If-modified-since for static resources not working.

[Piotr Czachur]

How to reproduce

1. Enable serving static files.
2. Request same file multiple times.
3. You always get HTTP "200", but it should be "304" after first request.

What causes the problem

django.views.static.was_modified_since() compares modification time taken from two sources:

• os.stat()
• HTTP_IF_MODIFIED_SINCE header.

os.stat() returns timestamp as float (Linux 64bit), and header contains integer timestamp, which results in such comparsions for unmodified files:

if 1331113576.68 > 1331113576:
     // assuming file was modified

which have incorrect result.

was_modified_since() should refuse comparing values of different types, or cast both to integers.

Notice that values returned by os.stat() can be OS-dependent.

[Simon Charette]

Running Linux 64 bit myself I can confirm os.stat returns a float. I'll write a patch with a failing testcase.

I guess no-one stumbled upon this issue since serve is only meant to be used in development.

[Preston Holmes]

is header_mtime always generated in a way that it is an int - or should it be cast at comparison time as well?

given that small caveat - I'd say this is RFC, I verified tests, and really far more people are using static serve in production behind things like gunicorn on heroku now.

[Simon Charette]

Looking at ​the code behind `djangp.utils.http.parse_http_date` it seems that header_mtime can only be an int since it returns the number of seconds since epoch from a datetime with no considerations towards units of magnitude lesser than seconds.

However the docstring clearly states: Returns an[sic] floating point number expressed in seconds since the epoch, in UTC

I think the parse_http_date's docstring should be changed and it's return result also cast to int?

[Claude Paroz <claude@…>]

In b3ee80a0cf0e60876f03b797d2bdc69505dbdfcb:

Fixed parse_http_date docstring and moved related tests

Refs #18675.

[Claude Paroz <claude@…>]

In 3cbe686af6b6a3f23a607b2eb7497c55d88e8345:

Fixed #18675 -- Fixed was_modified_since with floating-point mtime

Thanks Simon Charette for the patch.