ipn/ipnlocal: stop netmap expiry timer when resetting control client

This prevents a panic where we synthesize a new netmap in
setClientStatus after we've shut down and nil'd out the controlclient,
since that function expects to be called while connected to control.

Fixes #7392

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Ib631eb90f34f6afa008d69bbb386f70da145e102

ipn/ipnlocal/local.go

@@ -3682,6 +3682,21 @@ func (b *LocalBackend) resetControlClientLockedAsync() {
 	if b.cc == nil {
 		return
 	}
+
+	// When we clear the control client, stop any outstanding netmap expiry
+	// timer; synthesizing a new netmap while we don't have a control
+	// client will break things.
+	//
+	// See https://github.com/tailscale/tailscale/issues/7392
+	if b.nmExpiryTimer != nil {
+		b.nmExpiryTimer.Stop()
+		b.nmExpiryTimer = nil
+
+		// Also bump the epoch to ensure that if the timer started, it
+		// will abort.
+		b.numClientStatusCalls.Add(1)
+	}
+
 	go b.cc.Shutdown()
 	b.cc = nil
 	b.ccAuto = nil