control/controlclient, tailcfg: add Debug.SleepSeconds (mapver 19)

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

control/controlclient/direct.go

@@ -795,6 +795,45 @@ func (c *Direct) sendMapRequest(ctx context.Context, maxPolls int, cb func(*netm
 			}
 			setControlAtomic(&controlUseDERPRoute, resp.Debug.DERPRoute)
 			setControlAtomic(&controlTrimWGConfig, resp.Debug.TrimWGConfig)
+			if sleep := time.Duration(resp.Debug.SleepSeconds * float64(time.Second)); sleep > 0 {
+				const maxSleep = 5 * time.Minute
+				// SleepFor sleeps for d, but keeps the poll timeout from
+				// firing while we're sleeeping.
+				sleepFor := func(d time.Duration) {
+					ticker := time.NewTicker(pollTimeout / 2)
+					defer ticker.Stop()
+					timer := time.NewTimer(d)
+					defer timer.Stop()
+					for {
+						select {
+						case <-ctx.Done():
+							return
+						case <-timer.C:
+							return
+						case <-ticker.C:
+							select {
+							case timeoutReset <- struct{}{}:
+							case <-timer.C:
+								return
+							case <-ctx.Done():
+								return
+							}
+						}
+					}
+				}
+				if sleep > maxSleep {
+					c.logf("sleeping for %v, capped from server-requested %v ...", maxSleep, sleep)
+					sleepFor(maxSleep)
+				} else {
+					c.logf("sleeping for server-requested %v ...", sleep)
+					sleepFor(sleep)
+				}
+				select {
+				default:
+				case <-ctx.Done():
+					return ctx.Err()
+				}
+			}
 		}
 
 		nm := sess.netmapForResponse(&resp)

tailcfg/tailcfg.go

@@ -41,7 +41,8 @@ import (
 //    16: 2021-04-15: client understands Node.Online, MapResponse.OnlineChange
 //    17: 2021-04-18: MapResponse.Domain empty means unchanged
 //    18: 2021-04-19: MapResponse.Node nil means unchanged (all fields now omitempty)
-const CurrentMapRequestVersion = 18
+//    19: 2021-04-21: MapResponse.Debug.SleepSeconds
+const CurrentMapRequestVersion = 19
 
 type StableID string
 
@@ -1012,6 +1013,12 @@ type Debug struct {
 	// GoroutineDumpURL, if non-empty, requests that the client do
 	// a one-time dump of its active goroutines to the given URL.
 	GoroutineDumpURL string `json:",omitempty"`
+
+	// SleepSeconds requests that the client sleep for the
+	// provided number of seconds.
+	// The client can (and should) limit the value (such as 5
+	// minutes).
+	SleepSeconds float64 `json:",omitempty"`
 }
 
 func (k MachineKey) String() string                   { return fmt.Sprintf("mkey:%x", k[:]) }