net/tstun: add inner loop to poll

This avoids re-enqueuing to t.bufferConsumed,
which makes the code a bit clearer.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

net/tstun/wrap.go

@@ -181,6 +181,16 @@ func (t *Wrapper) Close() error {
 	return err
 }
 
+// isClosed reports whether t is closed.
+func (t *Wrapper) isClosed() bool {
+	select {
+	case <-t.closed:
+		return true
+	default:
+		return false
+	}
+}
+
 // pumpEvents copies events from t.tdev to t.eventsUpDown and t.eventsOther.
 // pumpEvents exits when t.tdev.events or t.closed is closed.
 // pumpEvents closes t.eventsUpDown and t.eventsOther when it exits.
@@ -266,28 +276,24 @@ func allowSendOnClosedChannel() {
 // so packets may be stuck in t.outbound if t.Read called t.tdev.Read directly.
 func (t *Wrapper) poll() {
 	defer allowSendOnClosedChannel() // for send to t.outbound
-	for {
-		<-t.bufferConsumed
-
+	for range t.bufferConsumed {
+		var n int
+		var err error
 		// Read may use memory in t.buffer before PacketStartOffset for mandatory headers.
 		// This is the rationale behind the tun.Wrapper.{Read,Write} interfaces
 		// and the reason t.buffer has size MaxMessageSize and not MaxContentSize.
-		n, err := t.tdev.Read(t.buffer[:], PacketStartOffset)
-		if err != nil {
-			t.outbound <- tunReadResult{err: err}
-			// In principle, read errors are not fatal (but wireguard-go disagrees).
-			t.bufferConsumed <- struct{}{}
-			continue
-		}
-
-		// Wireguard will skip an empty read,
-		// so we might as well do it here to avoid the send through t.outbound.
-		if n == 0 {
-			t.bufferConsumed <- struct{}{}
-			continue
+		// In principle, read errors are not fatal (but wireguard-go disagrees).
+		// We loop here until we get a non-empty (or failed) read.
+		// We don't need this loop for correctness,
+		// but wireguard-go will skip an empty read,
+		// so we might as well avoid the send through t.outbound.
+		for n == 0 && err == nil {
+			if t.isClosed() {
+				return
+			}
+			n, err = t.tdev.Read(t.buffer[:], PacketStartOffset)
 		}
-
-		t.outbound <- tunReadResult{data: t.buffer[PacketStartOffset : PacketStartOffset+n]}
+		t.outbound <- tunReadResult{data: t.buffer[PacketStartOffset : PacketStartOffset+n], err: err}
 	}
 }