Add a patch to libxml2 to close HTML comments on --!>

See guidance provided on incorrectly-closed comments here:

https://html.spec.whatwg.org/multipage/parsing.html#parse-error-incorrectly-closed-comment

patches/libxml2/0006-htmlParseComment-treat-as-if-it-closed-the-comment.patch

@@ -0,0 +1,55 @@
+From 15b28d8a99224e16ae8a7e7774ee3b6b6769dca3 Mon Sep 17 00:00:00 2001
+From: Mike Dalessio <[email protected]>
+Date: Mon, 3 Aug 2020 17:36:05 -0400
+Subject: [PATCH] htmlParseComment: treat `--!>` as if it closed the comment
+
+See guidance provided on incorrectly-closed comments here:
+
+https://html.spec.whatwg.org/multipage/parsing.html#parse-error-incorrectly-closed-comment
+
+This is a minimal patch to try to avoid merge conflicts.
+---
+ HTMLparser.c | 13 +++++++++++++
+ 1 file changed, 13 insertions(+)
+
+diff --git a/HTMLparser.c b/HTMLparser.c
+index 7b6d689..0046642 100644
+--- a/HTMLparser.c
++++ b/HTMLparser.c
+@@ -3301,6 +3301,7 @@ htmlParseComment(htmlParserCtxtPtr ctxt) {
+     int r, rl;
+     int cur, l;
+     xmlParserInputState state;
++    int possiblyIncorrectlyClosed = 0;
+
+     /*
+      * Check that there is a comment right here.
+@@ -3346,6 +3347,16 @@ htmlParseComment(htmlParserCtxtPtr ctxt) {
+ 	    buf = tmp;
+ 	}
+ 	COPY_BUF(ql,buf,len,q);
++
++	if (possiblyIncorrectlyClosed && cur == '>') {
++	  htmlParseErr(ctxt, XML_ERR_COMMENT_NOT_FINISHED,
++		       "Comment incorrectly closed by '--!>'", NULL, NULL);
++	  buf[len-2] = 0;
++	  goto recover ;
++	}
++
++	possiblyIncorrectlyClosed = (q == '-' && r == '-' && cur == '!') ? 1 : 0 ;
++
+ 	q = r;
+ 	ql = rl;
+ 	r = cur;
+@@ -3359,6 +3370,8 @@ htmlParseComment(htmlParserCtxtPtr ctxt) {
+ 	}
+     }
+     buf[len] = 0;
++
++recover:
+     if (IS_CHAR(cur)) {
+         NEXT;
+ 	if ((ctxt->sax != NULL) && (ctxt->sax->comment != NULL) &&
+-- 
+2.25.1
+

test/html/test_comments.rb

@@ -113,12 +113,29 @@ class TestComment < Nokogiri::TestCase
         let(:subject) { doc.at_css("div#under-test") }
         let(:inner_div) { doc.at_css("div#do-i-exist") }
 
-        it "behaves as if the comment encompasses the inner div" do # NON-COMPLIANT
-          assert_equal 1, subject.children.length
-          assert subject.children.first.comment?
-          assert !inner_div
-          assert_match(/id=do-i-exist/, subject.children.first.content)
-          assert_equal 0, doc.errors.length
+        if Nokogiri.uses_libxml? && Nokogiri::VersionInfo.instance.libxml2_using_packaged?
+          # see patches/libxml2/0006-htmlParseComment-treat-as-if-it-closed-the-comment.patch
+          it "behaves as if the comment is normally closed" do # COMPLIANT
+            assert_equal 3, subject.children.length
+            assert subject.children[0].comment?
+            assert_equal "foo", subject.children[0].content
+            assert inner_div
+            assert_equal inner_div, subject.children[1]
+            assert subject.children[2].comment?
+            assert_equal "bar", subject.children[2].content
+            assert_equal 1, doc.errors.length
+            assert_match(/Comment incorrectly closed/, doc.errors.first.to_s)
+          end
+        end
+
+        if Nokogiri.jruby? || Nokogiri::VersionInfo.instance.libxml2_using_system?
+          it "behaves as if the comment encompasses the inner div" do # NON-COMPLIANT
+            assert_equal 1, subject.children.length
+            assert subject.children.first.comment?
+            assert !inner_div
+            assert_match(/id=do-i-exist/, subject.children.first.content)
+            assert_equal 0, doc.errors.length
+          end
         end
       end