Allow UNLISTEN in hot-standby mode.

Since LISTEN is (still) disallowed, UNLISTEN must be a no-op in a
hot-standby session, and so there's no harm in allowing it.  This
change allows client code to not worry about whether it's connected
to a primary or standby server when performing session-state-reset
type activities.  (Note that DISCARD ALL, which includes UNLISTEN,
was already allowed, making it inconsistent to reject UNLISTEN.)

Per discussion, back-patch to all supported versions.

Shay Rojansky, reviewed by Mi Tar

Discussion: https://postgr.es/m/CADT4RqCf2gA_TJtPAjnGzkC3ZiexfBZiLmA-mV66e4UyuVv8bA@mail.gmail.com

doc/src/sgml/high-availability.sgml

@@ -1767,6 +1767,11 @@ if (!triggered)
        Plugins and extensions - <command>LOAD</command>
       </para>
      </listitem>
+     <listitem>
+      <para>
+       <command>UNLISTEN</command>
+      </para>
+     </listitem>
     </itemizedlist>
    </para>
 
@@ -1856,7 +1861,7 @@ if (!triggered)
      </listitem>
      <listitem>
       <para>
-       <command>LISTEN</command>, <command>UNLISTEN</command>, <command>NOTIFY</command>
+       <command>LISTEN</command>, <command>NOTIFY</command>
       </para>
      </listitem>
     </itemizedlist>
@@ -1864,8 +1869,8 @@ if (!triggered)
 
    <para>
     In normal operation, <quote>read-only</quote> transactions are allowed to
-    use <command>LISTEN</command>, <command>UNLISTEN</command>, and
-    <command>NOTIFY</command>, so Hot Standby sessions operate under slightly tighter
+    use <command>LISTEN</command> and <command>NOTIFY</command>,
+    so Hot Standby sessions operate under slightly tighter
     restrictions than ordinary read-only sessions.  It is possible that some
     of these restrictions might be loosened in a future release.
    </para>

src/backend/tcop/utility.c

@@ -629,7 +629,7 @@ standard_ProcessUtility(PlannedStmt *pstmt,
 			{
 				UnlistenStmt *stmt = (UnlistenStmt *) parsetree;
 
-				PreventCommandDuringRecovery("UNLISTEN");
+				/* we allow UNLISTEN during recovery, as it's a noop */
 				CheckRestrictedOperation("UNLISTEN");
 				if (stmt->conditionname)
 					Async_Unlisten(stmt->conditionname);

src/test/regress/expected/hs_standby_allowed.out

@@ -208,6 +208,9 @@ LOCK hs1 IN ACCESS SHARE MODE;
 LOCK hs1 IN ROW SHARE MODE;
 LOCK hs1 IN ROW EXCLUSIVE MODE;
 COMMIT;
+-- UNLISTEN
+UNLISTEN a;
+UNLISTEN *;
 -- LOAD
 -- should work, easier if there is no test for that...
 -- ALLOWED COMMANDS

src/test/regress/expected/hs_standby_disallowed.out

@@ -118,10 +118,6 @@ listen a;
 ERROR:  cannot execute LISTEN during recovery
 notify a;
 ERROR:  cannot execute NOTIFY during recovery
-unlisten a;
-ERROR:  cannot execute UNLISTEN during recovery
-unlisten *;
-ERROR:  cannot execute UNLISTEN during recovery
 -- disallowed commands
 ANALYZE hs1;
 ERROR:  cannot execute ANALYZE during recovery

src/test/regress/sql/hs_standby_allowed.sql

@@ -110,6 +110,10 @@ LOCK hs1 IN ROW SHARE MODE;
 LOCK hs1 IN ROW EXCLUSIVE MODE;
 COMMIT;
 
+-- UNLISTEN
+UNLISTEN a;
+UNLISTEN *;
+
 -- LOAD
 -- should work, easier if there is no test for that...
 

src/test/regress/sql/hs_standby_disallowed.sql

@@ -88,8 +88,6 @@ COMMIT;
 -- Listen
 listen a;
 notify a;
-unlisten a;
-unlisten *;
 
 -- disallowed commands