repmgr standby switchover: add sanity check for pg_rewind useability

pg_rewind will only be executed on a demoted primary if explictly
requested, to prevent transactions on the primary, which
were never replicated, from being automatically overwritten.

If --force-rewind is provided, we'll need to check pg_rewind
is actually useable before we need to use it.

dbutils.c

@@ -11,9 +11,9 @@
 
 #include "repmgr.h"
 #include "dbutils.h"
+#include "controldata.h"
 
-#include "catalog/pg_control.h"
-
+#include "dirutil.h"
 
 /* mainly for use by repmgrd */
 int			server_version_num = UNKNOWN_SERVER_VERSION_NUM;
@@ -1239,6 +1239,60 @@ get_replication_info(PGconn *conn, ReplInfo *replication_info)
 }
 
 
+bool
+can_use_pg_rewind(PGconn *conn, const char *data_directory, PQExpBufferData *reason)
+{
+	bool can_use = true;
+
+	if (guc_set(conn, "full_page_writes", "=", "off"))
+	{
+		if (can_use == false)
+			appendPQExpBuffer(reason, "; ");
+
+		appendPQExpBuffer(
+			reason,
+			_("\"full_page_writes\" must be set to \"on\""));
+
+		can_use = false;
+	}
+
+	/*
+	 * "wal_log_hints" off - are data checksums available?
+	 * Note: we're checking the local pg_control file here as the value will
+	 * be the same throughout the cluster and saves a round-trip to the
+	 * demotion candidate.
+	 */
+	if (guc_set(conn, "wal_log_hints", "=", "on") == false)
+	{
+		int data_checksum_version = get_data_checksum_version(data_directory);
+
+		if (data_checksum_version < 0)
+		{
+			if (can_use == false)
+				appendPQExpBuffer(reason, "; ");
+
+			appendPQExpBuffer(
+				reason,
+				_("\"wal_log_hints\" is set to \"off\" but unable to determine checksum version"));
+			can_use = false;
+		}
+		else if (data_checksum_version == 0)
+		{
+			if (can_use == false)
+				appendPQExpBuffer(reason, "; ");
+
+			appendPQExpBuffer(
+				reason,
+				_("\"wal_log_hints\" is set to \"off\" and checksums are disabled"));
+
+			can_use = false;
+		}
+	}
+
+	return can_use;
+}
+
+
 /* ================ */
 /* result functions */
 /* ================ */