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>

1 files changed, 104 insertions, 0 deletions

diff --git a/util/execqueue/execqueue.go b/util/execqueue/execqueue.go
new file mode 100644
index 000000000..889cea255
--- /dev/null
+++ b/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()
+	}
+}