Add support for reactor-tools debug agent

The `reactor-tools` dependency now brings a new Reactor Debug Agent
which instruments loaded classes for better Reactor stacktraces.
This commit removes the `spring.reactor.stacktrace-mode.enabled`
configuration property since the related Reactor Hook is about to be
removed.
As a replacement, we're introducing `spring.reactor.debug-agent.enabled`
which tells whether the Reactor Debug Agent should be loaded, given that
the `reactor-tools` dependency is available. This option is enabled by
default, since adding the dependency on classpath is a strong signal
already.

Fixes gh-17128

spring-boot-project/spring-boot/pom.xml

@@ -76,6 +76,11 @@
 			<artifactId>reactor-netty</artifactId>
 			<optional>true</optional>
 		</dependency>
+		<dependency>
+			<groupId>io.projectreactor</groupId>
+			<artifactId>reactor-tools</artifactId>
+			<optional>true</optional>
+		</dependency>
 		<dependency>
 			<groupId>io.netty</groupId>
 			<artifactId>netty-tcnative-boringssl-static</artifactId>

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/reactor/DebugAgentEnvironmentPostProcessor.java

@@ -0,0 +1,59 @@
+/*
+ * Copyright 2012-2019 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.reactor;
+
+import reactor.tools.agent.ReactorDebugAgent;
+
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.env.EnvironmentPostProcessor;
+import org.springframework.core.Ordered;
+import org.springframework.core.env.ConfigurableEnvironment;
+import org.springframework.util.ClassUtils;
+
+/**
+ * {@link EnvironmentPostProcessor} to enable the Reactor Debug Agent if available.
+ * <p>
+ * The debug agent is enabled by default, unless the
+ * {@code "spring.reactor.debug-agent.enabled"} configuration property is set to false. We
+ * are using here an {@link EnvironmentPostProcessor} instead of an auto-configuration
+ * class to enable the agent as soon as possible during the startup process.
+ *
+ * @author Brian Clozel
+ * @since 2.2.0
+ */
+public class DebugAgentEnvironmentPostProcessor implements EnvironmentPostProcessor, Ordered {
+
+	private static final String REACTOR_DEBUGAGENT_CLASS = "reactor.tools.agent.ReactorDebugAgent";
+
+	private static final String DEBUGAGENT_ENABLED_CONFIG_KEY = "spring.reactor.debug-agent.enabled";
+
+	@Override
+	public void postProcessEnvironment(ConfigurableEnvironment environment, SpringApplication application) {
+		if (ClassUtils.isPresent(REACTOR_DEBUGAGENT_CLASS, null)) {
+			Boolean agentEnabled = environment.getProperty(DEBUGAGENT_ENABLED_CONFIG_KEY, Boolean.class);
+			if (agentEnabled != Boolean.FALSE) {
+				ReactorDebugAgent.init();
+			}
+		}
+	}
+
+	@Override
+	public int getOrder() {
+		return Ordered.LOWEST_PRECEDENCE;
+	}
+
+}

spring-boot-project/spring-boot/src/main/resources/META-INF/additional-spring-configuration-metadata.json

@@ -719,6 +719,13 @@
       "sourceType": "org.springframework.boot.context.config.ConfigFileApplicationListener",
       "description": "Unconditionally activate the specified comma-separated list of profiles (or list of profiles if using YAML)."
     },
+    {
+      "name": "spring.reactor.debug-agent.enabled",
+      "type": "java.lang.Boolean",
+      "sourceType": "org.springframework.boot.reactor.DebugAgentEnvironmentPostProcessor",
+      "description": "Whether the Reactor Debug Agent should be enabled when reactor-tools is present.",
+      "defaultValue": true
+    },
     {
       "name": "trace",
       "type": "java.lang.Boolean",

spring-boot-project/spring-boot/src/main/resources/META-INF/spring.factories

@@ -34,7 +34,8 @@ org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener
 org.springframework.boot.env.EnvironmentPostProcessor=\
 org.springframework.boot.cloud.CloudFoundryVcapEnvironmentPostProcessor,\
 org.springframework.boot.env.SpringApplicationJsonEnvironmentPostProcessor,\
-org.springframework.boot.env.SystemEnvironmentPropertySourceEnvironmentPostProcessor
+org.springframework.boot.env.SystemEnvironmentPropertySourceEnvironmentPostProcessor,\
+org.springframework.boot.reactor.DebugAgentEnvironmentPostProcessor
 
 # Failure Analyzers
 org.springframework.boot.diagnostics.FailureAnalyzer=\

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/reactor/DebugAgentEnvironmentPostProcessorTests.java

@@ -0,0 +1,48 @@
+/*
+ * Copyright 2012-2019 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.reactor;
+
+import org.junit.jupiter.api.Test;
+import reactor.core.Scannable;
+import reactor.core.publisher.Flux;
+
+import org.springframework.mock.env.MockEnvironment;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link DebugAgentEnvironmentPostProcessor}.
+ *
+ * @author Brian Clozel
+ */
+class DebugAgentEnvironmentPostProcessorTests {
+
+	static {
+		MockEnvironment environment = new MockEnvironment();
+		DebugAgentEnvironmentPostProcessor postProcessor = new DebugAgentEnvironmentPostProcessor();
+		postProcessor.postProcessEnvironment(environment, null);
+	}
+
+	@Test
+	void enablesReactorDebugAgent() {
+		InstrumentedFluxProvider fluxProvider = new InstrumentedFluxProvider();
+		Flux<Integer> flux = fluxProvider.newFluxJust();
+		assertThat(Scannable.from(flux).stepName())
+				.startsWith("Flux.just ⇢ at org.springframework.boot.reactor.InstrumentedFluxProvider.newFluxJust");
+	}
+
+}

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/reactor/InstrumentedFluxProvider.java

@@ -0,0 +1,33 @@
+/*
+ * Copyright 2012-2019 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.reactor;
+
+import reactor.core.publisher.Flux;
+
+/**
+ * Utility class that should be instrumented by the reactor debug agent.
+ *
+ * @author Brian Clozel
+ * @see DebugAgentEnvironmentPostProcessorTests
+ */
+class InstrumentedFluxProvider {
+
+	Flux<Integer> newFluxJust() {
+		return Flux.just(1);
+	}
+
+}