Remove promote_trigger_file.

Previously, an idle startup (recovery) process would wake up every 5
seconds to have a chance to poll for promote_trigger_file, even if that
GUC was not configured.  That promotion triggering mechanism was
effectively superseded by pg_ctl promote and pg_promote() a long time
ago.  There probably aren't many users left and it's very easy to change
to the modern mechanisms, so we agreed to remove the feature.

This is part of a campaign to reduce wakeups on idle systems.

Author: Simon Riggs <simon.riggs@enterprisedb.com>
Reviewed-by: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Ian Lawrence Barwick <barwick@gmail.com>
Discussion: https://postgr.es/m/CANbhV-FsjnzVOQGBpQ589%3DnWuL1Ex0Ykn74Nh1hEjp2usZSR5g%40mail.gmail.com

doc/src/sgml/appendix-obsolete-recovery-config.sgml

@@ -34,14 +34,11 @@
    </para>
 
    <para>
-    The
-    <literal>trigger_file</literal>
-    <indexterm>
-     <primary>trigger_file</primary>
-     <see>promote_trigger_file</see>
-    </indexterm>
-    setting has been renamed to
-    <xref linkend="guc-promote-trigger-file"/>.
+    PostgreSQL 15 and below had a setting
+    <literal>promote_trigger_file</literal>, or
+    <literal>trigger_file</literal> before 12.
+    Use <command>pg_ctl promote</command> or call
+    <function>pg_promote()</function> to promote a standby instead.
    </para>
 
    <para>

doc/src/sgml/config.sgml

@@ -4610,24 +4610,6 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
         </listitem>
        </varlistentry>
 
-       <varlistentry id="guc-promote-trigger-file" xreflabel="promote_trigger_file">
-        <term><varname>promote_trigger_file</varname> (<type>string</type>)
-        <indexterm>
-          <primary><varname>promote_trigger_file</varname> configuration parameter</primary>
-        </indexterm>
-        </term>
-        <listitem>
-         <para>
-          Specifies a trigger file whose presence ends recovery in the
-          standby.  Even if this value is not set, you can still promote
-          the standby using <command>pg_ctl promote</command> or calling
-          <function>pg_promote()</function>.
-          This parameter can only be set in the <filename>postgresql.conf</filename>
-          file or on the server command line.
-         </para>
-        </listitem>
-       </varlistentry>
-
      <varlistentry id="guc-hot-standby" xreflabel="hot_standby">
       <term><varname>hot_standby</varname> (<type>boolean</type>)
       <indexterm>

doc/src/sgml/high-availability.sgml

@@ -653,11 +653,10 @@ protocol to make nodes agree on a serializable transactional order.
 
    <para>
     Standby mode is exited and the server switches to normal operation
-    when <command>pg_ctl promote</command> is run,
-    <function>pg_promote()</function> is called, or a trigger file is found
-    (<varname>promote_trigger_file</varname>). Before failover,
-    any WAL immediately available in the archive or in <filename>pg_wal</filename> will be
-    restored, but no attempt is made to connect to the primary.
+    when <command>pg_ctl promote</command> is run, or
+    <function>pg_promote()</function> is called. Before failover,
+    any WAL immediately available in the archive or in <filename>pg_wal</filename>
+    will be restored, but no attempt is made to connect to the primary.
    </para>
   </sect2>
 
@@ -1483,15 +1482,10 @@ synchronous_standby_names = 'ANY 2 (s1, s2, s3)'
 
    <para>
     To trigger failover of a log-shipping standby server, run
-    <command>pg_ctl promote</command>, call <function>pg_promote()</function>,
-    or create a trigger file with the file name and path specified by the
-    <varname>promote_trigger_file</varname>. If you're planning to use
-    <command>pg_ctl promote</command> or to call
-    <function>pg_promote()</function> to fail over,
-    <varname>promote_trigger_file</varname> is not required. If you're
-    setting up the reporting servers that are only used to offload read-only
-    queries from the primary, not for high availability purposes, you don't
-    need to promote it.
+    <command>pg_ctl promote</command> or call <function>pg_promote()</function>.
+    If you're setting up reporting servers that are only used to offload
+    read-only queries from the primary, not for high availability purposes,
+    you don't need to promote.
    </para>
   </sect1>