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

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 @@ public class JettyWebServerFactoryCustomizer
 	}
 
 	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 class NettyWebServerFactoryCustomizer
 	}
 
 	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 @@ public class TomcatWebServerFactoryCustomizer
 	}
 
 	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 @@ public class UndertowWebServerFactoryCustomizer
 	}
 
 	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 @@ class JettyWebServerFactoryCustomizerTests {
 		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 @@ class NettyWebServerFactoryCustomizerTests {
 		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 @@ class TomcatWebServerFactoryCustomizerTests {
 		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 @@ class UndertowWebServerFactoryCustomizerTests {
 		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);