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

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);
 	}

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -10825,3 +10825,35 @@ ERROR:  invalid value for integer option "batch_size": 100$%$#$#
 ALTER FOREIGN DATA WRAPPER postgres_fdw OPTIONS (nonexistent 'fdw');
 ERROR:  invalid option "nonexistent"
 HINT:  There are no valid options in this context.
+-- ===================================================================
+-- test postgres_fdw.application_name GUC
+-- ===================================================================
+-- Close all the existing cached connections so that new connection
+-- will be established with new setting of postgres_fdw.application_name.
+SELECT 1 FROM postgres_fdw_disconnect_all();
+ ?column? 
+----------
+        1
+(1 row)
+
+-- Add some escape sequences into postgres_fdw.application_name
+-- so as to test that they are replaced with status information expectedly.
+SET postgres_fdw.application_name TO '%a%u%d%p%%';
+BEGIN;
+SELECT 1 FROM ft6 LIMIT 1;
+ ?column? 
+----------
+        1
+(1 row)
+
+SELECT count(*) FROM pg_stat_activity
+  WHERE application_name = current_setting('application_name') ||
+    CURRENT_USER || current_database() || pg_backend_pid() || '%';
+ count 
+-------
+     1
+(1 row)
+
+COMMIT;
+--Clean up
+RESET postgres_fdw.application_name;

contrib/postgres_fdw/option.c

@@ -18,6 +18,7 @@
 #include "catalog/pg_user_mapping.h"
 #include "commands/defrem.h"
 #include "commands/extension.h"
+#include "libpq/libpq-be.h"
 #include "postgres_fdw.h"
 #include "utils/builtins.h"
 #include "utils/guc.h"
@@ -445,6 +446,67 @@ ExtractExtensionList(const char *extensionsString, bool warnOnMissing)
 	return extensionOids;
 }
 
+/*
+ * Replace escape sequences beginning with % character in the given
+ * application_name with status information, and return it.
+ *
+ * This function always returns a palloc'd string, so the caller is
+ * responsible for pfreeing it.
+ */
+char *
+process_pgfdw_appname(const char *appname)
+{
+	const char *p;
+	StringInfoData buf;
+
+	Assert(MyProcPort != NULL);
+
+	initStringInfo(&buf);
+
+	for (p = appname; *p != '\0'; p++)
+	{
+		if (*p != '%')
+		{
+			/* literal char, just copy */
+			appendStringInfoChar(&buf, *p);
+			continue;
+		}
+
+		/* must be a '%', so skip to the next char */
+		p++;
+		if (*p == '\0')
+			break;				/* format error - ignore it */
+		else if (*p == '%')
+		{
+			/* string contains %% */
+			appendStringInfoChar(&buf, '%');
+			continue;
+		}
+
+		/* process the option */
+		switch (*p)
+		{
+			case 'a':
+				appendStringInfoString(&buf, application_name);
+				break;
+			case 'd':
+				appendStringInfoString(&buf, MyProcPort->database_name);
+				break;
+			case 'p':
+				appendStringInfo(&buf, "%d", MyProcPid);
+				break;
+			case 'u':
+				appendStringInfoString(&buf, MyProcPort->user_name);
+				break;
+			default:
+				/* format error - ignore it */
+				break;
+		}
+	}
+
+	return buf.data;
+}
+
 /*
  * Module load callback
  */

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 *process_pgfdw_appname(const char *appname);
 extern char *pgfdw_application_name;
 
 /* in deparse.c */

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -3452,3 +3452,24 @@ CREATE FOREIGN TABLE inv_bsz (c1 int )
 
 -- No option is allowed to be specified at foreign data wrapper level
 ALTER FOREIGN DATA WRAPPER postgres_fdw OPTIONS (nonexistent 'fdw');
+
+-- ===================================================================
+-- test postgres_fdw.application_name GUC
+-- ===================================================================
+-- Close all the existing cached connections so that new connection
+-- will be established with new setting of postgres_fdw.application_name.
+SELECT 1 FROM postgres_fdw_disconnect_all();
+
+-- Add some escape sequences into postgres_fdw.application_name
+-- so as to test that they are replaced with status information expectedly.
+SET postgres_fdw.application_name TO '%a%u%d%p%%';
+
+BEGIN;
+SELECT 1 FROM ft6 LIMIT 1;
+SELECT count(*) FROM pg_stat_activity
+  WHERE application_name = current_setting('application_name') ||
+    CURRENT_USER || current_database() || pg_backend_pid() || '%';
+COMMIT;
+
+--Clean up
+RESET postgres_fdw.application_name;