pg_upgrade: issue helpful error message for use on standbys

Commit 777e6ddf checked for a shut down
message from a standby and allowed it to continue.  This patch reports a
helpful error message in these cases, suggesting to use rsync as
documented.

Diagnosed-by: Martín Marqués

Discussion: https://postgr.es/m/CAPdiE1xYCow-reLjrhJ9DqrMu-ppNq0ChUUEvVdxhdjGRD5_eA@mail.gmail.com

Backpatch-through: 9.3

src/bin/pg_upgrade/controldata.c

@@ -150,8 +150,14 @@ get_control_data(ClusterInfo *cluster, bool live_check)
 				/* remove leading spaces */
 				while (*p == ' ')
 					p++;
-				if (strcmp(p, "shut down\n") != 0 &&
-					strcmp(p, "shut down in recovery\n") != 0)
+				if (strcmp(p, "shut down in recovery\n") == 0)
+				{
+					if (cluster == &old_cluster)
+						pg_fatal("The source cluster was shut down while in recovery mode.  To upgrade, use \"rsync\" as documented or shut it down as a primary.\n");
+					else
+						pg_fatal("The target cluster was shut down while in recovery mode.  To upgrade, use \"rsync\" as documented or shut it down as a primary.\n");
+				}
+				else if (strcmp(p, "shut down\n") != 0)
 				{
 					if (cluster == &old_cluster)
 						pg_fatal("The source cluster was not shut down cleanly.\n");