Fix handling of escape sequences in postgres_fdw.application_name

postgres_fdw.application_name relies on MyProcPort to define the data
that should be added to escape sequences %u (user name) or %d (database
name).  However this code could be run in processes that lack a
MyProcPort, like an autovacuum process, causing crashes.

The code generating the application name is made more flexible with this
commit, so as it now generates no data for %u and %d if MyProcPort is
missing, and a simple "unknown" if MyProcPort exists, but the expected
fields are not set.

Reported-by: Alexander Lakhin
Author: Kyotaro Horiguchi, Michael Paquier
Reviewed-by: Hayato Kuroda, Masahiko Sawada
Discussion: https://postgr.es/m/17789-8b31c5a4672b74d9@postgresql.org
Backpatch-through: 15

contrib/postgres_fdw/option.c

@@ -485,8 +485,6 @@ process_pgfdw_appname(const char *appname)
 	const char *p;
 	StringInfoData buf;
 
-	Assert(MyProcPort != NULL);
-
 	initStringInfo(&buf);
 
 	for (p = appname; *p != '\0'; p++)
@@ -522,13 +520,29 @@ process_pgfdw_appname(const char *appname)
 				appendStringInfoString(&buf, cluster_name);
 				break;
 			case 'd':
-				appendStringInfoString(&buf, MyProcPort->database_name);
+				if (MyProcPort)
+				{
+					const char *dbname = MyProcPort->database_name;
+
+					if (dbname)
+						appendStringInfoString(&buf, dbname);
+					else
+						appendStringInfoString(&buf, "[unknown]");
+				}
 				break;
 			case 'p':
 				appendStringInfo(&buf, "%d", MyProcPid);
 				break;
 			case 'u':
-				appendStringInfoString(&buf, MyProcPort->user_name);
+				if (MyProcPort)
+				{
+					const char *username = MyProcPort->user_name;
+
+					if (username)
+						appendStringInfoString(&buf, username);
+					else
+						appendStringInfoString(&buf, "[unknown]");
+				}
 				break;
 			default:
 				/* format error - ignore it */