control/controlclient,util/execqueue: extract execqueue into a package

This is a useful primitive for asynchronous execution of ordered work I
want to use in another change.

Updates tailscale/corp#16833
Signed-off-by: James Tucker <james@tailscale.com>

control/controlclient/auto.go

@@ -22,6 +22,7 @@ import (
 	"tailscale.com/types/netmap"
 	"tailscale.com/types/persist"
 	"tailscale.com/types/structs"
+	"tailscale.com/util/execqueue"
 )
 
 type LoginGoal struct {
@@ -118,7 +119,7 @@ type Auto struct {
 	closed        bool
 	updateCh      chan struct{} // readable when we should inform the server of a change
 	observer      Observer      // called to update Client status; always non-nil
-	observerQueue execQueue
+	observerQueue execqueue.ExecQueue
 
 	unregisterHealthWatch func()
 
@@ -675,7 +676,7 @@ func (c *Auto) Shutdown() {
 	direct := c.direct
 	if !closed {
 		c.closed = true
-		c.observerQueue.shutdown()
+		c.observerQueue.Shutdown()
 		c.cancelAuthCtxLocked()
 		c.cancelMapCtxLocked()
 		for _, w := range c.unpauseWaiters {
@@ -696,7 +697,7 @@ func (c *Auto) Shutdown() {
 		}
 		ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 		defer cancel()
-		c.observerQueue.wait(ctx)
+		c.observerQueue.Wait(ctx)
 		c.logf("Client.Shutdown done.")
 	}
 }
@@ -737,95 +738,3 @@ func (c *Auto) DoNoiseRequest(req *http.Request) (*http.Response, error) {
 func (c *Auto) GetSingleUseNoiseRoundTripper(ctx context.Context) (http.RoundTripper, *tailcfg.EarlyNoise, error) {
 	return c.direct.GetSingleUseNoiseRoundTripper(ctx)
 }
-
-type execQueue struct {
-	mu         sync.Mutex
-	closed     bool
-	inFlight   bool          // whether a goroutine is running q.run
-	doneWaiter chan struct{} // non-nil if waiter is waiting, then closed
-	queue      []func()
-}
-
-func (q *execQueue) Add(f func()) {
-	q.mu.Lock()
-	defer q.mu.Unlock()
-	if q.closed {
-		return
-	}
-	if q.inFlight {
-		q.queue = append(q.queue, f)
-	} else {
-		q.inFlight = true
-		go q.run(f)
-	}
-}
-
-// RunSync waits for the queue to be drained and then synchronously runs f.
-// It returns an error if the queue is closed before f is run or ctx expires.
-func (q *execQueue) RunSync(ctx context.Context, f func()) error {
-	for {
-		if err := q.wait(ctx); err != nil {
-			return err
-		}
-		q.mu.Lock()
-		if q.inFlight {
-			q.mu.Unlock()
-			continue
-		}
-		defer q.mu.Unlock()
-		if q.closed {
-			return errors.New("closed")
-		}
-		f()
-		return nil
-	}
-}
-
-func (q *execQueue) run(f func()) {
-	f()
-
-	q.mu.Lock()
-	for len(q.queue) > 0 && !q.closed {
-		f := q.queue[0]
-		q.queue[0] = nil
-		q.queue = q.queue[1:]
-		q.mu.Unlock()
-		f()
-		q.mu.Lock()
-	}
-	q.inFlight = false
-	q.queue = nil
-	if q.doneWaiter != nil {
-		close(q.doneWaiter)
-		q.doneWaiter = nil
-	}
-	q.mu.Unlock()
-}
-
-func (q *execQueue) shutdown() {
-	q.mu.Lock()
-	defer q.mu.Unlock()
-	q.closed = true
-}
-
-// wait waits for the queue to be empty.
-func (q *execQueue) wait(ctx context.Context) error {
-	q.mu.Lock()
-	waitCh := q.doneWaiter
-	if q.inFlight && waitCh == nil {
-		waitCh = make(chan struct{})
-		q.doneWaiter = waitCh
-	}
-	q.mu.Unlock()
-
-	if waitCh == nil {
-		return nil
-	}
-
-	select {
-	case <-waitCh:
-		return nil
-	case <-ctx.Done():
-		return ctx.Err()
-	}
-}