django.template.loader is not thread-safe

[bear330]

In multithreaded environment, the django.template.loader is not thread-safe.

If browser sends many requests to django, sometimes it will raise TemplateDoesNotExist exception even the template actually existed.

This exception only raises in first requests, after that, everything will be fine.

This is because the django.template.loader use the global variable 'template_source_loaders'.

If many threads try to load template simultaneously, 'template_source_loaders' will not be initialized correctly.

I made a simple patch for that to fix this bug.

Thanks.

[bear330]

A simple patch to make sure template_source_loaders is initialized correctly.

[mrts]

An improved patch based on the findings of threading review. Initializes a temporary list instead of modifying the global list and assigns the temporary list in one "atomic" operation to the global list.

More to come to other parts of the template system.

[mrts]

minor stylistic improvements (tuple instead of list for const-correctness)

[mrts]

Marking as ready for checkin -- the last patch directly mimics source:django/trunk/django/template/context.py@7415#L86 , so it is the already prevalent way of doing things.

[Russell Keith-Magee]

(In [7563]) Fixed #6950 -- Modified initialization of template loaders to use a temporary variable instead of directly modifying the global loader list. Thanks, mrts.