fix: restore proper error handling in the SAX push parser

originally introduced in 771164d but broken in the recent commits.

This is an incomplete fix. We should adopt this same strategy of
save-and-restore everywhere we set the error handlers.

Author: flavorjones

ext/nokogiri/html_sax_push_parser.c

@@ -9,9 +9,10 @@
 static VALUE native_write(VALUE self, VALUE _chunk, VALUE _last_chunk)
 {
   xmlParserCtxtPtr ctx;
-  const char * chunk  = NULL;
-  int size            = 0;
-
+  const char * chunk = NULL;
+  int size = 0;
+  int status = 0;
+  libxmlStructuredErrorHandlerState handler_state;
 
   Data_Get_Struct(self, xmlParserCtxt, ctx);
 
@@ -20,13 +21,16 @@ static VALUE native_write(VALUE self, VALUE _chunk, VALUE _last_chunk)
     size = (int)RSTRING_LEN(_chunk);
   }
 
-  xmlSetStructuredErrorFunc(NULL, NULL);
+  Nokogiri_structured_error_func_save_and_set(&handler_state, NULL, NULL);
+
+  status = htmlParseChunk(ctx, chunk, size, Qtrue == _last_chunk ? 1 : 0);
+
+  Nokogiri_structured_error_func_restore(&handler_state);
 
-  if(htmlParseChunk(ctx, chunk, size, Qtrue == _last_chunk ? 1 : 0)) {
-    if (!(ctx->options & XML_PARSE_RECOVER)) {
-      xmlErrorPtr e = xmlCtxtGetLastError(ctx);
-      Nokogiri_error_raise(NULL, e);
-    }
+  if ((status != 0) && !(ctx->options & XML_PARSE_RECOVER)) {
+    // TODO: there appear to be no tests for this block
+    xmlErrorPtr e = xmlCtxtGetLastError(ctx);
+    Nokogiri_error_raise(NULL, e);
   }
 
   return self;

ext/nokogiri/xml_syntax_error.c

@@ -1,5 +1,28 @@
 #include <xml_syntax_error.h>
 
+void
+Nokogiri_structured_error_func_save(libxmlStructuredErrorHandlerState *handler_state)
+{
+  /* this method is tightly coupled to the implementation of xmlSetStructuredErrorFunc */
+  handler_state->user_data = xmlStructuredErrorContext;
+  handler_state->handler = xmlStructuredError;
+}
+
+void
+Nokogiri_structured_error_func_save_and_set(libxmlStructuredErrorHandlerState *handler_state,
+                                            void *user_data,
+                                            xmlStructuredErrorFunc handler)
+{
+  Nokogiri_structured_error_func_save(handler_state);
+  xmlSetStructuredErrorFunc(user_data, handler);
+}
+
+void
+Nokogiri_structured_error_func_restore(libxmlStructuredErrorHandlerState *handler_state)
+{
+  xmlSetStructuredErrorFunc(handler_state->user_data, handler_state->handler);
+}
+
 void Nokogiri_error_array_pusher(void * ctx, xmlErrorPtr error)
 {
   VALUE list = (VALUE)ctx;

ext/nokogiri/xml_syntax_error.h

@@ -3,11 +3,23 @@
 
 #include <nokogiri.h>
 
+typedef struct _libxmlStructuredErrorHandlerState {
+  void *user_data;
+  xmlStructuredErrorFunc handler;
+} libxmlStructuredErrorHandlerState ;
+
 void init_xml_syntax_error();
+
+void Nokogiri_structured_error_func_save(libxmlStructuredErrorHandlerState *handler_state);
+void Nokogiri_structured_error_func_save_and_set(libxmlStructuredErrorHandlerState *handler_state,
+                                                 void *user_data,
+                                                 xmlStructuredErrorFunc handler);
+void Nokogiri_structured_error_func_restore(libxmlStructuredErrorHandlerState *handler_state);
+
 VALUE Nokogiri_wrap_xml_syntax_error(xmlErrorPtr error);
-void Nokogiri_error_array_pusher(void * ctx, xmlErrorPtr error);
-NORETURN(void Nokogiri_error_raise(void * ctx, xmlErrorPtr error));
+void Nokogiri_error_array_pusher(void *ctx, xmlErrorPtr error);
+NORETURN(void Nokogiri_error_raise(void *ctx, xmlErrorPtr error));
 
 extern VALUE cNokogiriXmlSyntaxError;
-#endif
 
+#endif /* NOKOGIRI_XML_SYNTAX_ERROR */