postgres_fdw: Allow postgres_fdw.application_name to include escape sequences.

application_name that used when postgres_fdw establishes a connection to
a foreign server can be specified in either or both a connection parameter
of a server object and GUC postgres_fdw.application_name. This commit
allows those parameters to include escape sequences that begins with
% character. Then postgres_fdw replaces those escape sequences with
status information. For example, %d and %u are replaced with user name
and database name in local server, respectively. This feature enables us
to add information more easily to track remote transactions or queries,
into application_name of a remote connection.

Author: Hayato Kuroda
Reviewed-by: Kyotaro Horiguchi, Masahiro Ikeda, Hou Zhijie, Fujii Masao
Discussion: https://postgr.es/m/TYAPR01MB5866FAE71C66547C64616584F5EB9@TYAPR01MB5866.jpnprd01.prod.outlook.com
Discussion: https://postgr.es/m/TYCPR01MB5870D1E8B949DAF6D3B84E02F5F29@TYCPR01MB5870.jpnprd01.prod.outlook.com

1 files changed, 36 insertions, 0 deletions

diff --git a/contrib/postgres_fdw/connection.c b/contrib/postgres_fdw/connection.c
index 6bac4ad23eb..80db19e401c 100644
--- a/contrib/postgres_fdw/connection.c
+++ b/contrib/postgres_fdw/connection.c
@@ -348,6 +348,7 @@ connect_pg_server(ForeignServer *server, UserMapping *user)
 	{
 		const char **keywords;
 		const char **values;
+		char	   *appname = NULL;
 		int			n;
 
 		/*
@@ -383,6 +384,39 @@ connect_pg_server(ForeignServer *server, UserMapping *user)
 			n++;
 		}
 
+		/*
+		 * Search the parameter arrays to find application_name setting, and
+		 * replace escape sequences in it with status information if found.
+		 * The arrays are searched backwards because the last value is used if
+		 * application_name is repeatedly set.
+		 */
+		for (int i = n - 1; i >= 0; i--)
+		{
+			if (strcmp(keywords[i], "application_name") == 0 &&
+				*(values[i]) != '\0')
+			{
+				/*
+				 * Use this application_name setting if it's not empty string
+				 * even after any escape sequences in it are replaced.
+				 */
+				appname = process_pgfdw_appname(values[i]);
+				if (appname[0] != '\0')
+				{
+					values[i] = appname;
+					break;
+				}
+
+				/*
+				 * This empty application_name is not used, so we set
+				 * values[i] to NULL and keep searching the array to find the
+				 * next one.
+				 */
+				values[i] = NULL;
+				pfree(appname);
+				appname = NULL;
+			}
+		}
+
 		/* Use "postgres_fdw" as fallback_application_name */
 		keywords[n] = "fallback_application_name";
 		values[n] = "postgres_fdw";
@@ -452,6 +486,8 @@ connect_pg_server(ForeignServer *server, UserMapping *user)
 		/* Prepare new session for use */
 		configure_remote_session(conn);
 
+		if (appname != NULL)
+			pfree(appname);
 		pfree(keywords);
 		pfree(values);
 	}