control/controlclient: reset backoff in mapRoutine on netmap recv

We were never resetting the backoff in streaming mapResponses.
The call to `PollNetMap` always returns with an error. Changing that contract
is harder, so manually reset backoff when a netmap is received.

Updates tailscale/corp#12894

Signed-off-by: Maisem Ali <maisem@tailscale.com>

control/controlclient/auto.go

@@ -551,6 +551,8 @@ func (c *Auto) mapRoutine() {
 				if stillAuthed {
 					c.sendStatus("mapRoutine-got-netmap", nil, "", nm)
 				}
+				// Reset the backoff timer if we got a netmap.
+				bo.BackOff(ctx, nil)
 			})
 
 			health.SetInPollNetMap(false)

control/controlclient/direct.go

@@ -770,6 +770,8 @@ func (c *Direct) SetEndpoints(endpoints []tailcfg.Endpoint) (changed bool) {
 
 // PollNetMap makes a /map request to download the network map, calling cb with
 // each new netmap.
+// It always returns a non-nil error describing the reason for the failure
+// or why the request ended.
 func (c *Direct) PollNetMap(ctx context.Context, cb func(*netmap.NetworkMap)) error {
 	return c.sendMapRequest(ctx, -1, false, cb)
 }
@@ -798,7 +800,12 @@ func (c *Direct) SendLiteMapUpdate(ctx context.Context) error {
 // every minute.
 const pollTimeout = 120 * time.Second
 
-// cb nil means to omit peers.
+// sendMapRequest makes a /map request to download the network map, calling cb with
+// each new netmap. If maxPolls is -1, it will poll forever and only returns if
+// the context expires or the server returns an error/closes the connection and as
+// such always returns a non-nil error.
+//
+// If cb is nil, OmitPeers will be set to true.
 func (c *Direct) sendMapRequest(ctx context.Context, maxPolls int, readOnly bool, cb func(*netmap.NetworkMap)) error {
 	metricMapRequests.Add(1)
 	metricMapRequestsActive.Add(1)