forked from https://github.com/symfony/symfony
4bbd76d4fe
This PR was squashed before being merged into the 4.4 branch.
Discussion
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[HttpKernel] [HttpCache] Keep s-maxage=0 from ESI sub-responses
| Q | A
| ------------- | ---
| Branch? | 4.4
| Bug fix? | yes
| New feature? | no
| Deprecations? | no
| Tickets |
| License | MIT
| Doc PR |
When the `ResponseCacheStrategy` is merging ESI surrogates and the master response, it treats `s-maxage=0` as if no `s-maxage` has been set.
The result is that for a main and a surrogate response that both are `public, s-maxage=0`, the result will only be `public`, with no further expiration time.
https://datatracker.ietf.org/doc/html/rfc7234#section-4.2.2 allows caches to assign a _heuristic expiration time_ when no explicit expiration time has been given but the response has been marked as explicitly cacheable with `public`. Clearly, such a heuristic was _not_ intended or desired when `public, s-maxage=0` was given.
This PR ensures that `s-maxage=0` is passed along with the resulting response.
### Some notes on `s-maxage=0`
You might argue that `s-maxage=0` does not make sense on a response.
According to https://datatracker.ietf.org/doc/html/rfc7234#section-3.2, `s-maxage=0` is a valid setting to ensure that a cached response "cannot be used to satisfy a subsequent request without revalidating it on the origin server".
This setting can be used to keep responses in edge caches/CDNs, but to re-validate on every request. The bottom line result can still be faster (304 + response already at the edge vs. fetch response from origin).
To my understanding, the difference between `s-maxage=0` and `must-revalidate` is that a "disconnected" cache (one that cannot contact the origin server) _must not_ use a stale response when `must-revalidate` is used, but _is not prohibited_ from doing so for `s-maxage=0` (https://datatracker.ietf.org/doc/html/rfc7234#section-4.2.4). In other words, `must-revalidate` is not exactly the same as (or the "right" way instead of) `s-maxage=0`.
In the special case of ESI (composite) responses, revalidation is not possible (no `ETag`, no `Last-Modified`). But, as explained above, it is still important to pass on the explicit expiration time, instead of having no value for it.
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