Validation doesn't work

首先运行 Caddyfile 复现，并比较两个 curl 命令发出的上游请求。跟踪 Go 模块中的缓存验证/重新验证入口点，并确认已存储的 Last-Modified 元数据会作为条件请求发送；当上游收到该条件且响应行为符合 HTTP 验证时，即表示完成。

According to clause No. 4.3 of RFC 7234:

4.3. Validation
When a cache has one or more stored responses for a requested URI, but cannot serve any of them (e.g., because they are not fresh, or one cannot be selected; see Section 4.1), it can use the conditional request mechanism
...
This process is known as "validating" or "revalidating" the stored response.
4.3.1. Sending a Validation Request
When sending a conditional request for cache validation, a cache sends one or more precondition header fields containing validator metadata from its stored response(s), which is then compared by recipients to determine whether a stored response is equivalent to a current representation of the resource.
One such validator is the timestamp given in a Last-Modified header field...
Another validator is the entity-tag given in an ETag header field...

Thus, I expect caddy to cache the first response and subsequently validate cached data via a conditional request. However, caddy simply forwards a request to an upstream without any conditions.

I've tested it only with Last-Modified and no-cache, because of the bug I noticed with ETag. Additionally, I couldn't trigger revalidation using other directives like must-revalidate and stale-while-revalidate.

Reproduction steps:

{
    log
    cache {
        stale 60s
    }
}

http://localhost:80 {
    log
    cache
    reverse_proxy http://localhost:81
}

http://localhost:81 {
    log
    header Last-Modified "Mon, 15 Dec 2025 08:52:20 GMT"
    header Cache-Control "no-cache"
    respond "Hello world"
}

$ curl -ik localhost:80
HTTP/1.1 200 OK
Cache-Control: no-cache
Cache-Status: Souin; fwd=uri-miss; stored; key=GET-http-localhost-/
Last-Modified: Mon, 15 Dec 2025 08:52:20 GMT
...
$ curl -ik localhost:80
HTTP/1.1 200 OK
Cache-Control: no-cache
Cache-Status: Souin; fwd=request; fwd-status=200; key=GET-http-localhost-/; detail=REQUEST-REVALIDATION
Last-Modified: Mon, 15 Dec 2025 08:52:20 GMT
...

logs:

{"level":"info","ts":1766736377.4338582,"logger":"http.log.access","msg":"handled request","request":{"remote_ip":"127.0.0.1","remote_port":"48850","client_ip":"127.0.0.1","proto":"HTTP/1.1","method":"GET","host":"localhost","uri":"/","headers":{"X-Forwarded-Proto":["http"],"Accept-Encoding":["gzip"],"Accept":["*/*"],"Date":["Fri, 26 Dec 2025 08:06:17 UTC"],"Via":["1.1 Caddy"],"X-Forwarded-Host":["localhost"],"User-Agent":["curl/8.7.1"],"X-Forwarded-For":["172.16.249.1"]}},"bytes_read":0,"user_id":"","duration":0.000199667,"size":11,"status":200,"resp_headers":{"Content-Type":["text/plain; charset=utf-8"],"Server":["Caddy"],"Last-Modified":["Mon, 15 Dec 2025 08:52:20 GMT"],"Cache-Control":["no-cache"]}}
{"level":"info","ts":1766736377.4560177,"logger":"http.log.access","msg":"handled request","request":{"remote_ip":"172.16.249.1","remote_port":"57879","client_ip":"172.16.249.1","proto":"HTTP/1.1","method":"GET","host":"localhost","uri":"/","headers":{"User-Agent":["curl/8.7.1"],"Accept":["*/*"]}},"bytes_read":0,"user_id":"","duration":0.023146958,"size":11,"status":200,"resp_headers":{"Last-Modified":["Mon, 15 Dec 2025 08:52:20 GMT"],"Server":["Caddy"],"Content-Length":["11"],"Via":["1.1 Caddy"],"Date":["Fri, 26 Dec 2025 08:06:17 GMT"],"Cache-Control":["no-cache"],"Content-Type":["text/plain; charset=utf-8"],"Cache-Status":["Souin; fwd=uri-miss; stored; key=GET-http-localhost-/"]}}
{"level":"info","ts":1766736406.6892934,"logger":"http.log.access","msg":"handled request","request":{"remote_ip":"127.0.0.1","remote_port":"48850","client_ip":"127.0.0.1","proto":"HTTP/1.1","method":"GET","host":"localhost","uri":"/","headers":{"X-Forwarded-For":["172.16.249.1"],"X-Forwarded-Host":["localhost"],"X-Forwarded-Proto":["http"],"Accept":["*/*"],"Date":["Fri, 26 Dec 2025 08:06:46 UTC"],"Via":["1.1 Caddy"],"Accept-Encoding":["gzip"],"User-Agent":["curl/8.7.1"]}},"bytes_read":0,"user_id":"","duration":0.000023417,"size":11,"status":200,"resp_headers":{"Server":["Caddy"],"Last-Modified":["Mon, 15 Dec 2025 08:52:20 GMT"],"Cache-Control":["no-cache"],"Content-Type":["text/plain; charset=utf-8"]}}
{"level":"info","ts":1766736406.6932402,"logger":"http.log.access","msg":"handled request","request":{"remote_ip":"172.16.249.1","remote_port":"57892","client_ip":"172.16.249.1","proto":"HTTP/1.1","method":"GET","host":"localhost","uri":"/","headers":{"Accept":["*/*"],"User-Agent":["curl/8.7.1"]}},"bytes_read":0,"user_id":"","duration":0.019553791,"size":11,"status":200,"resp_headers":{"Content-Length":["11"],"Last-Modified":["Mon, 15 Dec 2025 08:52:20 GMT"],"Via":["1.1 Caddy"],"Cache-Control":["no-cache"],"Content-Type":["text/plain; charset=utf-8"],"Cache-Status":["Souin; fwd=request; fwd-status=200; key=GET-http-localhost-/; detail=REQUEST-REVALIDATION"],"Server":["Caddy"],"Date":["Fri, 26 Dec 2025 08:06:46 GMT"]}}

As we may see, the request made by caddy is not conditional.