fix(darwin): write without response canSendWriteWithoutResponse (#435)

* fix(darwin): write without response use IsReadyToSendWriteWithoutResponse

* poll every 15ms instead

Author: acouvreur

gattc_darwin.go

@@ -9,8 +9,8 @@ import (
 )
 
 var (
-	errCannotSendWriteWithoutResponse = errors.New("bluetooth: cannot send write without response (buffer full)")
-	errTimeoutEnableNotifications     = errors.New("timeout on EnableNotifications")
+	errWriteWithoutResponseTimeout = errors.New("bluetooth: write without response timed out waiting for buffer space")
+	errTimeoutEnableNotifications  = errors.New("timeout on EnableNotifications")
 )
 
 var (
@@ -243,13 +243,21 @@ func (c DeviceCharacteristic) Write(p []byte) (n int, err error) {
 // WriteWithoutResponse replaces the characteristic value with a new value. The
 // call will return before all data has been written. A limited number of such
 // writes can be in flight at any given time.
-// If the client is not ready to send write without response requests at this time (e.g. because the internal buffer is full), an error is returned.
+// If the peripheral's buffer is full, this method polls
+// CanSendWriteWithoutResponse every 15ms (one BLE connection interval) until
+// ready, with a 10-second timeout.
 func (c DeviceCharacteristic) WriteWithoutResponse(p []byte) (int, error) {
-	if !c.service.device.prph.CanSendWriteWithoutResponse() {
-		return 0, errCannotSendWriteWithoutResponse
+	dev := c.service.device
+
+	deadline := time.Now().Add(10 * time.Second)
+	for !dev.prph.CanSendWriteWithoutResponse() {
+		if time.Now().After(deadline) {
+			return 0, errWriteWithoutResponseTimeout
+		}
+		time.Sleep(15 * time.Millisecond)
 	}
 
-	c.service.device.prph.WriteCharacteristic(p, c.characteristic, false)
+	dev.prph.WriteCharacteristic(p, c.characteristic, false)
 
 	return len(p), nil
 }