Introduce a new GUC_REPORT setting "in_hot_standby".

Aside from being queriable via SHOW, this value is sent to the client
immediately at session startup, and again later on if the server gets
promoted to primary during the session.  The immediate report will be
used in an upcoming patch to avoid an extra round trip when trying to
connect to a primary server.

Haribabu Kommi, Greg Nancarrow, Tom Lane; reviewed at various times
by Laurenz Albe, Takayuki Tsunakawa, Peter Smith.

Discussion: https://postgr.es/m/CAF3+xM+8-ztOkaV9gHiJ3wfgENTq97QcjXQt+rbFQ6F7oNzt9A@mail.gmail.com

doc/src/sgml/config.sgml

@@ -9600,13 +9600,14 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
     <title>Preset Options</title>
 
     <para>
-     The following <quote>parameters</quote> are read-only, and are determined
-     when <productname>PostgreSQL</productname> is compiled or when it is
-     installed. As such, they have been excluded from the sample
+     The following <quote>parameters</quote> are read-only.
+     As such, they have been excluded from the sample
      <filename>postgresql.conf</filename> file.  These options report
      various aspects of <productname>PostgreSQL</productname> behavior
      that might be of interest to certain applications, particularly
      administrative front-ends.
+     Most of them are determined when <productname>PostgreSQL</productname>
+     is compiled or when it is installed.
     </para>
 
     <variablelist>
@@ -9651,10 +9652,11 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </term>
       <listitem>
        <para>
-        On Unix systems this parameter reports the permissions of the data
-        directory defined by (<xref linkend="guc-data-directory"/>) at startup.
+        On Unix systems this parameter reports the permissions the data
+        directory (defined by <xref linkend="guc-data-directory"/>)
+        had at server startup.
         (On Microsoft Windows this parameter will always display
-        <literal>0700</literal>). See
+        <literal>0700</literal>.) See
         <xref linkend="app-initdb-allow-group-access"/> for more information.
        </para>
       </listitem>
@@ -9695,6 +9697,23 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-in-hot-standby" xreflabel="in_hot_standby">
+      <term><varname>in_hot_standby</varname> (<type>boolean</type>)
+      <indexterm>
+       <primary><varname>in_hot_standby</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Reports whether the server is currently in hot standby mode.  When
+        this is <literal>on</literal>, all transactions are forced to be
+        read-only.  Within a session, this can change only if the server is
+        promoted to be primary.  See <xref linkend="hot-standby"/> for more
+        information.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-lc-collate" xreflabel="lc_collate">
       <term><varname>lc_collate</varname> (<type>string</type>)
       <indexterm>

doc/src/sgml/high-availability.sgml

@@ -1859,8 +1859,11 @@ if (!triggered)
    </para>
 
    <para>
-    Users will be able to tell whether their session is read-only by
-    issuing <command>SHOW transaction_read_only</command>.  In addition, a set of
+    Users can determine whether hot standby is currently active for their
+    session by issuing <command>SHOW in_hot_standby</command>.
+    (In server versions before 14, the <varname>in_hot_standby</varname>
+    parameter did not exist; a workable substitute method for older servers
+    is <command>SHOW transaction_read_only</command>.)  In addition, a set of
     functions (<xref linkend="functions-recovery-info-table"/>) allow users to
     access information about the standby server. These allow you to write
     programs that are aware of the current state of the database. These

doc/src/sgml/libpq.sgml

@@ -2150,6 +2150,7 @@ const char *PQparameterStatus(const PGconn *conn, const char *paramName);
        <varname>server_encoding</varname>,
        <varname>client_encoding</varname>,
        <varname>application_name</varname>,
+       <varname>in_hot_standby</varname>,
        <varname>is_superuser</varname>,
        <varname>session_authorization</varname>,
        <varname>DateStyle</varname>,
@@ -2162,7 +2163,10 @@ const char *PQparameterStatus(const PGconn *conn, const char *paramName);
        <varname>standard_conforming_strings</varname> was not reported by releases
        before 8.1;
        <varname>IntervalStyle</varname> was not reported by releases before 8.4;
-       <varname>application_name</varname> was not reported by releases before 9.0.)
+       <varname>application_name</varname> was not reported by releases before
+       9.0;
+       <varname>in_hot_standby</varname> was not reported by releases before
+       14.)
        Note that
        <varname>server_version</varname>,
        <varname>server_encoding</varname> and

doc/src/sgml/protocol.sgml

@@ -1278,6 +1278,7 @@ SELCT 1/0;<!-- this typo is intentional -->
     <varname>server_encoding</varname>,
     <varname>client_encoding</varname>,
     <varname>application_name</varname>,
+    <varname>in_hot_standby</varname>,
     <varname>is_superuser</varname>,
     <varname>session_authorization</varname>,
     <varname>DateStyle</varname>,
@@ -1290,7 +1291,10 @@ SELCT 1/0;<!-- this typo is intentional -->
     <varname>standard_conforming_strings</varname> was not reported by releases
     before 8.1;
     <varname>IntervalStyle</varname> was not reported by releases before 8.4;
-    <varname>application_name</varname> was not reported by releases before 9.0.)
+    <varname>application_name</varname> was not reported by releases before
+    9.0;
+    <varname>in_hot_standby</varname> was not reported by releases before
+    14.)
     Note that
     <varname>server_version</varname>,
     <varname>server_encoding</varname> and