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>

wgengine/userspace_test.go

@@ -11,6 +11,7 @@ import (
 
 	"go4.org/mem"
 	"tailscale.com/cmd/testwrapper/flakytest"
+	"tailscale.com/control/controlknobs"
 	"tailscale.com/net/dns"
 	"tailscale.com/net/netaddr"
 	"tailscale.com/net/tstun"
@@ -152,15 +153,16 @@ func TestUserspaceEngineReconfig(t *testing.T) {
 }
 
 func TestUserspaceEnginePortReconfig(t *testing.T) {
-	tstest.Replace(t, &randomizeClientPort, func() bool { return false })
-
 	flakytest.Mark(t, "https://github.com/tailscale/tailscale/issues/2855")
 	const defaultPort = 49983
+
+	var knobs controlknobs.Knobs
+
 	// Keep making a wgengine until we find an unused port
 	var ue *userspaceEngine
 	for i := 0; i < 100; i++ {
 		attempt := uint16(defaultPort + i)
-		e, err := NewFakeUserspaceEngine(t.Logf, attempt)
+		e, err := NewFakeUserspaceEngine(t.Logf, attempt, &knobs)
 		if err != nil {
 			t.Fatal(err)
 		}
@@ -199,7 +201,7 @@ func TestUserspaceEnginePortReconfig(t *testing.T) {
 		t.Errorf("no debug setting changed local port to %d from %d", got, startingPort)
 	}
 
-	randomizeClientPort = func() bool { return true }
+	knobs.RandomizeClientPort.Store(true)
 	if err := ue.Reconfig(cfg, routerCfg, &dns.Config{}); err != nil {
 		t.Fatal(err)
 	}
@@ -208,7 +210,7 @@ func TestUserspaceEnginePortReconfig(t *testing.T) {
 	}
 
 	lastPort := ue.magicConn.LocalPort()
-	randomizeClientPort = func() bool { return false }
+	knobs.RandomizeClientPort.Store(false)
 	if err := ue.Reconfig(cfg, routerCfg, &dns.Config{}); err != nil {
 		t.Fatal(err)
 	}