Use StoppableWorkers in the single-line encoder (#4147)

Also, make `start()` a private function, so you can't get into trouble by someone building one of these structs and calling `.Start()` on it multiple times.

Tried on the `grievance` rover: I didn't see any changes to behavior (and indeed, no changes to behavior are intended).

components/encoder/single/single_encoder.go

@@ -35,6 +35,7 @@ import (
 	"go.viam.com/rdk/components/encoder"
 	"go.viam.com/rdk/logging"
 	"go.viam.com/rdk/resource"
+	rdkutils "go.viam.com/rdk/utils"
 )
 
 var singleModel = resource.DefaultModelFamily.WithModel("single")
@@ -69,9 +70,7 @@ type Encoder struct {
 	positionType encoder.PositionType
 	logger       logging.Logger
 
-	cancelCtx               context.Context
-	cancelFunc              func()
-	activeBackgroundWorkers sync.WaitGroup
+	workers rdkutils.StoppableWorkers
 }
 
 // Pin describes the configuration of Pins for a Single encoder.
@@ -115,12 +114,9 @@ func NewSingleEncoder(
 	conf resource.Config,
 	logger logging.Logger,
 ) (encoder.Encoder, error) {
-	cancelCtx, cancelFunc := context.WithCancel(context.Background())
 	e := &Encoder{
 		Named:        conf.ResourceName().AsNamed(),
 		logger:       logger,
-		cancelCtx:    cancelCtx,
-		cancelFunc:   cancelFunc,
 		position:     0,
 		positionType: encoder.PositionTypeTicks,
 	}
@@ -163,9 +159,6 @@ func (e *Encoder) Reconfigure(
 		return nil
 	}
 	utils.UncheckedError(e.Close(ctx))
-	cancelCtx, cancelFunc := context.WithCancel(context.Background())
-	e.cancelCtx = cancelCtx
-	e.cancelFunc = cancelFunc
 
 	e.mu.Lock()
 	e.I = di
@@ -175,31 +168,39 @@ func (e *Encoder) Reconfigure(
 	atomic.StoreInt64(&e.position, 0)
 	e.mu.Unlock()
 
-	e.Start(ctx, board)
+	e.start(ctx, board)
 
 	return nil
 }
 
-// Start starts the Encoder background thread.
-func (e *Encoder) Start(ctx context.Context, b board.Board) {
+// start starts the Encoder background thread.
+func (e *Encoder) start(ctx context.Context, b board.Board) {
+	if e.workers != nil {
+		// We're already listening to an old interrupt. This should never happen! Stop that before
+		// we start listening to the new one.
+		utils.Logger.Error(
+			"starting an already-started encoder! Stopping the old interrupt stream...")
+		e.workers.Stop()
+	}
+	e.workers = rdkutils.NewStoppableWorkers()
+
 	encoderChannel := make(chan board.Tick)
-	err := b.StreamTicks(e.cancelCtx, []board.DigitalInterrupt{e.I}, encoderChannel, nil)
+	err := b.StreamTicks(e.workers.Context(), []board.DigitalInterrupt{e.I}, encoderChannel, nil)
 	if err != nil {
 		utils.Logger.Errorw("error getting interrupt ticks", "error", err)
 		return
 	}
-	e.activeBackgroundWorkers.Add(1)
 
-	utils.ManagedGo(func() {
+	e.workers.AddWorkers(func(cancelCtx context.Context) {
 		for {
 			select {
-			case <-e.cancelCtx.Done():
+			case <-cancelCtx.Done():
 				return
 			default:
 			}
 
 			select {
-			case <-e.cancelCtx.Done():
+			case <-cancelCtx.Done():
 				return
 			case <-encoderChannel:
 			}
@@ -216,7 +217,7 @@ func (e *Encoder) Start(ctx context.Context, b board.Board) {
 				e.logger.CDebug(ctx, "received tick for encoder that isn't connected to a motor; ignoring")
 			}
 		}
-	}, e.activeBackgroundWorkers.Done)
+	})
 }
 
 // Position returns the current position in terms of ticks or
@@ -250,7 +251,16 @@ func (e *Encoder) Properties(ctx context.Context, extra map[string]interface{})
 
 // Close shuts down the Encoder.
 func (e *Encoder) Close(ctx context.Context) error {
-	e.cancelFunc()
-	e.activeBackgroundWorkers.Wait()
+	// In unit tests, we construct encoders without calling NewSingleEncoder(), which means they
+	// might not have called e.start(), so might not have initialized e.workers. Don't crash if
+	// that happens.
+	if e.workers != nil {
+		e.workers.Stop() // This also shuts down the interrupt stream.
+	}
+
+	// During reconfiguration, we might call e.Close() and then e.start() to restart with a new
+	// interrupt pin. Remove the old StoppableWorkers so e.start() doesn't try adding workers to an
+	// already-stopped one.
+	e.workers = nil
 	return nil
 }