Cached template loader doesn't cache TemplateDoesNotExist

[German M. Bravo]

In cases where multiple fallback templates are requested to the template loader (i.e. django.template.loader.select_template()) to find the first one available, the cached template loader does not cache non-existent templates, making cached loader "slow" again if every time it has to try looking for a template that's been already found to be missing.

from django.template.loader import render_to_string
render_to_string(['missing_template_1.html', 'missing_template_2.html', ..., 'missing_template_N.html', 'existent_template.html'])

When using django.template.loaders.cached.Loader, and given that snippet above, and the fact that missing_template_1.html ... missing_template_N.html do not exist, the filesystem loader or other loaders get hit every time (trying and trying to find already known as missing templates).

I'm attaching a patch to fix this.

[Tim Graham]

It would be good to add a test for this.

[Susan Tan]

Updated PR: ​https://github.com/django/django/pull/1501/ Unit test and full test suite pass.

[Tim Graham]

Fixed in f33db5a09acfc3df3085235a5712c46094eb9a0d.

[Ramiro Morales]

I believe we need to work on this ticket a bit more.

The tests added, although correct because they verify something didn't test before, don't actually test the fix introduced and so didn't fail beforehand.

In particular these two lines:

​https://github.com/django/django/blob/master/django/template/loaders/cached.py#L74-L75

gives us a hint about the fact that this needs further investigation.

I've opened a PR with tests changes. We can discuss things there if you wish: ​https://github.com/django/django/pull/1511

It's not clear to me yet what the right fix would look like. Maybe we can reuse some code from the patch proposed by the OP (refactored to accompany the code evolution)?

Reopening.

[Susan Tan]

Thanks for catching the missing test. I forgot we had to test the cache after the loaders tries to load the missing template. I made a few comments on your PR.

[Ramiro Morales <cramm0@…>]

In b785a80d1934ef05165f222779c3f47c32889825:

Added further fixes, tests for #19949/f33db5a09a.

Thanks Susan Tan. Refs #19949.