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package main
import (
"log"
"net/http"
"strings"
"gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/v2/common/amp"
"gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/v2/common/messages"
)
// ampClientOffers is the AMP-speaking endpoint for client poll messages,
// intended for access via an AMP cache. In contrast to the other clientOffers,
// the client's encoded poll message is stored in the URL path rather than the
// HTTP request body (because an AMP cache does not support POST), and the
// encoded client poll response is sent back as AMP-armored HTML.
func ampClientOffers(i *IPC, w http.ResponseWriter, r *http.Request) {
// The encoded client poll message immediately follows the /amp/client/
// path prefix, so this function unfortunately needs to be aware of and
// remote its own routing prefix.
path := strings.TrimPrefix(r.URL.Path, "/amp/client/")
if path == r.URL.Path {
// The path didn't start with the expected prefix. This probably
// indicates an internal bug.
log.Println("ampClientOffers: unexpected prefix in path")
w.WriteHeader(http.StatusInternalServerError)
return
}
var encPollReq []byte
var response []byte
var err error
encPollReq, err = amp.DecodePath(path)
if err == nil {
arg := messages.Arg{
Body: encPollReq,
RemoteAddr: "",
RendezvousMethod: messages.RendezvousAmpCache,
}
err = i.ClientOffers(arg, &response)
} else {
response, err = (&messages.ClientPollResponse{
Error: "cannot decode URL path",
}).EncodePollResponse()
}
if err != nil {
// We couldn't even construct a JSON object containing an error
// message :( Nothing to do but signal an error at the HTTP
// layer. The AMP cache will translate this 500 status into a
// 404 status.
// https://amp.dev/documentation/guides-and-tutorials/learn/amp-caches-and-cors/amp-cache-urls/#redirect-%26-error-handling
log.Printf("ampClientOffers: %v", err)
w.WriteHeader(http.StatusInternalServerError)
return
}
w.Header().Set("Content-Type", "text/html")
// Attempt to hint to an AMP cache not to waste resources caching this
// document. "The Google AMP Cache considers any document fresh for at
// least 15 seconds."
// https://developers.google.com/amp/cache/overview#google-amp-cache-updates
w.Header().Set("Cache-Control", "max-age=15")
w.WriteHeader(http.StatusOK)
enc, err := amp.NewArmorEncoder(w)
if err != nil {
log.Printf("amp.NewArmorEncoder: %v", err)
return
}
defer enc.Close()
if _, err := enc.Write(response); err != nil {
log.Printf("ampClientOffers: unable to write answer: %v", err)
}
}
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