diff -urN old/postgresql-10.1/src/bin/pg_upgrade/test.sh new/postgresql-10.1/src/bin/pg_upgrade/test.sh
--- old/postgresql-10.1/src/bin/pg_upgrade/test.sh	2017-11-07 00:46:52.000000000 +0000
+++ new/postgresql-10.1/src/bin/pg_upgrade/test.sh	2018-01-12 18:50:04.038190911 +0000
@@ -20,7 +20,7 @@
 # Run a given "initdb" binary and overlay the regression testing
 # authentication configuration.
 standard_initdb() {
-	"$1" -N
+	"$1" -U postgres -N
 	if [ -n "$TEMP_CONFIG" -a -r "$TEMP_CONFIG" ]
 	then
 		cat "$TEMP_CONFIG" >> "$PGDATA/postgresql.conf"
@@ -153,6 +153,9 @@
 # enable echo so the user can see what is being executed
 set -x
 
+PGUSER=postgres
+export PGUSER
+
 standard_initdb "$oldbindir"/initdb
 "$oldbindir"/pg_ctl start -l "$logdir/postmaster1.log" -o "$POSTMASTER_OPTS" -w
 
@@ -252,6 +255,8 @@
 	*)	    sh ./delete_old_cluster.sh ;;
 esac
 
+unset PGUSER
+
 if diff "$temp_root"/dump1.sql "$temp_root"/dump2.sql >/dev/null; then
 	echo PASSED
 	exit 0
diff -urN old/postgresql-10.1/src/test/regress/pg_regress.c new/postgresql-10.1/src/test/regress/pg_regress.c
--- old/postgresql-10.1/src/test/regress/pg_regress.c	2017-11-07 00:46:52.000000000 +0000
+++ new/postgresql-10.1/src/test/regress/pg_regress.c	2018-01-12 18:03:56.732603484 +0000
@@ -829,6 +829,9 @@
 			sprintf(s, "%d", port);
 			doputenv("PGPORT", s);
 		}
+
+		/* use user "postgres" for temp instance */
+		doputenv("PGUSER", "postgres");
 	}
 	else
 	{
@@ -2246,7 +2249,7 @@
 		/* initdb */
 		header(_("initializing database system"));
 		snprintf(buf, sizeof(buf),
-				 "\"%s%sinitdb\" -D \"%s/data\" --no-clean --no-sync%s%s > \"%s/log/initdb.log\" 2>&1",
+				 "\"%s%sinitdb\" -D \"%s/data\" -U postgres --no-clean --no-sync%s%s > \"%s/log/initdb.log\" 2>&1",
 				 bindir ? bindir : "",
 				 bindir ? "/" : "",
 				 temp_instance,
@@ -2317,7 +2320,7 @@
 		 * Check if there is a postmaster running already.
 		 */
 		snprintf(buf2, sizeof(buf2),
-				 "\"%s%spsql\" -X postgres <%s 2>%s",
+				 "\"%s%spsql\" -X -U postgres <%s 2>%s",
 				 bindir ? bindir : "",
 				 bindir ? "/" : "",
 				 DEVNULL, DEVNULL);