magicsock: call RequestStatus when DERP connects

Without this, a freshly started ipn client will be stuck in the
"Starting" state until something triggers a call to RequestStatus.
Usually a UI does this, but until then we can sit in this state
until poked by an external event, as is evidenced by our e2e tests
locking up when DERP is attached.

(This only recently became a problem when we enabled lazy handshaking
everywhere, otherwise the wireugard tunnel creation would also
trigger a RequestStatus.)

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -1206,6 +1206,7 @@ func (c *Conn) derpWriteChanOfAddr(addr netaddr.IPPort, peer key.Public) chan<-
 
 	go c.runDerpReader(ctx, addr, dc, wg, startGate)
 	go c.runDerpWriter(ctx, dc, ch, wg, startGate)
+	go c.epFunc(nil) // call RequestStatus with DERPs change
 
 	return ad.writeCh
 }

wgengine/userspace.go

@@ -229,10 +229,11 @@ func newUserspaceEngineAdvanced(conf EngineConfig) (_ Engine, reterr error) {
 	e.linkMon = mon
 
 	endpointsFn := func(endpoints []string) {
-		e.mu.Lock()
-		e.endpoints = append(e.endpoints[:0], endpoints...)
-		e.mu.Unlock()
-
+		if len(endpoints) != 0 {
+			e.mu.Lock()
+			e.endpoints = append(e.endpoints[:0], endpoints...)
+			e.mu.Unlock()
+		}
 		e.RequestStatus()
 	}
 	magicsockOpts := magicsock.Options{