control/controlknobs, all: add plumbed Knobs type, not global variables

Previously two tsnet nodes in the same process couldn't have disjoint
sets of controlknob settings from control as both would overwrite each
other's global variables.

This plumbs a new controlknobs.Knobs type around everywhere and hangs
the knobs sent by control on that instead.

Updates #9351

Change-Id: I75338646d36813ed971b4ffad6f9a8b41ec91560
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlknobs/controlknobs.go

@@ -8,22 +8,30 @@ package controlknobs
 import (
 	"sync/atomic"
 
-	"tailscale.com/envknob"
+	"tailscale.com/syncs"
+	"tailscale.com/types/opt"
 )
 
-// disableUPnP indicates whether to attempt UPnP mapping.
-var disableUPnPControl atomic.Bool
+// Knobs is the set of knobs that the control plane's coordination server can
+// adjust at runtime.
+type Knobs struct {
+	// DisableUPnP indicates whether to attempt UPnP mapping.
+	DisableUPnP atomic.Bool
 
-var disableUPnpEnv = envknob.RegisterBool("TS_DISABLE_UPNP")
+	// DisableDRPO is whether control says to disable the
+	// DERP route optimization (Issue 150).
+	DisableDRPO atomic.Bool
 
-// DisableUPnP reports the last reported value from control
-// whether UPnP portmapping should be disabled.
-func DisableUPnP() bool {
-	return disableUPnPControl.Load() || disableUPnpEnv()
-}
-
-// SetDisableUPnP sets whether control says that UPnP should be
-// disabled.
-func SetDisableUPnP(v bool) {
-	disableUPnPControl.Store(v)
+	// KeepFullWGConfig is whether we should disable the lazy wireguard
+	// programming and instead give WireGuard the full netmap always, even for
+	// idle peers.
+	KeepFullWGConfig atomic.Bool
+
+	// RandomizeClientPort is whether control says we should randomize
+	// the client port.
+	RandomizeClientPort atomic.Bool
+
+	// OneCGNAT is whether the the node should make one big CGNAT route
+	// in the OS rather than one /32 per peer.
+	OneCGNAT syncs.AtomicValue[opt.Bool]
 }