[libxml2] Patch to forbid epsilon-reduction of final states

[Nick Wellnhofer <nwellnhof src gnome org>]

commit ec6e3efb06d7b15cf5a2328fabd3845acea4c815
Author: Arne Becker <arne_becker genua de>
Date:   Tue Jul 6 21:56:04 2021 +0200

    Patch to forbid epsilon-reduction of final states
    
    When building the internal representation of a regexp, it is possible
    that a lot of empty transitions are created. Therefore there is a step
    to reduce them in the function xmlFAEliminateSimpleEpsilonTransitions.
    
    There is an error there for this case:
    
    * State 1 has a transition with an atom (in this case "a") to state 2.
    * State 2 is final and has an epsilon transition to state 1.
    
    After reduction it looked like:
    * State 1 has a transition with an atom (in this case "a") to itself
      and is final.
    
    In other words, the empty string is accepted when it shouldn't be.
    
    The attached patch skips the reduction step for final states.
    An alternative would be to insert or increment counters when reducing a
    final state, but this seemed error prone and unnecessary, since there
    aren't that many final states.
    
    Fixes #282

 xmlregexp.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)
---
diff --git a/xmlregexp.c b/xmlregexp.c
index 40dabb20..8d01c2ba 100644
--- a/xmlregexp.c
+++ b/xmlregexp.c
@@ -1892,6 +1892,12 @@ xmlFAReduceEpsilonTransitions(xmlRegParserCtxtPtr ctxt, int fromnr,
  * then X and Y are semantically equivalent and X can be eliminated
  * If X is the start state then make Y the start state, else replace the
  * target of all transitions to X by transitions to Y.
+ *
+ * If X is a final state, skip it.
+ * Otherwise it would be necessary to manipulate counters for this case when
+ * eliminating state 2:
+ * State 1 has a transition with an atom to state 2.
+ * State 2 is final and has an epsilon transition to state 1.
  */
 static void
 xmlFAEliminateSimpleEpsilonTransitions(xmlRegParserCtxtPtr ctxt) {
@@ -1904,7 +1910,8 @@ xmlFAEliminateSimpleEpsilonTransitions(xmlRegParserCtxtPtr ctxt) {
            continue;
        if (state->nbTrans != 1)
            continue;
-       if (state->type == XML_REGEXP_UNREACH_STATE)
+       if (state->type == XML_REGEXP_UNREACH_STATE ||
+           state->type == XML_REGEXP_FINAL_STATE)
            continue;
        /* is the only transition out a basic transition */
        if ((state->trans[0].atom == NULL) &&