Make Template.generate() reentrant

[cboos]

As I understand it, it is currently not possible to reuse a template from different threads. For example, Trac would require this if we were to store a TemplateLoader in the environment.

This is because there are several directives which currently maintain an internal state, during __call__():

• the choose/when/otherwise logic is done by checking and setting the self.matched and self.value attributes of the ChooseDirective.
  However, that ChooseDirective is itself stored in the context (choose = ctxt['_choose']). Instead, the matched and value should be stored directly in the context (ctxt['_choose_matched'] and ctxt['_choose_value'] for example).
• the MatchDirective stores a copy of the stream in self.stream but also appends it to the ctxt._match_templates. Isn't the latter operation enough, as I don't see where that self.stream is being used?

If that is all there is to it, then I could write a patch myself... But I prefer to ask first if there's is anything more complex which I overlooked!

[cmlenz]

Very good point. These things should all be done through the context as you propose. I don't think there's anything you've overlooked, but I'm not 100% sure.

[cboos]

Proposed implementation for the feature, on top of r250.

[cboos]

Cache the template loader in Trac -- makes it easy to stress test reentrancy issues.

[cboos]

OK, so it appears that's all there is to it ;)

In Trac's ​Markup branch, with attachment:env_load_template-r3652.patch​ applied, all seems well for over 30 simultaneous requests on the same complex template (​ticket_view.html), when using the proposed implementation (attachment:template_generate_now_reentrant-r250.patch​).

Not to mention it's way faster, as that was to be expected ;)

[cmlenz]

Applied slightly modified patch in [252]. Thanks!