Template engine

Since Louis factored out all HTML from qanda and added a configuration file to the templates, it's a small task to add a template engine, replace the PHP views with e.g. Twig templates.

Use-case

• Template providers don't have to worry about core changes in the question view files
• Template providers can apply any CSS framework specific HTML they want, like Foundation

Requirements

Must have

• Template files in template zip packages can override question view files
• Template zip package can upload template files in template editor

Should have

• Possible to edit Twig/Mustache templates in template editor

Could have

?

Comparison between Twig and Mustache

Function	Twig	Mustache
Echo variable	{{var}}	{{var}}
Echo field in variable	{{var.field}}, {{var['field']}}	{{#var}} {{field}} {{/var}}
Loop variable	{% for user in users %} ... {% endfor %}	{{#users}} ... {{/users}}
Branch on boolean	{% if trueOrFalse %} ... {% endif %}	{{#trueOfFalse}} ... {{/trueOrFalse}}
Translate		Bind eT to lambda and then {{#eT}} "String to translate"" {{/eT}}
Sandbox	Yes	Not needed

Security

Longer article about template injection: http://blog.portswigger.net/2015/08/server-side-template-injection.html

The lowest risk approach is to simply use a trivial template engine such as Mustache

About CPU load: It wold be easy for a template designer to make a loop within a loop within a loop etc, to exhaust the CPU power of the server. This vulnerability is in all template engines where you can loop an array (Mustache, Twig). To protect against this, the server should have a max script runtime set.

BUT: Rendering PDF takes a long time, how to know the right limit?