Improve pg_upgrade's checks for required executables.

Don't insist on pg_dumpall and psql being present in the old cluster,
since they are not needed.  Do insist on pg_resetxlog being present
(in both old and new), since we need it.  Also check for pg_config,
but only in the new cluster.  Remove the useless attempt to call
pg_config in the old cluster; we don't need to know the old value of
--pkglibdir.  (In the case of a stripped-down migration installation
there might be nothing there to look at anyway, so any future change
that might reintroduce that need would have to be considered carefully.)

Per my attempts to build a minimal previous-version installation to support
pg_upgrade.

contrib/pg_upgrade/exec.c

@@ -14,7 +14,7 @@
 
 
 static void check_data_dir(const char *pg_data);
-static void check_bin_dir(ClusterInfo *cluster);
+static void check_bin_dir(ClusterInfo *cluster, Cluster whichCluster);
 static int	check_exec(const char *dir, const char *cmdName);
 static const char *validate_exec(const char *path);
 
@@ -99,7 +99,7 @@ verify_directories(void)
 	check_ok();
 
 	prep_status("Checking old bin directory (%s)", old_cluster.bindir);
-	check_bin_dir(&old_cluster);
+	check_bin_dir(&old_cluster, CLUSTER_OLD);
 	check_ok();
 
 	prep_status("Checking new data directory (%s)", new_cluster.pgdata);
@@ -107,7 +107,7 @@ verify_directories(void)
 	check_ok();
 
 	prep_status("Checking new bin directory (%s)", new_cluster.bindir);
-	check_bin_dir(&new_cluster);
+	check_bin_dir(&new_cluster, CLUSTER_NEW);
 	check_ok();
 }
 
@@ -158,12 +158,18 @@ check_data_dir(const char *pg_data)
  *	exit().
  */
 static void
-check_bin_dir(ClusterInfo *cluster)
+check_bin_dir(ClusterInfo *cluster, Cluster whichCluster)
 {
 	check_exec(cluster->bindir, "postgres");
-	check_exec(cluster->bindir, "psql");
 	check_exec(cluster->bindir, "pg_ctl");
-	check_exec(cluster->bindir, "pg_dumpall");
+	check_exec(cluster->bindir, "pg_resetxlog");
+	if (whichCluster == CLUSTER_NEW)
+	{
+		/* these are only needed in the new cluster */
+		check_exec(cluster->bindir, "pg_config");
+		check_exec(cluster->bindir, "psql");
+		check_exec(cluster->bindir, "pg_dumpall");
+	}
 }
 
 

contrib/pg_upgrade/option.c

@@ -310,7 +310,11 @@ validateDirectoryOption(char **dirpath,
 static void
 get_pkglibdirs(void)
 {
-	old_cluster.libpath = get_pkglibdir(old_cluster.bindir);
+	/*
+	 * we do not need to know the libpath in the old cluster, and might not
+	 * have a working pg_config to ask for it anyway.
+	 */
+	old_cluster.libpath = NULL;
 	new_cluster.libpath = get_pkglibdir(new_cluster.bindir);
 }