[libxml2] Fix corner case with empty xi:fallback

[Nick Wellnhofer <nwellnhof src gnome org>]

commit d88df4bd48ba4ce9a68040a2427b4a665d5ff891
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Sun Aug 16 23:38:48 2020 +0200

    Fix corner case with empty xi:fallback
    
    xi:fallback could become empty after recursive expansion. Use a flag
    to track whether nodes should be skipped.

 result/XInclude/fallback6.xml     |  1 +
 result/XInclude/fallback6.xml.rdr |  0
 test/XInclude/docs/fallback6.xml  |  6 ++++++
 xinclude.c                        | 11 ++++-------
 4 files changed, 11 insertions(+), 7 deletions(-)
---
diff --git a/result/XInclude/fallback6.xml b/result/XInclude/fallback6.xml
new file mode 100644
index 000000000..2b5d41167
--- /dev/null
+++ b/result/XInclude/fallback6.xml
@@ -0,0 +1 @@
+<?xml version="1.0"?>
diff --git a/result/XInclude/fallback6.xml.rdr b/result/XInclude/fallback6.xml.rdr
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/XInclude/docs/fallback6.xml b/test/XInclude/docs/fallback6.xml
new file mode 100644
index 000000000..fd00a03f2
--- /dev/null
+++ b/test/XInclude/docs/fallback6.xml
@@ -0,0 +1,6 @@
+<?xml version="1.0"?>
+<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"; href="b.xml">
+    <xi:fallback><xi:include href="c.xml">
+        <xi:fallback/>
+    </xi:include></xi:fallback>
+</xi:include>
diff --git a/xinclude.c b/xinclude.c
index ff265eb94..e9d3af5ef 100644
--- a/xinclude.c
+++ b/xinclude.c
@@ -60,7 +60,7 @@ struct _xmlXIncludeRef {
     int                   xml; /* xml or txt */
     int                 count; /* how many refs use that specific doc */
     xmlXPathObjectPtr    xptr; /* the xpointer if needed */
-    int                      emptyFb; /* flag to show fallback empty */
+    int                         skip; /* skip in case of errors */
 };
 
 struct _xmlXIncludeCtxt {
@@ -2007,7 +2007,6 @@ xmlXIncludeLoadFallback(xmlXIncludeCtxtPtr ctxt, xmlNodePtr fallback, int nr) {
                                                   fallback->children);
     } else {
         ctxt->incTab[nr]->inc = NULL;
-       ctxt->incTab[nr]->emptyFb = 1;  /* flag empty callback */
     }
     return(ret);
 }
@@ -2164,13 +2163,13 @@ xmlXIncludeLoadNode(xmlXIncludeCtxtPtr ctxt, int nr) {
                ((xmlStrEqual(children->ns->href, XINCLUDE_NS)) ||
                 (xmlStrEqual(children->ns->href, XINCLUDE_OLD_NS)))) {
                ret = xmlXIncludeLoadFallback(ctxt, children, nr);
-               if (ret == 0)
-                   break;
+               break;
            }
            children = children->next;
        }
     }
     if (ret < 0) {
+        ctxt->incTab[nr]->skip = 1;
        xmlXIncludeErr(ctxt, ctxt->incTab[nr]->ref,
                       XML_XINCLUDE_NO_FALLBACK,
                       "could not load %s, and no fallback was found\n",
@@ -2468,9 +2467,7 @@ xmlXIncludeDoProcess(xmlXIncludeCtxtPtr ctxt, xmlDocPtr doc, xmlNodePtr tree,
      *
      */
     for (i = ctxt->incBase;i < ctxt->incNr; i++) {
-       if ((ctxt->incTab[i]->inc != NULL) ||
-               (ctxt->incTab[i]->xptr != NULL) ||
-               (ctxt->incTab[i]->emptyFb != 0))        /* (empty fallback) */
+       if (ctxt->incTab[i]->skip == 0)
            xmlXIncludeIncludeNode(ctxt, i);
     }