[libxml2] Add TODO comment in xinclude.c

[Nick Wellnhofer <nwellnhof src gnome org>]

commit d63cfeca3577dfa2e658b33c56889dbab7de8ff5
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Mon Aug 17 15:40:06 2020 +0200

    Add TODO comment in xinclude.c
    
    Add some thoughts on the major remaining problems with the XInclude
    implementation.

 xinclude.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)
---
diff --git a/xinclude.c b/xinclude.c
index 36bdfaed3..3c810ca14 100644
--- a/xinclude.c
+++ b/xinclude.c
@@ -2381,6 +2381,23 @@ xmlXIncludeDoProcess(xmlXIncludeCtxtPtr ctxt, xmlDocPtr doc, xmlNodePtr tree,
     }
     start = ctxt->incNr;
 
+    /*
+     * TODO: The phases must run separately for recursive inclusions.
+     *
+     * - Phase 1 should start with top-level XInclude nodes, load documents,
+     *   execute XPointer expressions, then process only the result nodes
+     *   (not whole document, see bug #324081) and only for phase 1
+     *   recursively. We will need a backreference from xmlNodes to
+     *   xmlIncludeRefs to detect references that were already visited.
+     *   This can also be used for proper cycle detection, see bug #344240.
+     *
+     * - Phase 2 should visit all top-level XInclude nodes and expand
+     *   possible subreferences in the replacement recursively.
+     *
+     * - Phase 3 should finally replace the top-level XInclude nodes.
+     *   It could also be run together with phase 2.
+     */
+
     /*
      * First phase: lookup the elements in the document
      */