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

1 files changed, 34 insertions, 1 deletions

diff --git a/contrib/postgres_fdw/option.c b/contrib/postgres_fdw/option.c
index c574ca2cf39..5bb1af4084a 100644
--- a/contrib/postgres_fdw/option.c
+++ b/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
  *
@@ -46,6 +46,13 @@ static PgFdwOption *postgres_fdw_options;
 static PQconninfoOption *libpq_options;
 
 /*
+ * GUC parameters
+ */
+char	   *pgfdw_application_name = NULL;
+
+void		_PG_init(void);
+
+/*
  * Helper functions
  */
 static void InitPgFdwOptions(void);
@@ -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);
+}