INT-4491: (S)FTP inbound rotate dirs/servers

JIRA: https://jira.spring.io/browse/INT-4491

Add Rotating Server/Directory Polling Advice.

**cherry-pick to 5.0.x**

* Polishing - PR Comments.

* Polishing

* Polishing; revert `KeyDirectory`; WARN about `TaskExecutor` and `MessageSoureMutator`(s).

* More polishing - PR comments

* Apply stashed changes.

* Fix WARN log - the `SyncTaskExecutor` is wrapped.

# Conflicts:
#	spring-integration-core/src/main/java/org/springframework/integration/endpoint/SourcePollingChannelAdapter.java
#	src/reference/asciidoc/whats-new.adoc

spring-integration-core/src/main/java/org/springframework/integration/aop/AbstractMessageSourceAdvice.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2015 the original author or authors.
+ * Copyright 2015-2018 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.
@@ -27,9 +27,10 @@ import org.springframework.messaging.Message;
  * should be ignored and/or take action after the receive.
  *
  * @author Gary Russell
+ *
  * @since 4.2
  */
-public abstract class AbstractMessageSourceAdvice implements MethodInterceptor {
+public abstract class AbstractMessageSourceAdvice implements MethodInterceptor, MessageSourceMutator {
 
 	@Override
 	public final Object invoke(MethodInvocation invocation) throws Throwable {
@@ -45,20 +46,4 @@ public abstract class AbstractMessageSourceAdvice implements MethodInterceptor {
 		return afterReceive(result, (MessageSource<?>) target);
 	}
 
-	/**
-	 * Subclasses can decide whether to proceed with this poll.
-	 * @param source the message source.
-	 * @return true to proceed.
-	 */
-	public abstract boolean beforeReceive(MessageSource<?> source);
-
-	/**
-	 * Subclasses can take actions based on the result of the poll; e.g.
-	 * adjust the {@code trigger}. The message can also be replaced with a new one.
-	 * @param result the received message.
-	 * @param source the message source.
-	 * @return a message to continue to process the result, null to discard whatever the poll returned.
-	 */
-	public abstract Message<?> afterReceive(Message<?> result, MessageSource<?> source);
-
 }

spring-integration-core/src/main/java/org/springframework/integration/aop/CompoundTriggerAdvice.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2015-2016 the original author or authors.
+ * Copyright 2015-2018 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.
@@ -47,11 +47,6 @@ public class CompoundTriggerAdvice extends AbstractMessageSourceAdvice {
 		this.override = overrideTrigger;
 	}
 
-	@Override
-	public boolean beforeReceive(MessageSource<?> source) {
-		return true;
-	}
-
 	@Override
 	public Message<?> afterReceive(Message<?> result, MessageSource<?> source) {
 		if (result == null) {

spring-integration-core/src/main/java/org/springframework/integration/aop/MessageSourceMutator.java

@@ -0,0 +1,52 @@
+/*
+ * Copyright 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.
+ * You may obtain a copy of the License at
+ *
+ *      http://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.integration.aop;
+
+import org.springframework.integration.core.MessageSource;
+import org.springframework.messaging.Message;
+
+/**
+ * An object that can mutate a {@link MessageSource} before and/or after
+ * {@link MessageSource#receive()} is called.
+ *
+ * @author Gary Russell
+ *
+ * @since 5.0.7.
+ *
+ */
+@FunctionalInterface
+public interface MessageSourceMutator {
+
+	/**
+	 * Subclasses can decide whether to proceed with this poll.
+	 * @param source the message source.
+	 * @return true to proceed (default).
+	 */
+	default boolean beforeReceive(MessageSource<?> source) {
+		return true;
+	}
+
+	/**
+	 * Subclasses can take actions based on the result of the poll; e.g.
+	 * adjust the {@code trigger}. The message can also be replaced with a new one.
+	 * @param result the received message.
+	 * @param source the message source.
+	 * @return a message to continue to process the result, null to discard whatever the poll returned.
+	 */
+	Message<?> afterReceive(Message<?> result, MessageSource<?> source);
+
+}

spring-integration-core/src/main/java/org/springframework/integration/aop/SimpleActiveIdleMessageSourceAdvice.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2015 the original author or authors.
+ * Copyright 2015-2018 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.
@@ -61,11 +61,6 @@ public class SimpleActiveIdleMessageSourceAdvice extends AbstractMessageSourceAd
 		this.activePollPeriod = activePollPeriod;
 	}
 
-	@Override
-	public boolean beforeReceive(MessageSource<?> source) {
-		return true;
-	}
-
 	@Override
 	public Message<?> afterReceive(Message<?> result, MessageSource<?> source) {
 		if (result == null) {

spring-integration-core/src/main/java/org/springframework/integration/dsl/PollerSpec.java

@@ -75,7 +75,7 @@ public final class PollerSpec extends IntegrationComponentSpec<PollerSpec, Polle
 	 * to the {@link org.springframework.integration.util.ErrorHandlingTaskExecutor}.
 	 * @param errorHandler the {@link ErrorHandler} to use.
 	 * @return the spec.
-	 * @see #taskExecutor
+	 * @see #taskExecutor(Executor)
 	 */
 	public PollerSpec errorHandler(ErrorHandler errorHandler) {
 		this.target.setErrorHandler(errorHandler);

spring-integration-core/src/main/java/org/springframework/integration/endpoint/AbstractPollingEndpoint.java

@@ -60,17 +60,25 @@ import org.springframework.util.ErrorHandler;
  */
 public abstract class AbstractPollingEndpoint extends AbstractEndpoint implements BeanClassLoaderAware {
 
-	private volatile Executor taskExecutor = new SyncTaskExecutor();
+	private final Object initializationMonitor = new Object();
 
-	private volatile ErrorHandler errorHandler;
+	private Executor taskExecutor = new SyncTaskExecutor();
 
-	private volatile boolean errorHandlerIsDefault;
+	private boolean syncExecutor = true;
 
-	private volatile Trigger trigger = new PeriodicTrigger(10);
+	private ErrorHandler errorHandler;
 
-	private volatile List<Advice> adviceChain;
+	private boolean errorHandlerIsDefault;
 
-	private volatile ClassLoader beanClassLoader = ClassUtils.getDefaultClassLoader();
+	private Trigger trigger = new PeriodicTrigger(10);
+
+	private List<Advice> adviceChain;
+
+	private ClassLoader beanClassLoader = ClassUtils.getDefaultClassLoader();
+
+	private long maxMessagesPerPoll = -1;
+
+	private TransactionSynchronizationFactory transactionSynchronizationFactory;
 
 	private volatile ScheduledFuture<?> runningTask;
 
@@ -78,18 +86,23 @@ public abstract class AbstractPollingEndpoint extends AbstractEndpoint implement
 
 	private volatile boolean initialized;
 
-	private volatile long maxMessagesPerPoll = -1;
-
-	private final Object initializationMonitor = new Object();
-
-	private volatile TransactionSynchronizationFactory transactionSynchronizationFactory;
-
 	public AbstractPollingEndpoint() {
 		this.setPhase(Integer.MAX_VALUE / 2);
 	}
 
 	public void setTaskExecutor(Executor taskExecutor) {
 		this.taskExecutor = (taskExecutor != null ? taskExecutor : new SyncTaskExecutor());
+		this.syncExecutor = this.taskExecutor instanceof SyncTaskExecutor
+				|| (this.taskExecutor instanceof ErrorHandlingTaskExecutor
+						&& ((ErrorHandlingTaskExecutor) this.taskExecutor).isSyncExecutor());
+	}
+
+	protected Executor getTaskExecutor() {
+		return this.taskExecutor;
+	}
+
+	protected boolean isSyncExecutor() {
+		return this.syncExecutor;
 	}
 
 	public void setTrigger(Trigger trigger) {

spring-integration-core/src/main/java/org/springframework/integration/endpoint/SourcePollingChannelAdapter.java

@@ -27,7 +27,7 @@ import org.springframework.aop.support.AopUtils;
 import org.springframework.aop.support.NameMatchMethodPointcutAdvisor;
 import org.springframework.beans.factory.BeanCreationException;
 import org.springframework.context.Lifecycle;
-import org.springframework.integration.aop.AbstractMessageSourceAdvice;
+import org.springframework.integration.aop.MessageSourceMutator;
 import org.springframework.integration.context.ExpressionCapable;
 import org.springframework.integration.core.MessageSource;
 import org.springframework.integration.core.MessagingTemplate;
@@ -137,7 +137,7 @@ public class SourcePollingChannelAdapter extends AbstractPollingEndpoint
 
 	@Override
 	protected boolean isReceiveOnlyAdvice(Advice advice) {
-		return advice instanceof AbstractMessageSourceAdvice;
+		return advice instanceof MessageSourceMutator;
 	}
 
 	@Override
@@ -159,6 +159,13 @@ public class SourcePollingChannelAdapter extends AbstractPollingEndpoint
 			}
 			this.appliedAdvices.clear();
 			this.appliedAdvices.addAll(chain);
+			if (!(isSyncExecutor()) && logger.isWarnEnabled()) {
+				logger.warn(getComponentName() + ": A task executor is supplied and " + chain.size()
+						+ "MessageSourceMutator(s) is/are provided. If an advice mutates the source, such "
+						+ "mutations are not thread safe and could cause unexpected results, especially with "
+						+ "high frequency pollers. Consider using a downstream ExecutorChannel instead of "
+						+ "adding an executor to the poller");
+			}
 		}
 	}
 

spring-integration-core/src/main/java/org/springframework/integration/util/ErrorHandlingTaskExecutor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -18,6 +18,7 @@ package org.springframework.integration.util;
 
 import java.util.concurrent.Executor;
 
+import org.springframework.core.task.SyncTaskExecutor;
 import org.springframework.core.task.TaskExecutor;
 import org.springframework.util.Assert;
 import org.springframework.util.ErrorHandler;
@@ -45,6 +46,9 @@ public class ErrorHandlingTaskExecutor implements TaskExecutor {
 		this.errorHandler = errorHandler;
 	}
 
+	public boolean isSyncExecutor() {
+		return this.executor instanceof SyncTaskExecutor;
+	}
 
 	@Override
 	public void execute(final Runnable task) {