portlist: unexport all Poller fields, removing unused one, rework channels

Poller.C and Poller.c were duplicated for one caller. Add an accessor
returning the receive-only version instead. It'll inline.

Poller.Err was unused. Remove.

Then Poller is opaque.

The channel usage and shutdown was a bit sketchy. Clean it up.

And document some things.

Change-Id: I5669e54f51a6a13492cf5485c83133bda7ea3ce9
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -1530,7 +1530,7 @@ func dnsMapsEqual(new, old *netmap.NetworkMap) bool {
 func (b *LocalBackend) readPoller() {
 	n := 0
 	for {
-		ports, ok := <-b.portpoll.C
+		ports, ok := <-b.portpoll.Updates()
 		if !ok {
 			return
 		}

portlist/poller.go

@@ -15,22 +15,18 @@ import (
 // Poller scans the systems for listening ports periodically and sends
 // the results to C.
 type Poller struct {
-	// C received the list of ports periodically. It's closed when
-	// Run completes, after which Err can be checked.
-	C <-chan List
+	c chan List // unbuffered
 
-	// Err is the error from the final GetList call. It is only
-	// valid to read once C has been closed. Err is nil if Close
-	// is called or the context is canceled.
-	Err error
+	// closeCtx is the context that's canceled on Close.
+	closeCtx       context.Context
+	closeCtxCancel context.CancelFunc
+
+	runDone chan struct{} // closed when Run completes
 
 	// scatch is memory for Poller.getList to reuse between calls.
 	scratch []Port
 
-	c chan List // the unconstrained version of the exported C above
-
-	quitCh chan struct{} // close this to force exit
-	prev   List          // most recent data
+	prev List // most recent data
 }
 
 // NewPoller returns a new portlist Poller. It returns an error
@@ -40,10 +36,10 @@ func NewPoller() (*Poller, error) {
 		return nil, errors.New("not available on iOS")
 	}
 	p := &Poller{
-		c:      make(chan List),
-		quitCh: make(chan struct{}),
+		c:       make(chan List),
+		runDone: make(chan struct{}),
 	}
-	p.C = p.c
+	p.closeCtx, p.closeCtxCancel = context.WithCancel(context.Background())
 
 	// Do one initial poll synchronously so we can return an error
 	// early.
@@ -55,49 +51,64 @@ func NewPoller() (*Poller, error) {
 	return p, nil
 }
 
+// Updates return the channel that receives port list updates.
+//
+// The channel is closed when the Poller is closed.
+func (p *Poller) Updates() <-chan List { return p.c }
+
+// Close closes the Poller.
+// Run will return with a nil error.
 func (p *Poller) Close() error {
+	p.closeCtxCancel()
+	<-p.runDone
+	return nil
+}
+
+// send sends pl to p.c and returns whether it was successfully sent.
+func (p *Poller) send(ctx context.Context, pl List) (sent bool, err error) {
 	select {
-	case <-p.quitCh:
-		return nil
-	default:
+	case p.c <- pl:
+		return true, nil
+	case <-ctx.Done():
+		return false, ctx.Err()
+	case <-p.closeCtx.Done():
+		return false, nil
 	}
-	close(p.quitCh)
-	<-p.C
-	return nil
 }
 
 // Run runs the Poller periodically until either the context
 // is done, or the Close is called.
+//
+// Run may only be called once.
 func (p *Poller) Run(ctx context.Context) error {
+	defer close(p.runDone)
 	defer close(p.c)
+
 	tick := time.NewTicker(pollInterval)
 	defer tick.Stop()
 
-	// Send out the pre-generated initial value
-	p.c <- p.prev
+	// Send out the pre-generated initial value.
+	if sent, err := p.send(ctx, p.prev); !sent {
+		return err
+	}
 
 	for {
 		select {
 		case <-tick.C:
 			pl, err := p.getList()
 			if err != nil {
-				p.Err = err
 				return err
 			}
 			if pl.sameInodes(p.prev) {
 				continue
 			}
 			p.prev = pl
-			select {
-			case p.c <- pl:
-			case <-ctx.Done():
-				return ctx.Err()
-			case <-p.quitCh:
-				return nil
+			if sent, err := p.send(ctx, p.prev); !sent {
+				return err
 			}
 		case <-ctx.Done():
 			return ctx.Err()
-		case <-p.quitCh:
+		case <-p.closeCtx.Done():
 			return nil
 		}
 	}