feat(pgs): initial support for conditional requests

[mac-chaffee]

This PR provides initial support for conditional requests.

I copied the important RFC-compliant bits from https://cs.opensource.google/go/go/+/refs/tags/go1.23.4:src/net/http/fs.go and used that to parse and set the right headers and status codes for conditional requests.

The alternative would be using http.ServeContent, but that requires an io.ReadSeeker which wouldn't work with files obtained from imgproxy since that returns an io.ReadCloser.

Ref: #177

• Looks like Souin ignores If-Not-Modified headers (Revalidation with If-Modified-Since not working (always returning 200) darkweak/souin#588), but this issue is technically not a blocker because browsers generally send both If-Modified-Since and If-None-Match, where the If-None-Match takes precedence (which souin does handle), making the conditional request flow work as expected. This is all allowed by the RFCs as far as I can tell.

[mac-chaffee]

To test, follow the new instructions in dev.md to sign up and upload a file. Then:

# This example hits pgs/web directly since the file isn't cached

$ curl -iH "Host: picouser-test.pgs.dev.pico.sh" -H "If-Modified-Since: Sat, 21 Dec 2026 03:03:03 GMT" -H 'If-None-Match: "a9a83095955b3d3efacc7f833a103cb2-1"'  localhost:3000/file.txt
HTTP/1.1 304 Not Modified
Cache-Control: max-age=600
Cache-Status: Souin; fwd=uri-miss; key=GET-http-picouser-test.pgs.dev.pico.sh-/file.txt; detail=UNCACHEABLE-STATUS-CODE
Etag: "a9a83095955b3d3efacc7f833a103cb2-1"
Surrogate-Key: picouser-test
Date: Sat, 21 Dec 2024 15:33:53 GMT

# Cache the file
$ curl -iH "Host: picouser-test.pgs.dev.pico.sh"  localhost:3000/file.txt
HTTP/1.1 200 OK
Cache-Control: max-age=600
Cache-Status: Souin; fwd=uri-miss; stored; key=GET-http-picouser-test.pgs.dev.pico.sh-/file.txt
Content-Length: 14
Content-Type: text/plain
Etag: "a9a83095955b3d3efacc7f833a103cb2-1"
Last-Modified: Sat, 21 Dec 2024 03:03:10 GMT
Surrogate-Key: picouser-test
Date: Sat, 21 Dec 2024 15:34:31 GMT

Hello, World!

# Now test conditional requests coming from souin
$ curl -iH "Host: picouser-test.pgs.dev.pico.sh" -H "If-Modified-Since: Sat, 21 Dec 2026 03:03:03 GMT" -H 'If-None-Match: "a9a83095955b3d3efacc7f833a103cb2-1"'  localhost:3000/file.txt
HTTP/1.1 304 Not Modified
Age: 63
Cache-Control: max-age=600
Cache-Status: Souin; hit; ttl=537; key=GET-http-picouser-test.pgs.dev.pico.sh-/file.txt; detail=OTTER
Date: Sat, 21 Dec 2024 15:34:31 GMT
Etag: "a9a83095955b3d3efacc7f833a103cb2-1"
Last-Modified: Sat, 21 Dec 2024 03:03:10 GMT
Surrogate-Key: picouser-test

[neurosnap]

Thanks so much for your contribution! I think we might want to take a stab at getting http.ServeContent to work with pobj. I know this can get tricky so I can try to see how feasible it is.

I'd really like align pobj and pgs with what golang's stdlib expects.

Thoughts?

[neurosnap]

Alright, I quickly looked into it, I think we should just continue with your approach because converting io.ReadCloser to io.ReadSeeker might be prohibitive.

[mac-chaffee]

Right if you update pobj to always return a ReadSeeker, we still couldn't use http.ServeContent in pgs because the file returned from imgproxy doesn't come through pobj; it comes from http.Get.

[neurosnap]

Overall looks good since most of the changes are from fs.go.

How hard would it be to add a test to web_test.go that ensures things are working as intended?

[mac-chaffee]

Sure I can work on a test, thanks for reviewing!

[mac-chaffee]

Tests added!

[neurosnap]

Looks great, thanks!