INT-3132 Add minSubscribers to Pub/SubChannel

Send is deemed to be successful if sent to at least this
number of subscribers (default 0).

Note: when using a task executor, if there is at least one
subscriber, a send is always good, regardless of success
or failure of invoking the handler.

Polishing - Docs - PR Comments

src/reference/docbook/channel.xml

@@ -81,6 +81,20 @@
         must be a <interfacename>MessageHandler</interfacename> itself, and the subscriber's
         <methodname>handleMessage(Message)</methodname> method will be invoked in turn.
       </para>
+      <para>
+        Prior to version 3.0, invoking the send method on a <classname>PublishSubscribeChannel</classname> that
+        had no subscribers returned <code>false</code>. When used in conjunction with a
+        <classname>MessagingTemplate</classname>, a <classname>MessageDeliveryException</classname>
+        was thrown. Starting with version 3.0, the behavior has changed such that a send is
+        always considered successful if at least the minimum subscribers are present (and successfully
+        handle the message). This behavior can be modified by setting the <code>minSubscribers</code>
+        property, which defaults to <code>0</code>.
+      </para>
+      <note>
+        If a <classname>TaskExecutor</classname> is used, only the presence of the correct number
+        of subscribers is used for this determination, because the actual handling of the message
+        is performed asynchronously.
+      </note>
     </section>
     <section id="channel-implementations-queuechannel">
       <title>QueueChannel</title>

src/reference/docbook/whats-new.xml

@@ -363,5 +363,18 @@
 				For more information see <xref linkend="http"/>.
 			</para>
 		</section>
+		<section id="3.0-pub-sub">
+			<title>PublishSubscribeChannel Behavior</title>
+			<para>
+				Previously, sending to a &lt;publish-subscribe-channel/&gt; that had
+				no subscribers would return a <code>false</code> result. If used in conjunction with
+				a <classname>MessagingTemplate</classname>, this would result in an exception being thrown.
+				Now, the <classname>PublishSubscribeChannel</classname> has a property
+				<code>minSubscribers</code> (default 0). If the message is sent to at least the minimum
+				number of subscribers, the send is deemed to be successful (even if zero). If an application
+				is expecting to get an exception under these conditions, set the minimum subscribers to at
+				least 1.
+			</para>
+		</section>
 	</section>
 </chapter>