registry: Start using libxml2 contextual API

Contextual functions are safer because they do not rely on a global
state.

meson.build

@@ -383,6 +383,13 @@ endif
 # libxkbregistry
 if get_option('enable-xkbregistry')
     dep_libxml = dependency('libxml-2.0')
+    if cc.has_header_symbol(
+        'libxml/xmlerror.h',
+        'xmlCtxtSetErrorHandler',
+        prefix: system_ext_define
+    )
+        configh_data.set10('HAVE_XML_CTXT_SET_ERRORHANDLER', true)
+    endif
     deps_libxkbregistry = [dep_libxml]
     libxkbregistry_sources = [
         'src/registry.c',

src/registry.c

@@ -1167,6 +1167,14 @@ xml_error_func(void *ctx, const char *msg, ...)
     }
 }
 
+#ifdef HAVE_XML_CTXT_SET_ERRORHANDLER
+static void
+xml_structured_error_func(void *userData, const xmlError * error)
+{
+    xmlFormatError(error, xml_error_func, userData);
+}
+#endif
+
 static bool
 validate(struct rxkb_context *ctx, xmlDoc *doc)
 {
@@ -1211,7 +1219,7 @@ validate(struct rxkb_context *ctx, xmlDoc *doc)
     /* Note: do not use xmlParserInputBufferCreateStatic, it generates random
      * DTD validity errors for unknown reasons */
     buf = xmlParserInputBufferCreateMem(dtdstr, sizeof(dtdstr),
-                                        XML_CHAR_ENCODING_UTF8);
+                                        XML_CHAR_ENCODING_NONE);
     if (!buf)
         return false;
 
@@ -1246,9 +1254,19 @@ parse(struct rxkb_context *ctx, const char *path,
 
     LIBXML_TEST_VERSION
 
+    xmlParserCtxtPtr xmlCtxt = xmlNewParserCtxt();
+    if (!xmlCtxt)
+        return false;
+
+#ifdef HAVE_XML_CTXT_SET_ERRORHANDLER
+    /* Prefer contextual handler whenever possible. It takes precedence over
+     * the global generic handler. */
+    xmlCtxtSetErrorHandler(xmlCtxt, xml_structured_error_func, ctx);
+#endif
+    /* This is still unconditionnally needed for the DTD validation (for now) */
     xmlSetGenericErrorFunc(ctx, xml_error_func);
 
-    doc = xmlParseFile(path);
+    doc = xmlCtxtReadFile(xmlCtxt, path, NULL, 0);
     if (!doc)
         goto parse_error;
 
@@ -1274,8 +1292,11 @@ parse(struct rxkb_context *ctx, const char *path,
      *     rxkb_context_parse();
      *     rxkb_context_unref();
      */
-    /* TODO: use the new API xmlCtxtSetErrorHandler */
     xmlSetGenericErrorFunc(NULL, NULL);
+#ifdef HAVE_XML_CTXT_SET_ERRORHANDLER
+    xmlCtxtSetErrorHandler(xmlCtxt, NULL, NULL);
+#endif
+    xmlFreeParserCtxt(xmlCtxt);
 
     return success;
 }