tailcfg, wgengine, controlclient: add control-side gating of lazy WG config

control/controlclient/direct.go

@@ -629,10 +629,8 @@ func (c *Direct) PollNetMap(ctx context.Context, maxPolls int, cb func(*NetworkM
 			if resp.Debug.LogHeapPprof {
 				go logheap.LogHeap(resp.Debug.LogHeapURL)
 			}
-			newv := resp.Debug.DERPRoute
-			if old, ok := controlUseDERPRoute.Load().(opt.Bool); !ok || old != newv {
-				controlUseDERPRoute.Store(newv)
-			}
+			setControlAtomic(&controlUseDERPRoute, resp.Debug.DERPRoute)
+			setControlAtomic(&controlTrimWGConfig, resp.Debug.TrimWGConfig)
 		}
 		// Temporarily (2020-06-29) support removing all but
 		// discovery-supporting nodes during development, for
@@ -964,7 +962,18 @@ func cloneNodes(v1 []*tailcfg.Node) []*tailcfg.Node {
 	return v2
 }
 
-var controlUseDERPRoute atomic.Value
+// opt.Bool configs from control.
+var (
+	controlUseDERPRoute atomic.Value
+	controlTrimWGConfig atomic.Value
+)
+
+func setControlAtomic(dst *atomic.Value, v opt.Bool) {
+	old, ok := dst.Load().(opt.Bool)
+	if !ok || old != v {
+		dst.Store(v)
+	}
+}
 
 // DERPRouteFlag reports the last reported value from control for whether
 // DERP route optimization (Issue 150) should be enabled.
@@ -972,3 +981,10 @@ func DERPRouteFlag() opt.Bool {
 	v, _ := controlUseDERPRoute.Load().(opt.Bool)
 	return v
 }
+
+// TrimWGConfig reports the last reported value from control for whether
+// we should do lazy wireguard configuration.
+func TrimWGConfig() opt.Bool {
+	v, _ := controlTrimWGConfig.Load().(opt.Bool)
+	return v
+}

tailcfg/tailcfg.go

@@ -570,6 +570,10 @@ type Debug struct {
 	// highest priority (if set), then this (if set), then the
 	// binary default value.
 	DERPRoute opt.Bool `json:",omitempty"`
+
+	// TrimWGConfig controls whether Tailscale does lazy, on-demand
+	// wireguard configuration of peers.
+	TrimWGConfig opt.Bool `json:",omitempty"`
 }
 
 func (k MachineKey) String() string                   { return fmt.Sprintf("mkey:%x", k[:]) }

wgengine/userspace.go

@@ -574,7 +574,10 @@ func (e *userspaceEngine) pinger(peerKey wgcfg.Key, ips []wgcfg.IP) {
 	p.run(ctx, peerKey, ips, srcIP)
 }
 
-var debugTrimWireguard, _ = strconv.ParseBool(os.Getenv("TS_DEBUG_TRIM_WIREGUARD"))
+var (
+	debugTrimWireguardEnv = os.Getenv("TS_DEBUG_TRIM_WIREGUARD")
+	debugTrimWireguard, _ = strconv.ParseBool(debugTrimWireguardEnv)
+)
 
 // forceFullWireguardConfig reports whether we should give wireguard
 // our full network map, even for inactive peers
@@ -584,9 +587,13 @@ func (e *userspaceEngine) pinger(peerKey wgcfg.Key, ips []wgcfg.IP) {
 // and we haven't got enough time testing it.
 func forceFullWireguardConfig(numPeers int) bool {
 	// Did the user explicitly enable trimmming via the environment variable knob?
-	if debugTrimWireguard {
-		return false
+	if debugTrimWireguardEnv != "" {
+		return !debugTrimWireguard
 	}
+	if opt := controlclient.TrimWGConfig(); opt != "" {
+		return !opt.EqualBool(true)
+	}
+
 	// On iOS with large networks, it's critical, so turn on trimming.
 	// Otherwise we run out of memory from wireguard-go goroutine stacks+buffers.
 	// This will be the default later for all platforms and network sizes.