[libxml2] Don't load external entity from xmlSAX2GetEntity

[Nick Wellnhofer <nwellnhof src gnome org>]

commit eddfbc38fa7e84ccd480eab3738e40d1b2c83979
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Wed Jan 22 22:03:45 2020 +0100

    Don't load external entity from xmlSAX2GetEntity
    
    Despite the comment, I can't see a reason why external entities must be
    loaded in the SAX handler. For external entities, the handler is
    typically first invoked via xmlParseReference which will later load the
    entity on its own if it wasn't loaded yet.
    
    The old code also lead to duplicated SAX events which makes it
    basically impossible to reuse xmlSAX2GetEntity for a custom SAX parser.
    See the change to the expected test output.
    
    Note that xmlSAX2GetEntity was loading the entity via
    xmlParseCtxtExternalEntity while xmlParseReference uses
    xmlParseExternalEntityPrivate. In the previous commit, the two
    functions were merged, trying to compensate for some slight differences
    between the two mostly identical implementations.
    
    But the more urgent reason for this change is that xmlParseReference
    has the facility to abort early when recursive entities are detected,
    avoiding what could practically amount to an infinite loop.
    
    If you want to backport this change, note that the previous three
    commits are required as well:
    
    f9ea1a24 Fix copying of entities in xmlParseReference
    5c7e0a9a Copy some XMLReader option flags to parser context
    1a3e584a Merge code paths loading external entities
    
    Found by OSS-Fuzz.

 SAX2.c                 | 30 ------------------------------
 result/noent/ent2.sax2 |  7 -------
 2 files changed, 37 deletions(-)
---
diff --git a/SAX2.c b/SAX2.c
index 5f141f9a..6045ca13 100644
--- a/SAX2.c
+++ b/SAX2.c
@@ -590,36 +590,6 @@ xmlSAX2GetEntity(void *ctx, const xmlChar *name)
     } else {
        ret = xmlGetDocEntity(ctxt->myDoc, name);
     }
-    if ((ret != NULL) &&
-       ((ctxt->validate) || (ctxt->replaceEntities)) &&
-       (ret->children == NULL) &&
-       (ret->etype == XML_EXTERNAL_GENERAL_PARSED_ENTITY)) {
-       int val;
-
-       /*
-        * for validation purposes we really need to fetch and
-        * parse the external entity
-        */
-       xmlNodePtr children;
-       unsigned long oldnbent = ctxt->nbentities;
-
-        val = xmlParseCtxtExternalEntity(ctxt, ret->URI,
-                                        ret->ExternalID, &children);
-       if (val == 0) {
-           xmlAddChildList((xmlNodePtr) ret, children);
-       } else {
-           xmlFatalErrMsg(ctxt, XML_ERR_ENTITY_PROCESSING,
-                          "Failure to process entity %s\n", name, NULL);
-           ctxt->validate = 0;
-           return(NULL);
-       }
-       ret->owner = 1;
-       if (ret->checked == 0) {
-           ret->checked = (ctxt->nbentities - oldnbent + 1) * 2;
-           if ((ret->content != NULL) && (xmlStrchr(ret->content, '<')))
-               ret->checked |= 1;
-       }
-    }
     return(ret);
 }
 
diff --git a/result/noent/ent2.sax2 b/result/noent/ent2.sax2
index 88c6aa6b..d17f2ffe 100644
--- a/result/noent/ent2.sax2
+++ b/result/noent/ent2.sax2
@@ -17,13 +17,6 @@ SAX.characters(my title, 8)
 SAX.endElementNs(title, NULL, NULL)
 SAX.characters(
 , 1)
-SAX.ignorableWhitespace(
-, 1)
-SAX.startElementNs(title, NULL, NULL, 0, 0, 0)
-SAX.characters(my title, 8)
-SAX.endElementNs(title, NULL, NULL)
-SAX.characters(
-, 1)
 SAX.characters(
   This text is about XML, the, 31)
 SAX.getEntity(xml)