postgres_fdw: Allow application_name of remote connection to be set via GUC.

This commit adds postgres_fdw.application_name GUC which specifies
a value for application_name configuration parameter used
when postgres_fdw establishes a connection to a foreign server.
This GUC setting always overrides application_name option of
the foreign server object. This GUC is useful when we want to
specify our own application_name per remote connection.

Previously application_name of a remote connection could be set
basically only via options of a server object. But which meant that
every session connecting to the same foreign server basically
should use the same application_name. Also if we want to change
the setting, we had to execute "ALTER SERVER ... OPTIONS ..." command.
It was inconvenient.

Author: Hayato Kuroda
Reviewed-by: Masahiro Ikeda, Fujii Masao
Discussion: https://postgr.es/m/TYCPR01MB5870D1E8B949DAF6D3B84E02F5F29@TYCPR01MB5870.jpnprd01.prod.outlook.com

contrib/postgres_fdw/connection.c

@@ -353,10 +353,11 @@ connect_pg_server(ForeignServer *server, UserMapping *user)
 		/*
 		 * Construct connection params from generic options of ForeignServer
 		 * and UserMapping.  (Some of them might not be libpq options, in
-		 * which case we'll just waste a few array slots.)  Add 3 extra slots
+		 * which case we'll just waste a few array slots.)  Add 4 extra slots
-		 * for fallback_application_name, client_encoding, end marker.
+		 * for application_name, fallback_application_name, client_encoding,
+		 * end marker.
 		 */
-		n = list_length(server->options) + list_length(user->options) + 3;
+		n = list_length(server->options) + list_length(user->options) + 4;
 		keywords = (const char **) palloc(n * sizeof(char *));
 		values = (const char **) palloc(n * sizeof(char *));
 
@@ -366,7 +367,23 @@ connect_pg_server(ForeignServer *server, UserMapping *user)
 		n += ExtractConnectionOptions(user->options,
 									  keywords + n, values + n);
 
-		/* Use "postgres_fdw" as fallback_application_name. */
+		/*
+		 * Use pgfdw_application_name as application_name if set.
+		 *
+		 * PQconnectdbParams() processes the parameter arrays from start to
+		 * end. If any key word is repeated, the last value is used. Therefore
+		 * note that pgfdw_application_name must be added to the arrays after
+		 * options of ForeignServer are, so that it can override
+		 * application_name set in ForeignServer.
+		 */
+		if (pgfdw_application_name && *pgfdw_application_name != '\0')
+		{
+			keywords[n] = "application_name";
+			values[n] = pgfdw_application_name;
+			n++;
+		}
+
+		/* Use "postgres_fdw" as fallback_application_name */
 		keywords[n] = "fallback_application_name";
 		values[n] = "postgres_fdw";
 		n++;

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -10761,3 +10761,82 @@ ERROR:  invalid value for integer option "fetch_size": 100$%$#$#
 CREATE FOREIGN TABLE inv_bsz (c1 int )
 	SERVER loopback OPTIONS (batch_size '100$%$#$#');
 ERROR:  invalid value for integer option "batch_size": 100$%$#$#
+-- ===================================================================
+-- test postgres_fdw.application_name GUC
+-- ===================================================================
+-- Turn debug_discard_caches off for this test to make that
+-- the remote connection is alive when checking its application_name.
+-- For each test, close all the existing cached connections manually and
+-- establish connection with new setting of application_name.
+SET debug_discard_caches = 0;
+-- If appname is set as GUC but not as options of server object,
+-- the GUC setting is used as application_name of remote connection.
+SET postgres_fdw.application_name TO 'fdw_guc_appname';
+SELECT 1 FROM postgres_fdw_disconnect_all();
+ ?column? 
+----------
+        1
+(1 row)
+
+SELECT 1 FROM ft6 LIMIT 1;
+ ?column? 
+----------
+        1
+(1 row)
+
+SELECT application_name FROM pg_stat_activity
+       WHERE application_name IN ('loopback2', 'fdw_guc_appname');
+ application_name 
+------------------
+ fdw_guc_appname
+(1 row)
+
+-- If appname is set as options of server object but not as GUC,
+-- appname of server object is used.
+RESET postgres_fdw.application_name;
+ALTER SERVER loopback2 OPTIONS (ADD application_name 'loopback2');
+SELECT 1 FROM postgres_fdw_disconnect_all();
+ ?column? 
+----------
+        1
+(1 row)
+
+SELECT 1 FROM ft6 LIMIT 1;
+ ?column? 
+----------
+        1
+(1 row)
+
+SELECT application_name FROM pg_stat_activity
+       WHERE application_name IN ('loopback2', 'fdw_guc_appname');
+ application_name 
+------------------
+ loopback2
+(1 row)
+
+-- If appname is set both as GUC and as options of server object,
+-- the GUC setting overrides appname of server object and is used.
+SET postgres_fdw.application_name TO 'fdw_guc_appname';
+SELECT 1 FROM postgres_fdw_disconnect_all();
+ ?column? 
+----------
+        1
+(1 row)
+
+SELECT 1 FROM ft6 LIMIT 1;
+ ?column? 
+----------
+        1
+(1 row)
+
+SELECT application_name FROM pg_stat_activity
+       WHERE application_name IN ('loopback2', 'fdw_guc_appname');
+ application_name 
+------------------
+ fdw_guc_appname
+(1 row)
+
+--Clean up
+ALTER SERVER loopback2 OPTIONS (DROP application_name);
+RESET postgres_fdw.application_name;
+RESET debug_discard_caches;

contrib/postgres_fdw/option.c

@@ -1,7 +1,7 @@
 /*-------------------------------------------------------------------------
  *
  * option.c
- *		  FDW option handling for postgres_fdw
+ *		  FDW and GUC option handling for postgres_fdw
  *
  * Portions Copyright (c) 2012-2021, PostgreSQL Global Development Group
  *
@@ -45,6 +45,13 @@ static PgFdwOption *postgres_fdw_options;
  */
 static PQconninfoOption *libpq_options;
 
+/*
+ * GUC parameters
+ */
+char	   *pgfdw_application_name = NULL;
+
+void		_PG_init(void);
+
 /*
  * Helper functions
  */
@@ -435,3 +442,29 @@ ExtractExtensionList(const char *extensionsString, bool warnOnMissing)
 	list_free(extlist);
 	return extensionOids;
 }
+
+/*
+ * Module load callback
+ */
+void
+_PG_init(void)
+{
+	/*
+	 * Unlike application_name GUC, don't set GUC_IS_NAME flag nor check_hook
+	 * to allow postgres_fdw.application_name to be any string more than
+	 * NAMEDATALEN characters and to include non-ASCII characters. Instead,
+	 * remote server truncates application_name of remote connection to less
+	 * than NAMEDATALEN and replaces any non-ASCII characters in it with a '?'
+	 * character.
+	 */
+	DefineCustomStringVariable("postgres_fdw.application_name",
+							   "Sets the application name to be used on the remote server.",
+							   NULL,
+							   &pgfdw_application_name,
+							   NULL,
+							   PGC_USERSET,
+							   0,
+							   NULL,
+							   NULL,
+							   NULL);
+}

contrib/postgres_fdw/postgres_fdw.h

@@ -158,6 +158,7 @@ extern int	ExtractConnectionOptions(List *defelems,
 									 const char **values);
 extern List *ExtractExtensionList(const char *extensionsString,
 								  bool warnOnMissing);
+extern char *pgfdw_application_name;
 
 /* in deparse.c */
 extern void classifyConditions(PlannerInfo *root,

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -3422,3 +3422,42 @@ CREATE FOREIGN TABLE inv_fsz (c1 int )
 -- Invalid batch_size option
 CREATE FOREIGN TABLE inv_bsz (c1 int )
 	SERVER loopback OPTIONS (batch_size '100$%$#$#');
+
+-- ===================================================================
+-- test postgres_fdw.application_name GUC
+-- ===================================================================
+-- Turn debug_discard_caches off for this test to make that
+-- the remote connection is alive when checking its application_name.
+-- For each test, close all the existing cached connections manually and
+-- establish connection with new setting of application_name.
+SET debug_discard_caches = 0;
+
+-- If appname is set as GUC but not as options of server object,
+-- the GUC setting is used as application_name of remote connection.
+SET postgres_fdw.application_name TO 'fdw_guc_appname';
+SELECT 1 FROM postgres_fdw_disconnect_all();
+SELECT 1 FROM ft6 LIMIT 1;
+SELECT application_name FROM pg_stat_activity
+       WHERE application_name IN ('loopback2', 'fdw_guc_appname');
+
+-- If appname is set as options of server object but not as GUC,
+-- appname of server object is used.
+RESET postgres_fdw.application_name;
+ALTER SERVER loopback2 OPTIONS (ADD application_name 'loopback2');
+SELECT 1 FROM postgres_fdw_disconnect_all();
+SELECT 1 FROM ft6 LIMIT 1;
+SELECT application_name FROM pg_stat_activity
+       WHERE application_name IN ('loopback2', 'fdw_guc_appname');
+
+-- If appname is set both as GUC and as options of server object,
+-- the GUC setting overrides appname of server object and is used.
+SET postgres_fdw.application_name TO 'fdw_guc_appname';
+SELECT 1 FROM postgres_fdw_disconnect_all();
+SELECT 1 FROM ft6 LIMIT 1;
+SELECT application_name FROM pg_stat_activity
+       WHERE application_name IN ('loopback2', 'fdw_guc_appname');
+
+--Clean up
+ALTER SERVER loopback2 OPTIONS (DROP application_name);
+RESET postgres_fdw.application_name;
+RESET debug_discard_caches;

doc/src/sgml/postgres-fdw.sgml

@@ -905,6 +905,31 @@ postgres=# SELECT postgres_fdw_disconnect_all();
   </para>
  </sect2>
 
+ <sect2>
+  <title>Configuration Parameters</title>
+
+  <variablelist>
+   <varlistentry>
+    <term>
+     <varname>postgres_fdw.application_name</varname> (<type>string</type>)
+     <indexterm>
+      <primary><varname>postgres_fdw.application_name</varname> configuration parameter</primary>
+     </indexterm>
+    </term>
+    <listitem>
+     <para>
+      Specifies a value for <xref linkend="guc-application-name"/>
+      configuration parameter used when <filename>postgres_fdw</filename>
+      establishes a connection to a foreign server.  This overrides
+      <varname>application_name</varname> option of the server object.
+      Note that change of this parameter doesn't affect any existing
+      connections until they are re-established.
+     </para>
+    </listitem>
+   </varlistentry>
+  </variablelist>
+ </sect2>
+
  <sect2>
   <title>Examples</title>