fix(gateway): Cache-Control header for UnixFS directories

[acejam]

Cache-Control headers are currently not being returned when viewing the directory index page of a UnixFS directory. This is because rq.ttl returns 0.

For example, the following request path is missing a Cache-Control header: /ipfs/<cid>/

According to the Path Gateway Specification:

Cache-Control: public, max-age=29030400, immutable must be returned for every immutable resource under /ipfs/ namespace.

https://specs.ipfs.tech/http-gateways/path-gateway/#cache-control-response-header

This PR sets the appropriate header.

[lidel]

Thank you for raising this.

This is bit more tricky, because HTML listing is quality-of-life improvement for web browsers, and not the actual immutable data.

Generated HTML listing of UnixFS directory is NOT immutable and should not be cached forever.

• Proper fix for /ipns/ is to implement TTL handling.
• Proper fix for /ipfs/ may not exist. A fair compromise may be to cache for a week, and also allow for serving stale cache while check for update happens asynchronously. This ensures CDNs serve content fast, but we dont cache generated HTML forever. See proposed value inline.

Would you have time to submit related PRs to

• https://github.com/ipfs/specs/blob/main/src/http-gateways/path-gateway.md#cache-control-response-header
• https://github.com/ipfs/gateway-conformance

?

We need to update conformance tests before this can be shipped. (CI is currently failing).

[acejam]

@lidel gateway-conformance PR here: ipfs/gateway-conformance#218

[lidel]

Thanks! Had to push some test fixes but your fork does not allow it, so continuing in #643