device: prevent spurious errors while closing a device

When closing a device, packets that are in flight
can make it to SendBuffer, which then returns an error.
Those errors add noise but no light;
they do not reflect an actual problem.

Adding the synchronization required to prevent
this from occurring is currently expensive and error-prone.
Instead, quietly drop such packets instead of
returning an error.

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

device/peer.go

@@ -140,6 +140,11 @@ func (peer *Peer) SendBuffer(buffer []byte) error {
 	defer peer.device.net.RUnlock()
 
 	if peer.device.net.bind == nil {
+		// Packets can leak through to SendBuffer while the device is closing.
+		// When that happens, drop them silently to avoid spurious errors.
+		if peer.device.isClosed.Get() {
+			return nil
+		}
 		return errors.New("no bind")
 	}