
+		ConfigurableUndertowWebServerFactory factory = mock(ConfigurableUndertowWebServerFactory.class);
+		this.customizer.customize(factory);
+		verify(factory).setUseForwardHeaders(true);
+	}
+
+	@Test
+	void forwardHeadersWhenStrategyIsNoneShouldNotConfigureValve() {
+		this.environment.setProperty("DYNO", "-");
+		this.serverProperties.setForwardHeadersStrategy(ServerProperties.ForwardHeadersStrategy.NONE);
+		ConfigurableUndertowWebServerFactory factory = mock(ConfigurableUndertowWebServerFactory.class);
+		this.customizer.customize(factory);
+		verify(factory).setUseForwardHeaders(false);
+	}
+
 	private <T> T boundServerOption(Option<T> option) {
 		Builder builder = Undertow.builder();
 		ConfigurableUndertowWebServerFactory factory = mockFactory(builder);