transport/local: fix "client-provided callback did block on send" crash

fixes #173

(amendment of 0e419c11df2bc45feb6edb7d82a309b0fe174875)

transport/local/serve_local.go

@@ -61,7 +61,9 @@ func (l *LocalListener) Connect(dialCtx context.Context, clientIdentity string)
 	// place request
 	req := connectRequest{
 		clientIdentity: clientIdentity,
-		callback:       make(chan connectResult),
+		// ensure non-blocking send in Accept, we don't necessarily read from callback before
+		// Accept writes to it
+		callback: make(chan connectResult, 1),
 	}
 	select {
 	case l.connects <- req:
@@ -93,19 +95,35 @@ func (l *LocalListener) Addr() net.Addr { return localAddr{"<listening>"} }
 
 func (l *LocalListener) Accept(ctx context.Context) (*transport.AuthConn, error) {
 	respondToRequest := func(req connectRequest, res connectResult) (err error) {
+
 		transport.GetLogger(ctx).
 			WithField("res.conn", res.conn).WithField("res.err", res.err).
 			Debug("responding to client request")
+
+		// contract bewteen Connect and Accept is that Connect sends a req.callback
+		// into which we can send one result non-blockingly.
+		// We want to panic if that contract is violated (impl error)
+		//
+		// However, Connect also supports timeouts through context cancellation:
+		// Connect closes the channel into which we might send, which will panic.
+		//
+		// ==> distinguish those cases in defer
+		const clientCallbackBlocked = "client-provided callback did block on send"
 		defer func() {
 			errv := recover()
+			if errv == clientCallbackBlocked {
+				// this would be a violation of contract betwee Connect and Accept, see above
+				panic(clientCallbackBlocked)
+			} else {
 				transport.GetLogger(ctx).WithField("recover_err", errv).
 					Debug("panic on send to client callback, likely a legitimate client-side timeout")
+			}
 		}()
 		select {
 		case req.callback <- res:
 			err = nil
 		default:
-			err = fmt.Errorf("client-provided callback did block on send")
+			panic(clientCallbackBlocked)
 		}
 		close(req.callback)
 		return err