p2p: Use JSServer for comms rather than GoJsConn (#888)

* p2p: Use JSServer for comms rather than GoJsConn

This has several benefits:
 - it fixes a bug whereby you could not transmit >4k bytes to/from JS/Go land.
 - it more clearly exposes the interface point between Go and JS: a single
   global function call.
 - it presents a nicer API shape than the previous `net.Conn`.
 - it doesn't needlessly 'stream' data which is already sitting in-memory.

This is currently only active for local CS API traffic, another PR will
add Federation P2P support.

* Typo

cmd/dendritejs/jsServer.go

@@ -0,0 +1,104 @@
+// Copyright 2020 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build wasm
+
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"syscall/js"
+)
+
+// JSServer exposes an HTTP-like server interface which allows JS to 'send' requests to it.
+type JSServer struct {
+	// The router which will service requests
+	Mux *http.ServeMux
+}
+
+// OnRequestFromJS is the function that JS will invoke when there is a new request.
+// The JS function signature is:
+//   function(reqString: string): Promise<{result: string, error: string}>
+// Usage is like:
+//   const res = await global._go_js_server.fetch(reqString);
+//   if (res.error) {
+//     // handle error: this is a 'network' error, not a non-2xx error.
+//   }
+//   const rawHttpResponse = res.result;
+func (h *JSServer) OnRequestFromJS(this js.Value, args []js.Value) interface{} {
+	// we HAVE to spawn a new goroutine and return immediately or else Go will deadlock
+	// if this request blocks at all e.g for /sync calls
+	httpStr := args[0].String()
+	promise := js.Global().Get("Promise").New(js.FuncOf(func(pthis js.Value, pargs []js.Value) interface{} {
+		// The initial callback code for new Promise() is also called on the critical path, which is why
+		// we need to put this in an immediately invoked goroutine.
+		go func() {
+			resolve := pargs[0]
+			fmt.Println("Received request:")
+			fmt.Printf("%s\n", httpStr)
+			resStr, err := h.handle(httpStr)
+			errStr := ""
+			if err != nil {
+				errStr = err.Error()
+			}
+			fmt.Println("Sending response:")
+			fmt.Printf("%s\n", resStr)
+			resolve.Invoke(map[string]interface{}{
+				"result": resStr,
+				"error":  errStr,
+			})
+		}()
+		return nil
+	}))
+	return promise
+}
+
+// handle invokes the http.ServeMux for this request and returns the raw HTTP response.
+func (h *JSServer) handle(httpStr string) (resStr string, err error) {
+	req, err := http.ReadRequest(bufio.NewReader(strings.NewReader(httpStr)))
+	if err != nil {
+		return
+	}
+	w := httptest.NewRecorder()
+
+	h.Mux.ServeHTTP(w, req)
+
+	res := w.Result()
+	var resBuffer strings.Builder
+	err = res.Write(&resBuffer)
+	return resBuffer.String(), err
+}
+
+// ListenAndServe registers a variable in JS-land with the given namespace. This variable is
+// a function which JS-land can call to 'send' HTTP requests. The function is attached to
+// a global object called "_go_js_server". See OnRequestFromJS for more info.
+func (h *JSServer) ListenAndServe(namespace string) {
+	globalName := "_go_js_server"
+	// register a hook in JS-land for it to invoke stuff
+	server := js.Global().Get(globalName)
+	if !server.Truthy() {
+		server = js.Global().Get("Object").New()
+		js.Global().Set(globalName, server)
+	}
+
+	server.Set(namespace, js.FuncOf(h.OnRequestFromJS))
+
+	fmt.Printf("Listening for requests from JS on function %s.%s\n", globalName, namespace)
+	// Block forever to mimic http.ListenAndServe
+	select {}
+}

cmd/dendritejs/main.go

@@ -156,10 +156,10 @@ func main() {
 	// Expose the matrix APIs via fetch - for local traffic
 	go func() {
 		logrus.Info("Listening for service-worker fetch traffic")
-
-		listener := go_http_js_libp2p.NewFetchListener()
-		s := &http.Server{}
-		go s.Serve(listener)
+		s := JSServer{
+			Mux: http.DefaultServeMux,
+		}
+		s.ListenAndServe("fetch")
 	}()
 
 	// We want to block forever to let the fetch and libp2p handler serve the APIs