markup/goldmark: Fix panic on stray attribute nodes

markup/goldmark/internal/extensions/attributes/attributes.go

@@ -117,8 +117,9 @@ func (a *transformer) isFragmentNode(n ast.Node) bool {
 
 func (a *transformer) Transform(node *ast.Document, reader text.Reader, pc parser.Context) {
 	var attributes []ast.Node
+	var solitaryAttributeNodes []ast.Node
 	if a.cfg.Attribute.Block {
-		attributes = make([]ast.Node, 0, 500)
+		attributes = make([]ast.Node, 0, 100)
 	}
 	ast.Walk(node, func(node ast.Node, entering bool) (ast.WalkStatus, error) {
 		if !entering {
@@ -141,8 +142,7 @@ func (a *transformer) Transform(node *ast.Document, reader text.Reader, pc parse
 				attributes = append(attributes, node)
 				return ast.WalkSkipChildren, nil
 			} else {
-				// remove attributes node
-				node.Parent().RemoveChild(node.Parent(), node)
+				solitaryAttributeNodes = append(solitaryAttributeNodes, node)
 			}
 		}
 
@@ -161,6 +161,11 @@ func (a *transformer) Transform(node *ast.Document, reader text.Reader, pc parse
 		// remove attributes node
 		attr.Parent().RemoveChild(attr.Parent(), attr)
 	}
+
+	// Remove any solitary attribute nodes.
+	for _, n := range solitaryAttributeNodes {
+		n.Parent().RemoveChild(n.Parent(), n)
+	}
 }
 
 func (a *transformer) generateAutoID(n ast.Node, reader text.Reader, pc parser.Context) {

markup/goldmark/internal/extensions/attributes/attributes_integration_test.go

@@ -75,3 +75,36 @@ Second line
 		"|Identifiers: [a-a-a-a-a-a-c-c-c-c-c-c-c-c-d base-name base-name-1 example-hyperlink-in-a-header foo-something-bar foobar my-title my-title-1 second-line term title-with-id title-with-id]|",
 	)
 }
+
+func TestSolitaryAttributesCrash(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+[markup.goldmark.parser.attribute]
+block = true
+-- layouts/_default/single.html --
+Content: {{ .Content }}
+-- content/p1.md --
+---
+title: "Title"
+---
+
+1. a
+
+{.x}
+
+1. b
+
+{.x}
+
+
+
+`
+
+	b := hugolib.Test(t, files)
+
+	b.AssertFileContent("public/p1/index.html",
+		` <li>a</li>`,
+	)
+}