Fix user mention processing

When mentioning a user, the markup post-processor did not handle the
case where the mentioned user did not exist well: it tried to skip to
the next node, which in turn, ended up skipping the rest of the line.

To fix this, lets skip just the mentioned, but non-existing user, and
continue processing the current node from there.

Fixes #3535.

Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>

modules/markup/html.go

@@ -623,10 +623,10 @@ func mentionProcessor(ctx *RenderContext, node *html.Node) {
 		if DefaultProcessorHelper.IsUsernameMentionable != nil && DefaultProcessorHelper.IsUsernameMentionable(ctx.Ctx, mentionedUsername) {
 			replaceContent(node, loc.Start, loc.End, createLink(util.URLJoin(ctx.Links.Prefix(), mentionedUsername), mention, "mention"))
 			node = node.NextSibling.NextSibling
+			start = 0
 		} else {
-			node = node.NextSibling
+			start = loc.End
 		}
-		start = 0
 	}
 }
 

modules/templates/util_render_test.go

@@ -50,6 +50,11 @@ func TestApostrophesInMentions(t *testing.T) {
 	assert.EqualValues(t, template.HTML("<p><a href=\"/mention-user\" rel=\"nofollow\">@mention-user</a>&#39;s comment</p>\n"), rendered)
 }
 
+func TestNonExistantUserMention(t *testing.T) {
+	rendered := RenderMarkdownToHtml(context.Background(), "@ThisUserDoesNotExist @mention-user")
+	assert.EqualValues(t, template.HTML("<p>@ThisUserDoesNotExist <a href=\"/mention-user\" rel=\"nofollow\">@mention-user</a></p>\n"), rendered)
+}
+
 func TestRenderCommitBody(t *testing.T) {
 	type args struct {
 		ctx   context.Context