py-libxml2: work around a problem in error handling

Restore part of a patch lost in the last libxml2 update which is still
relevant. Reapplying it fixes segfaults caused by itstool, e.g., when
building editors/pluma, which is PR pkg/56229 from Andrius V.

Quoting from wiz@'s original commit from Jan 9, 2019, which covers
everything else:

"In some cases, invalid UTF-8 strings were returned which caused
Python interpreter crashes. See

itstool/itstool#22

Use a variant of the patch that was used in Fedora.

Bump PKGREVISION."

Fedora is still carrying this patch as-is.

(Also, evidently distinfo was not regenerated properly after the last
update, so there's a diff applied to it unrelated to this change set.)


(gutteridge)

$NetBSD: patch-python_libxml.c,v 1.4 2021/06/12 17:02:51 gutteridge Exp $

Avoid returning invalid UTF-8 strings to Python.
Based on https://bugzilla.opensuse.org/attachment.cgi?id=746044&action=edit
Fixes https://github.com/itstool/itstool/issues/22

--- python/libxml.c.orig	2016-06-07 10:04:14.000000000 +0000
+++ python/libxml.c
@@ -1620,6 +1620,7 @@ libxml_xmlErrorFuncHandler(ATTRIBUTE_UNU
     PyObject *message;
     PyObject *result;
     char str[1000];
+    unsigned char *ptr = (unsigned char *)str;
 
 #ifdef DEBUG_ERROR
     printf("libxml_xmlErrorFuncHandler(%p, %s, ...) called\n", ctx, msg);
@@ -1636,10 +1637,14 @@ libxml_xmlErrorFuncHandler(ATTRIBUTE_UNU
 	    str[999] = 0;
         va_end(ap);
 
+        /* Ensure the error string doesn't start at UTF8 continuation. */
+        while (*ptr && (*ptr & 0xc0) == 0x80)
+	    ptr++;
+
         list = PyTuple_New(2);
         PyTuple_SetItem(list, 0, libxml_xmlPythonErrorFuncCtxt);
         Py_XINCREF(libxml_xmlPythonErrorFuncCtxt);
-        message = libxml_charPtrConstWrap(str);
+        message = libxml_charPtrConstWrap(ptr);
         PyTuple_SetItem(list, 1, message);
         result = PyEval_CallObject(libxml_xmlPythonErrorFuncHandler, list);
         Py_XDECREF(list);