# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1655494230 -7200
# Node ID da7cfb64d1897cf66566e0a2a24fd254fd3766e7
# Parent  65230d933d16b59b210834dd91823819531ddd1d
Introduced support for omitting optional flexpage results from messages. This is
useful for the system-level page fault handler which may not explicitly return a
flexpage to resolve a page fault because it may have arranged a mapping through
interactions with other components.

diff -r 65230d933d16 -r da7cfb64d189 libipc/lib/src/message.c
--- a/libipc/lib/src/message.c	Mon Jun 13 17:32:24 2022 +0200
+++ b/libipc/lib/src/message.c	Fri Jun 17 21:30:30 2022 +0200
@@ -254,7 +254,11 @@
 
 void ipc_message_add_fpage(ipc_message_t *msg, l4_snd_fpage_t fpage)
 {
-  ipc_message_export_fpage(msg, msg->items++, fpage);
+  /* Test for {0, l4_fpage_invalid()} as the flexpage to avoid setting an
+     optional flexpage result, useful for the page fault handler. */
+
+  if (fpage.snd_base || fpage.fpage.raw)
+    ipc_message_export_fpage(msg, msg->items++, fpage);
 }
 
 /* Add a receive window flexpage item to the message. */