Close mail folder if no messages to produce

Related to https://stackoverflow.com/questions/71667731/spring-integration-mimemessage-gmail-folder-is-not-open-exception

If no mail messages pulled from the folder or all of them are filtered out,
there is nothing to produce downstream.
Therefore, always close the folder in the end of `AbstractMailReceiver.receive()`
when no messages and even if `autoCloseFolder == false`

**Cherry-pick to `5.5.x`**

Author: artembilan

spring-integration-mail/src/main/java/org/springframework/integration/mail/AbstractMailReceiver.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 the original author or authors.
+ * Copyright 2002-2022 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.
@@ -53,6 +53,7 @@
 import org.springframework.util.Assert;
 import org.springframework.util.FileCopyUtils;
 import org.springframework.util.MimeTypeUtils;
+import org.springframework.util.ObjectUtils;
 
 /**
  * Base class for {@link MailReceiver} implementations.
@@ -362,6 +363,7 @@ private Folder obtainFolderInstance() throws MessagingException {
 	public Object[] receive() throws javax.mail.MessagingException {
 		this.folderReadLock.lock(); // NOSONAR - guarded with the getReadHoldCount()
 		try {
+			Object[] messagesToReturn = null;
 			try {
 				Folder folderToCheck = getFolder();
 				if (folderToCheck == null || !folderToCheck.isOpen()) {
@@ -375,10 +377,11 @@ public Object[] receive() throws javax.mail.MessagingException {
 						this.folderWriteLock.unlock();
 					}
 				}
-				return convertMessagesIfNecessary(searchAndFilterMessages());
+				messagesToReturn = convertMessagesIfNecessary(searchAndFilterMessages());
+				return messagesToReturn;
 			}
 			finally {
-				if (this.autoCloseFolder) {
+				if (this.autoCloseFolder || ObjectUtils.isEmpty(messagesToReturn)) {
 					closeFolder();
 				}
 			}

spring-integration-mail/src/test/java/org/springframework/integration/mail/MailReceiverTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2014-2021 the original author or authors.
+ * Copyright 2014-2022 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.mail;
 
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.ArgumentMatchers.anyBoolean;
 import static org.mockito.Mockito.doNothing;
 import static org.mockito.Mockito.doReturn;
 import static org.mockito.Mockito.mock;
@@ -46,31 +48,37 @@
 public class MailReceiverTests {
 
 	@Test
-	public void testStoreConnect() throws Exception {
+	public void testStoreConnectAndFolderCloseWhenNoMessages() throws Exception {
 		AbstractMailReceiver receiver = new AbstractMailReceiver() {
 
 			@Override
 			protected Message[] searchForNewMessages() {
-				return null;
+				return new Message[0];
 			}
 
 		};
 		Properties props = new Properties();
 		Session session = Session.getInstance(props);
 		receiver.setSession(session);
 		receiver.setProtocol("imap");
+		receiver.setAutoCloseFolder(false);
 		Store store = session.getStore("imap");
 		store = spy(store);
 		new DirectFieldAccessor(receiver).setPropertyValue("store", store);
 		when(store.isConnected()).thenReturn(false);
 		Folder folder = mock(Folder.class);
 		when(folder.exists()).thenReturn(true);
-		when(folder.isOpen()).thenReturn(false);
+		when(folder.isOpen()).thenReturn(false, true);
 		doReturn(folder).when(store).getFolder((URLName) null);
 		doNothing().when(store).connect();
 		receiver.openFolder();
 		receiver.openFolder();
 		verify(store, times(2)).connect();
+
+		Object[] receive = receiver.receive();
+		assertThat(receive).isEmpty();
+
+		verify(folder).close(anyBoolean());
 	}
 
 }

src/reference/asciidoc/mail.adoc

@@ -140,6 +140,9 @@ This functionality is enabled with this combination of options: no `headerMapper
 The `MimeMessage` is present as the payload of the Spring message produced.
 In this case, the only header populated is the above mentioned `IntegrationMessageHeaderAccessor.CLOSEABLE_RESOURCE` for the folder which must be closed when processing of the `MimeMessage` is complete.
 
+Starting with version 5.5.11, the folder is closed automatically after `AbstractMailReceiver.receive()` if no messages received or all of them are filtered out independently of the `autoCloseFolder` flag.
+In this case there is nothing to produce downstream for possible logic around `IntegrationMessageHeaderAccessor.CLOSEABLE_RESOURCE` header.
+
 [[mail-mapping]]
 === Inbound Mail Message Mapping