DAV module does not respect if-unmodified-since

[Marijn Haverbeke]

I.e. if you PUT or DELETE a resource with an if-unmodified-since header, the overwrite or delete will go through happily even if the header should have prevented it.

(This is a common use case, where you've previously a version of a resource, and you know its modified date, and then, when updating it or deleting it, you want to check for race conditions with other clients, and can use if-unmodified-since to get an error back if someone else messed with the resource in the meantime.)

Find a patch for this attached (also at ​https://gist.github.com/4013062). It's my first Nginx contribution -- feel free to point out style mistakes or general wrong-headedness.

I did not find a clean way to make the existing code in ngx_http_not_modified_filter_module.c handle this. It looks directly at the last-modified header, and, as a header filter, will only run *after* the actions for the request have already been taken.

I also did not add code for if-match, which is analogous, and code for which could probably be added to the ngx_http_test_if_unmodified function I added (which would be renamed in that case). But I don't really understand handling of etags by nginx yet, so I didn't touch that.

[Marijn Haverbeke]

Path that solves the problem for me