wgengine/magicsock: deflake some tests with an ugly hack

Starting with fe68841dc7, some e2e tests
got flaky. Rather than debug them (they're gnarly), just revert to the old
behavior as far as those tests are concerned. The tests were somehow
using magicsock without a private key and expecting it to do ... something.

My goal with fe68841dc7 was to stop log spam
and unnecessary work I saw on the iOS app when when stopping the app.

Instead, only stop doing that work on any transition from
once-had-a-private-key to no-longer-have-a-private-key. That fixes
what I wanted to fix while still making the mysterious e2e tests
happy.

wgengine/magicsock/magicsock.go

@@ -175,6 +175,7 @@ type Conn struct {
 	derpMap     *tailcfg.DERPMap // nil (or zero regions/nodes) means DERP is disabled
 	netMap      *controlclient.NetworkMap
 	privateKey  key.Private
+	everHadKey  bool               // whether we ever had a non-zero private key
 	myDerp      int                // nearest DERP region ID; 0 means none/unknown
 	derpStarted chan struct{}      // closed on first connection to DERP; for tests & cleaner Close
 	activeDerp  map[int]activeDerp // DERP regionID -> connection to a node in that region
@@ -1815,6 +1816,7 @@ func (c *Conn) SetPrivateKey(privateKey wgcfg.PrivateKey) error {
 	c.privateKey = newKey
 
 	if oldKey.IsZero() {
+		c.everHadKey = true
 		c.logf("magicsock: SetPrivateKey called (init)")
 		go c.ReSTUN("set-private-key")
 	} else if newKey.IsZero() {
@@ -2184,8 +2186,19 @@ func (c *Conn) ReSTUN(why string) {
 		// raced with a shutdown.
 		return
 	}
-	if c.privateKey.IsZero() {
-		c.logf("magicsock: ReSTUN(%q) ignored; no private key", why)
+
+	// If the user stopped the app, stop doing work. (When the
+	// user stops Tailscale via the GUI apps, ipn/local.go
+	// reconfigures the engine with a zero private key.)
+	//
+	// This used to just check c.privateKey.IsZero, but that broke
+	// some end-to-end tests tests that didn't ever set a private
+	// key somehow. So for now, only stop doing work if we ever
+	// had a key, which helps real users, but appeases tests for
+	// now. TODO: rewrite those tests to be less brittle or more
+	// realistic.
+	if c.privateKey.IsZero() && c.everHadKey {
+		c.logf("magicsock: ReSTUN(%q) ignored; stopped, no private key", why)
 		return
 	}