INT-4477: Add getRole() to LeaderInitiator Context

JIRA: https://jira.spring.io/browse/INT-4477

When running multiple roles, it is useful to be able to determine the
role for a particular context, e.g. in an event.

* Docs

**cherry-pick to 5.0.x**

(cherry picked from commit 73dbdb8)

spring-integration-core/src/main/java/org/springframework/integration/leader/Context.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2014-2017 the original author or authors.
+ * Copyright 2014-2018 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.
@@ -16,6 +16,8 @@
 
 package org.springframework.integration.leader;
 
+import org.springframework.lang.Nullable;
+
 /**
  * Interface that defines the context for candidate leadership.
  * Instances of this object are passed to {@link Candidate candidates}
@@ -27,6 +29,7 @@ package org.springframework.integration.leader;
  * @author Patrick Peralta
  * @author Janne Valkealahti
  * @author Artem Bilan
+ * @author Gary Russell
  *
  */
 @FunctionalInterface
@@ -49,4 +52,14 @@ public interface Context {
 		// no-op
 	}
 
+	/**
+	 * Get the role for the {@link Candidate}.
+	 * @return the role.
+	 * @since 5.0.6
+	 */
+	@Nullable
+	default String getRole() {
+		return null;
+	}
+
 }

spring-integration-core/src/main/java/org/springframework/integration/support/leader/LockRegistryLeaderInitiator.java

@@ -58,6 +58,7 @@ import org.springframework.util.Assert;
  * @author Vedran Pavic
  * @author Glenn Renfro
  * @author Kiel Boatman
+ * @author Gary Russell
  *
  * @since 4.3.1
  */
@@ -69,8 +70,6 @@ public class LockRegistryLeaderInitiator implements SmartLifecycle, DisposableBe
 
 	private static final Log logger = LogFactory.getLog(LockRegistryLeaderInitiator.class);
 
-	private static final Context NULL_CONTEXT = () -> false;
-
 	private final Object lifecycleMonitor = new Object();
 
 	/**
@@ -87,6 +86,20 @@ public class LockRegistryLeaderInitiator implements SmartLifecycle, DisposableBe
 	 */
 	private final Candidate candidate;
 
+	private final Context nullContext = new Context() {
+
+		@Override
+		public boolean isLeader() {
+			return false;
+		}
+
+		@Override
+		public String getRole() {
+			return LockRegistryLeaderInitiator.this.candidate.getRole();
+		}
+
+	};
+
 	/**
 	 * Executor service for running leadership daemon.
 	 */
@@ -243,11 +256,11 @@ public class LockRegistryLeaderInitiator implements SmartLifecycle, DisposableBe
 	}
 
 	/**
-	 * @return the context (or null if not running)
+	 * @return the context.
 	 */
 	public Context getContext() {
 		if (this.leaderSelector == null) {
-			return NULL_CONTEXT;
+			return this.nullContext;
 		}
 		return this.leaderSelector.context;
 	}
@@ -520,6 +533,11 @@ public class LockRegistryLeaderInitiator implements SmartLifecycle, DisposableBe
 			}
 		}
 
+		@Override
+		public String getRole() {
+			return LockRegistryLeaderInitiator.this.candidate.getRole();
+		}
+
 		@Override
 		public String toString() {
 			return "LockContext{role=" + LockRegistryLeaderInitiator.this.candidate.getRole() +