Broker /amp/client route (AMP cache client registration).

broker/amp.go

@@ -0,0 +1,76 @@
+package main
+
+import (
+	"log"
+	"net/http"
+	"strings"
+
+	"git.torproject.org/pluggable-transports/snowflake.git/common/amp"
+	"git.torproject.org/pluggable-transports/snowflake.git/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: "",
+		}
+		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)
+	}
+}