linux: call startDiscovery async as it can block if adapter powered off

gap.go

@@ -8,6 +8,7 @@ import (
 var (
 	errScanning                  = errors.New("bluetooth: a scan is already in progress")
 	errNotScanning               = errors.New("bluetooth: there is no scan in progress")
+	errScanStopped               = errors.New("bluetooth: scan was stopped unexpectedly")
 	errAdvertisementPacketTooBig = errors.New("bluetooth: advertisement packet overflows")
 	errNotYetImplmented          = errors.New("bluetooth: not implemented")
 )

gap_linux.go

@@ -142,6 +142,27 @@ func (a *Adapter) Scan(callback func(*Adapter, ScanResult)) error {
 		return errScanning
 	}
 
+	signal := make(chan *dbus.Signal)
+	a.bus.Signal(signal)
+	defer a.bus.RemoveSignal(signal)
+
+	propertiesChangedMatchOptions := []dbus.MatchOption{dbus.WithMatchInterface("org.freedesktop.DBus.Properties")}
+	a.bus.AddMatchSignal(propertiesChangedMatchOptions...)
+	defer a.bus.RemoveMatchSignal(propertiesChangedMatchOptions...)
+
+	newObjectMatchOptions := []dbus.MatchOption{dbus.WithMatchInterface("org.freedesktop.DBus.ObjectManager")}
+	a.bus.AddMatchSignal(newObjectMatchOptions...)
+	defer a.bus.RemoveMatchSignal(newObjectMatchOptions...)
+
+	// Check if the adapter is powered on.
+	powered, err := a.adapter.GetProperty("org.bluez.Adapter1.Powered")
+	if err != nil {
+		return err
+	}
+	if !powered.Value().(bool) {
+		return errAdaptorNotPowered
+	}
+
 	// Channel that will be closed when the scan is stopped.
 	// Detecting whether the scan is stopped can be done by doing a non-blocking
 	// read from it. If it succeeds, the scan is stopped.
@@ -150,25 +171,13 @@ func (a *Adapter) Scan(callback func(*Adapter, ScanResult)) error {
 
 	// This appears to be necessary to receive any BLE discovery results at all.
 	defer a.adapter.Call("org.bluez.Adapter1.SetDiscoveryFilter", 0)
-	err := a.adapter.Call("org.bluez.Adapter1.SetDiscoveryFilter", 0, map[string]interface{}{
+	err = a.adapter.Call("org.bluez.Adapter1.SetDiscoveryFilter", 0, map[string]interface{}{
 		"Transport": "le",
 	}).Err
 	if err != nil {
 		return err
 	}
 
-	signal := make(chan *dbus.Signal)
-	a.bus.Signal(signal)
-	defer a.bus.RemoveSignal(signal)
-
-	propertiesChangedMatchOptions := []dbus.MatchOption{dbus.WithMatchInterface("org.freedesktop.DBus.Properties")}
-	a.bus.AddMatchSignal(propertiesChangedMatchOptions...)
-	defer a.bus.RemoveMatchSignal(propertiesChangedMatchOptions...)
-
-	newObjectMatchOptions := []dbus.MatchOption{dbus.WithMatchInterface("org.freedesktop.DBus.ObjectManager")}
-	a.bus.AddMatchSignal(newObjectMatchOptions...)
-	defer a.bus.RemoveMatchSignal(newObjectMatchOptions...)
-
 	// Go through all connected devices and present the connected devices as
 	// scan results. Also save the properties so that the full list of
 	// properties is known on a PropertiesChanged signal. We can't present the
@@ -200,10 +209,9 @@ func (a *Adapter) Scan(callback func(*Adapter, ScanResult)) error {
 	}
 
 	// Instruct BlueZ to start discovering.
-	err = a.adapter.Call("org.bluez.Adapter1.StartDiscovery", 0).Err
-	if err != nil {
-		return err
-	}
+	// NOTE: We must call Go here, not Call, because it can block if adapter is
+	// powered off, or was recently powered off.
+	startDiscovery := a.adapter.Go("org.bluez.Adapter1.StartDiscovery", 0, nil)
 
 	for {
 		// Check whether the scan is stopped. This is necessary to avoid a race
@@ -217,6 +225,12 @@ func (a *Adapter) Scan(callback func(*Adapter, ScanResult)) error {
 		}
 
 		select {
+		case <-startDiscovery.Done:
+			if startDiscovery.Err != nil {
+				close(cancelChan)
+				a.scanCancelChan = nil
+				return startDiscovery.Err
+			}
 		case sig := <-signal:
 			// This channel receives anything that we watch for, so we'll have
 			// to check for signals that are relevant to us.
@@ -236,14 +250,16 @@ func (a *Adapter) Scan(callback func(*Adapter, ScanResult)) error {
 				case "org.bluez.Adapter1":
 					// check power state
 					changes := sig.Body[1].(map[string]dbus.Variant)
-					for k, v := range changes {
-						if k == "Powered" && !v.Value().(bool) {
-							// adapter is powered off, stop the scan
-							close(cancelChan)
-							close(a.scanCancelChan)
-							a.scanCancelChan = nil
-							return errAdaptorNotPowered
-						}
+					if powered, ok := changes["Powered"]; ok && !powered.Value().(bool) {
+						// adapter is powered off, stop the scan
+						close(cancelChan)
+						a.scanCancelChan = nil
+						return errAdaptorNotPowered
+					} else if discovering, ok := changes["Discovering"]; ok && !discovering.Value().(bool) {
+						// adapter stopped discovering unexpectedly (e.g. due to external event)
+						close(cancelChan)
+						a.scanCancelChan = nil
+						return errScanStopped
 					}
 
 				case "org.bluez.Device1":