ipn/ipnlocal: remove all the weird locking (LockedOnEntry, UnlockEarly, etc)

Fixes #11649
Updates #16369

Co-authored-by: James Sanderson <jsanderson@tailscale.com>
Change-Id: I63eaa18fe870ddf81d84b949efac4d1b44c3db86
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/execqueue/execqueue.go

@@ -12,6 +12,8 @@ import (
 
 type ExecQueue struct {
 	mu         sync.Mutex
+	ctx        context.Context    // context.Background + closed on Shutdown
+	cancel     context.CancelFunc // closes ctx
 	closed     bool
 	inFlight   bool          // whether a goroutine is running q.run
 	doneWaiter chan struct{} // non-nil if waiter is waiting, then closed
@@ -24,6 +26,7 @@ func (q *ExecQueue) Add(f func()) {
 	if q.closed {
 		return
 	}
+	q.initCtxLocked()
 	if q.inFlight {
 		q.queue = append(q.queue, f)
 	} else {
@@ -79,18 +82,32 @@ func (q *ExecQueue) Shutdown() {
 	q.mu.Lock()
 	defer q.mu.Unlock()
 	q.closed = true
+	if q.cancel != nil {
+		q.cancel()
+	}
 }
 
-// Wait waits for the queue to be empty.
+func (q *ExecQueue) initCtxLocked() {
+	if q.ctx == nil {
+		q.ctx, q.cancel = context.WithCancel(context.Background())
+	}
+}
+
+// Wait waits for the queue to be empty or shut down.
 func (q *ExecQueue) Wait(ctx context.Context) error {
 	q.mu.Lock()
+	q.initCtxLocked()
 	waitCh := q.doneWaiter
 	if q.inFlight && waitCh == nil {
 		waitCh = make(chan struct{})
 		q.doneWaiter = waitCh
 	}
+	closed := q.closed
 	q.mu.Unlock()
 
+	if closed {
+		return errors.New("execqueue shut down")
+	}
 	if waitCh == nil {
 		return nil
 	}
@@ -98,6 +115,8 @@ func (q *ExecQueue) Wait(ctx context.Context) error {
 	select {
 	case <-waitCh:
 		return nil
+	case <-q.ctx.Done():
+		return errors.New("execqueue shut down")
 	case <-ctx.Done():
 		return ctx.Err()
 	}