fix(test): clobber libxml2's global error handler before every test

to check that we're setting error handlers everywhere we need to.

Related to #2168

ext/nokogiri/nokogiri.c

@@ -1,5 +1,7 @@
 #include <nokogiri.h>
 
+void init_test_global_handlers(); /* in lieu of test_global_handlers.h */
+
 VALUE mNokogiri ;
 VALUE mNokogiriXml ;
 VALUE mNokogiriHtml ;
@@ -132,4 +134,5 @@ void Init_nokogiri()
   init_xml_relax_ng();
   init_nokogiri_io();
   init_xml_encoding_handler();
+  init_test_global_handlers();
 }

ext/nokogiri/test_global_handlers.c

@@ -0,0 +1,41 @@
+#include <nokogiri.h>
+#include "libxml/xmlerror.h"
+
+static VALUE foreign_error_handler_block = Qnil;
+
+static void foreign_error_handler(void* user_data, xmlErrorPtr c_error)
+{
+  rb_funcall(foreign_error_handler_block, rb_intern("call"), 0);
+}
+
+/*
+ * call-seq:
+ *   __foreign_error_handler { ... } -> nil
+ *
+ * Override libxml2's global error handlers to call the block. This method thus has very little
+ * value except to test that Nokogiri is properly setting error handlers elsewhere in the code. See
+ * test/helper.rb for how this is being used.
+ */
+static VALUE
+rb_foreign_error_handler(VALUE klass)
+{
+  rb_need_block();
+  foreign_error_handler_block = rb_block_proc();
+  xmlSetStructuredErrorFunc(NULL, foreign_error_handler);
+  return Qnil;
+}
+
+/*
+ *  Document-module: Nokogiri::Test
+ *
+ *  The Nokogiri::Test module should only be used for testing Nokogiri.
+ *  Do NOT use this outside of the Nokogiri test suite.
+ */
+void
+init_test_global_handlers()
+{
+  VALUE mNokogiri = rb_define_module("Nokogiri");
+  VALUE mNokogiriTest = rb_define_module_under(mNokogiri, "Test");
+
+  rb_define_singleton_method(mNokogiriTest, "__foreign_error_handler", rb_foreign_error_handler, 0);
+}

test/helper.rb

@@ -58,7 +58,16 @@ class TestCase < MiniTest::Spec
     XSLT_FILE           = File.join(ASSETS_DIR, 'staff.xslt')
     XPATH_FILE          = File.join(ASSETS_DIR, 'slow-xpath.xml')
 
+    def setup
+      @fake_error_handler_called = false
+      Nokogiri::Test.__foreign_error_handler do
+        @fake_error_handler_called = true
+      end if Nokogiri.uses_libxml?
+    end
+
     def teardown
+      refute(@fake_error_handler_called, "the fake error handler should never get called") if Nokogiri.uses_libxml?
+
       if ENV['NOKOGIRI_GC']
         STDOUT.putc '!'
         if RUBY_PLATFORM =~ /java/
@@ -135,6 +144,11 @@ class Doc < XML::SAX::Document
         attr_reader :xmldecls
         attr_reader :processing_instructions
 
+        def initialize
+          @errors = []
+          super
+        end
+
         def xmldecl version, encoding, standalone
           @xmldecls = [version, encoding, standalone].compact
           super