Nixpkgs: nginx serving nix store paths sets last-modified header to unix+1 timestamp

[domenkozar]

If you serve files from /nix/store, nginx will set last-modified header to unix timestamp 1. The result is that browser will cache indefinitely.

Not sure what we should do about it really, following nginx configuration helps:

          extraConfig = ''
            if_modified_since off;
            add_header Last-Modified "";
            etag off;
          '';

[chris-martin]

Why do you need to turn etag off?

[chris-martin]

Ah, I see. nginx's etags aren't hashes. https://serverfault.com/questions/690341/algorithm-behind-nginx-etag-generation/690374#690374

[chris-martin]

I wonder if we could find a way to pull the hash out of the nix store path and put it into an etag header.

[samdroid-apps]

Thanks for finding this. I've been having issues with broken caching on my nginx/nixos server. Even when the files on the server were updated (files in the nix store), the browser was told that no change had happened.

If I'm understanding this correctly, since nginx's etags are based on filesystem modification time; you can't have proper cache invalidation (ie. it will always send 304 not modified). Or is there a workaround?

[greglearns]

+1 this is a problem. And turning off etag seems like overkill. Is there some way to 1) set the date on a file so that Nginx can pick it up, or 2) do something else awesome?

[chris-martin]

Is the zero-timestamp rule set in stone in Nix, or is there some way to override it if you really want a file in /nix/store to have a particular timestamp?

[domenkozar]

I think we could get away with:

nix-repl> builtins.currentTime 
1527419011

It's not perfect, as it will change on each Nix evaluation instead of being content addressed. Another way would be to turn the store path passed to nginx root directive to translate into an etag.

[yorickvP]

The etag function is at ngx_http_core_module.c#L1582
called from ngx_http_gzip_static_module and ngx_http_static_module.

Making it dependent on the realpath of the file seems doable.

[Akii]

Explanation how to configure nginx to use the hash from the nix path: https://nixos.wiki/wiki/Nginx#Correct_Caching_when_Serving_Static_Files_from_.2Fnix.2Fstore

[yorickvP]

In my testing, this sends the correct etag, but does not send the 304 not
modified, when the etag matches.
Also, the header gets stripped out when compression is enabled.

On Sun, Jul 29, 2018, 14:54 Akii [e-mail] wrote:

Explanation how to configure nginx to use the hash from the nix path:
https://nixos.wiki/wiki/Nginx#Correct_Caching_when_Serving_Static_Files_from_.2Fnix.2Fstore

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[Akii]

It's working perfectly for me. I've gzip compression enabled, the etag header is sent and the browser gets a 304 when querying the If-None-Match header.

[domenkozar]

Nice! An example with a directory would be more common:

locations."/var/www" = let path = /var/www; { alias = path; extraConfig = '' etag off; add_header etag "${builtins.substring 11 32 "${folder}"}"; ''; }

[Akii]

I've to correct what I said. @yorickvP your statement seems to be correct. What happens in my case is that Cloudflare is actually handling the caching of the ETag correctly, not nginx.

So this approach is completely useless without external caching. I'll investigate further and see if I can come up with a solution.

[yegortimoshenko]

Here is a patch: https://gitlab.com/yegortimoshenko/patches/blob/cc67b35dc7cf68a18ba98043b54ed3d10374c486/nginx/nix-etag-1.15.4.patch

If real path (after dereferencing symlinks) of root is in Nix store, it will use path's hash as ETag. For example:

# nginx root: /tmp/result/hello
$ realpath /tmp/result
/nix/store/wnrhnnpdj3x50j5xz38zp1qxs1ygwccw-site
$ curl --head localhost
HTTP/1.1 200 OK
Server: nginx
Date: Fri, 28 Sep 2018 06:09:25 GMT
Content-Type: text/html
Content-Length: 50
Last-Modified: Thu, 01 Jan 1970 00:00:01 GMT
Connection: keep-alive
ETag: "wnrhnnpdj3x50j5xz38zp1qxs1ygwccw"
Accept-Ranges: bytes
$ curl --head --header 'If-None-Match: "wnrhnnpdj3x50j5xz38zp1qxs1ygwccw"' localhost
HTTP/1.1 304 Not Modified
Server: nginx
Date: Fri, 28 Sep 2018 06:13:07 GMT
Last-Modified: Thu, 01 Jan 1970 00:00:01 GMT
Connection: keep-alive
ETag: "wnrhnnpdj3x50j5xz38zp1qxs1ygwccw"

I'll take some time to double check everything and only then send a PR. However, feel free to test:

```nix
{ pkgs, ... }:

{
services.nginx = {
enable = true;
package = with pkgs; nginx.overrideAttrs (super: {
patches = (super.patches or []) ++ [(fetchpatch {
url = https://gitlab.com/yegortimoshenko/patches/raw/cc67b35dc7cf68a18ba98043b54ed3d10374c486/nginx/nix-etag-1.15.4.patch;
sha256 = "16pa1vwcm7kibkjsxpk3szaa2vnxdinml6v0fp4038i2qaav113k";
})];
});
};
}

[chris-martin]

This is really great! @yegortimoshenko, @domenkozar, or anyone, do you have a moment to write an explanation of this new feature and its practical ramifications for anyone configuring an nginx server? Since it isn't typical nginx behavior, I think the nixos manual should explain it somewhere so it doesn't catch people by surprise.

[domenkozar]

I think https://twitter.com/chris__martin/status/1123050969271554048 is a good explanation, just needs to be put into the manual.

[chris-martin]

Cool, submitted #60578.