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>

cmd/tailscaled/depaware.txt

@@ -348,6 +348,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
      💣 tailscale.com/util/deephash                                  from tailscale.com/ipn/ipnlocal+
    L 💣 tailscale.com/util/dirwalk                                   from tailscale.com/metrics+
         tailscale.com/util/dnsname                                   from tailscale.com/hostinfo+
+        tailscale.com/util/execqueue                                 from tailscale.com/control/controlclient
         tailscale.com/util/goroutines                                from tailscale.com/ipn/ipnlocal
         tailscale.com/util/groupmember                               from tailscale.com/ipn/ipnauth+
      💣 tailscale.com/util/hashx                                     from tailscale.com/util/deephash

control/controlclient/auto.go

@@ -22,6 +22,7 @@
 	"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()
-	}
-}

util/execqueue/execqueue.go

@@ -0,0 +1,104 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package execqueue implements an ordered asynchronous queue for executing functions.
+package execqueue
+
+import (
+	"context"
+	"errors"
+	"sync"
+)
+
+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()
+}
+
+// Shutdown asynchronously signals the queue to stop.
+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()
+	}
+}

util/execqueue/execqueue_test.go

@@ -0,0 +1,22 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package execqueue
+
+import (
+	"context"
+	"sync/atomic"
+	"testing"
+)
+
+func TestExecQueue(t *testing.T) {
+	ctx := context.Background()
+	var n atomic.Int32
+	q := &ExecQueue{}
+	defer q.Shutdown()
+	q.Add(func() { n.Add(1) })
+	q.Wait(ctx)
+	if got := n.Load(); got != 1 {
+		t.Errorf("n=%d; want 1", got)
+	}
+}