Add a handler for federation transactions

[anoadragon453]

Add a handler which can accept a federation transaction, validate the PDUs and return errors in the case of failing to process a given PDU.

EDUs are accepted but we currently don't do anything with them.

This grew out of trying to fix TestOutboundFederationIgnoresMissingEventWithBadJSONForRoomVersion6 for Synapse. The test would fail because as soon as Synapse joined the room, it would start trying to send presence EDUs to the new server. Rather than disabling presence, I thought it would be better to write a proper handler for it.

I'll use this handler for that test (and possibly others) in a separate PR.

[anoadragon453]

I've just realised that a proto version of this exists here:

complement/tests/federation_room_send_test.go

Lines 45 to 65 in a7d6117

	// TODO: Have a nicer api shape than just http.Handler
	srv.Mux().Handle("/_matrix/federation/v1/send/{txnID}", http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
	defer waiter.Finish()
	var body gomatrixserverlib.Transaction
	if err := json.NewDecoder(req.Body).Decode(&body); err != nil {
	t.Errorf("failed to decode /send body: %s", err)
	w.WriteHeader(500)
	return
	}
	if len(body.PDUs) != 1 {
	t.Fatalf("got %d pdus, want %d", len(body.PDUs), 1)
	}
	ev, err := gomatrixserverlib.NewEventFromUntrustedJSON(body.PDUs[0], ver)
	if err != nil {
	t.Fatalf("PDU failed NewEventFromUntrustedJSON checks: %s", err)
	}
	if ev.Type() != wantEventType {
	t.Errorf("Wrong event type, got %s want %s", ev.Type(), wantEventType)
	}
	w.WriteHeader(200)
	})).Methods("PUT")

, but perhaps this change is better still?

[kegsay]

Nil check is important.

[kegsay]

Might be cute to have a callback function here which is passed with valid gmsl.Event structs, since it's fairly common to want to process stuff on /send. E.g:

func HandleTransactionRequests(cb func(gmsl.Event)) func(*Server) {
    // when ready..
    cb(event)
}

[anoadragon453]

This is what I was thinking, yeah. Then we could perhaps augment TestOutboundFederationSend to use this instead, while slimming down the test at the same time.

[anoadragon453]

Shall I add another callback method for EDUs? Edit: have done so.

stale

[anoadragon453]

Realised there was a bug in that we were reading from req.Body which had already been read from during gmsl.VerifyHTTPRequest. So now we read from fedReq.Content() instead.

Additionally I added nil checks for the pdu and edu handler functions.

[kegsay]

Logging needs to be more explicit, and the panic on error needs to be fixed, otherwise LGTM

Review items addressed.