Replace the pg_listener-based LISTEN/NOTIFY mechanism with an in-memory queue.

In addition, add support for a "payload" string to be passed along with
each notify event.

This implementation should be significantly more efficient than the old one,
and is also more compatible with Hot Standby usage.  There is not yet any
facility for HS slaves to receive notifications generated on the master,
although such a thing is possible in future.

Joachim Wieland, reviewed by Jeff Davis; also hacked on by me.

doc/src/sgml/func.sgml

@@ -1,4 +1,4 @@
-<!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.503 2010/02/16 21:18:01 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.504 2010/02/16 22:34:42 tgl Exp $ -->
 
  <chapter id="functions">
   <title>Functions and Operators</title>
@@ -11529,6 +11529,12 @@ postgres=# select * from unnest2(array[[1,2],[3,4]]);
        </entry>
       </row>
 
+      <row>
+       <entry><literal><function>pg_listening_channels</function>()</literal></entry>
+       <entry><type>setof text</type></entry>
+       <entry>channel names that the session is currently listening on</entry>
+      </row>
+
       <row>
        <entry><literal><function>inet_client_addr</function>()</literal></entry>
        <entry><type>inet</type></entry>
@@ -11674,6 +11680,16 @@ SET search_path TO <replaceable>schema</> <optional>, <replaceable>schema</>, ..
     </para>
    </note>
 
+   <indexterm>
+    <primary>pg_listening_channels</primary>
+   </indexterm>
+
+   <para>
+    <function>pg_listening_channels</function> returns a set of names of
+    channels that the current session is listening to.  See <xref
+    linkend="sql-listen" endterm="sql-listen-title"> for more information.
+   </para>
+
    <indexterm>
     <primary>inet_client_addr</primary>
    </indexterm>