Fix PostgresSubscribableChannel.notifyUpdate()

When transaction is configured for the `PostgresSubscribableChannel.notifyUpdate()`
and it is rolled back, the next poll in that loop will return the same message.
Again and again if transaction is always rolled back.
This leads to the condition when we never leave this loop even if
we fully unsubscribed from this channel.

The issue has need spotted after introducing `SKIP LOCKED`
for `PostgresChannelMessageStoreQueryProvider` which leads to
the locked record in DB in the mentioned above transaction.

* Introduce `PostgresSubscribableChannel.hasHandlers` flag
to check in the `notifyUpdate()` before performing poll query in DB.

**Cherry-pick to `6.1.x` & `6.0.x`**

spring-integration-jdbc/src/main/java/org/springframework/integration/jdbc/channel/PostgresSubscribableChannel.java

@@ -67,6 +67,8 @@ public class PostgresSubscribableChannel extends AbstractSubscribableChannel
 
 	private Executor executor;
 
+	private volatile boolean hasHandlers;
+
 	/**
 	 * Create a subscribable channel for a Postgres database.
 	 * @param jdbcChannelMessageStore The message store to use for the relevant region.
@@ -128,6 +130,7 @@ public class PostgresSubscribableChannel extends AbstractSubscribableChannel
 		boolean subscribed = super.subscribe(handler);
 		if (this.dispatcher.getHandlerCount() == 1) {
 			this.messageTableSubscriber.subscribe(this);
+			this.hasHandlers = true;
 			notifyUpdate();
 		}
 		return subscribed;
@@ -138,6 +141,7 @@ public class PostgresSubscribableChannel extends AbstractSubscribableChannel
 		boolean unsubscribed = super.unsubscribe(handle);
 		if (this.dispatcher.getHandlerCount() == 0) {
 			this.messageTableSubscriber.unsubscribe(this);
+			this.hasHandlers = false;
 		}
 		return unsubscribed;
 	}
@@ -159,18 +163,7 @@ public class PostgresSubscribableChannel extends AbstractSubscribableChannel
 			try {
 				Optional<Message<?>> dispatchedMessage;
 				do {
-					if (this.transactionTemplate != null) {
-						dispatchedMessage =
-								this.retryTemplate.execute(context ->
-										this.transactionTemplate.execute(status ->
-												pollMessage()
-														.map(this::dispatch)));
-					}
-					else {
-						dispatchedMessage =
-								pollMessage()
-										.map(message -> this.retryTemplate.execute(context -> dispatch(message)));
-					}
+					dispatchedMessage = askForMessage();
 				} while (dispatchedMessage.isPresent());
 			}
 			catch (Exception ex) {
@@ -179,6 +172,20 @@ public class PostgresSubscribableChannel extends AbstractSubscribableChannel
 		});
 	}
 
+	private Optional<Message<?>> askForMessage() {
+		if (this.hasHandlers) {
+			if (this.transactionTemplate != null) {
+				return this.retryTemplate.execute(context ->
+						this.transactionTemplate.execute(status -> pollMessage().map(this::dispatch)));
+			}
+			else {
+				return pollMessage()
+						.map(message -> this.retryTemplate.execute(context -> dispatch(message)));
+			}
+		}
+		return Optional.empty();
+	}
+
 	private Optional<Message<?>> pollMessage() {
 		return Optional.ofNullable(this.jdbcChannelMessageStore.pollMessageFromGroup(this.groupId));
 	}

spring-integration-jdbc/src/test/java/org/springframework/integration/jdbc/channel/PostgresChannelMessageTableSubscriberTests.java

@@ -45,6 +45,7 @@ import org.springframework.jdbc.datasource.DataSourceTransactionManager;
 import org.springframework.jdbc.datasource.init.DataSourceInitializer;
 import org.springframework.jdbc.datasource.init.ResourceDatabasePopulator;
 import org.springframework.jdbc.datasource.init.ScriptUtils;
+import org.springframework.messaging.MessageHandler;
 import org.springframework.messaging.support.GenericMessage;
 import org.springframework.retry.support.RetryTemplate;
 import org.springframework.test.annotation.DirtiesContext;
@@ -172,14 +173,16 @@ public class PostgresChannelMessageTableSubscriberTests implements PostgresConta
 		postgresSubscribableChannel.setTransactionManager(transactionManager);
 
 		postgresChannelMessageTableSubscriber.start();
-		postgresSubscribableChannel.subscribe(message -> {
+		MessageHandler messageHandler =
+				message -> {
 			try {
 				throw new RuntimeException("An error has occurred");
 			}
 			finally {
 				latch.countDown();
 			}
-		});
+		};
+		postgresSubscribableChannel.subscribe(messageHandler);
 
 		messageStore.addMessageToGroup(groupId, new GenericMessage<>("1"));
 		messageStore.addMessageToGroup(groupId, new GenericMessage<>("2"));
@@ -188,6 +191,7 @@ public class PostgresChannelMessageTableSubscriberTests implements PostgresConta
 
 		// Stop subscriber to unlock records from TX for the next verification
 		postgresChannelMessageTableSubscriber.stop();
+		postgresSubscribableChannel.unsubscribe(messageHandler);
 
 		assertThat(messageStore.messageGroupSize(groupId)).isEqualTo(2);
 		assertThat(messageStore.pollMessageFromGroup(groupId).getPayload()).isEqualTo("1");