wgengine,util/execqueue: wait for in-flight linkChange before closing

ExecQueue.Shutdown does not wait for a function that is already
executing, so Close could tear down magicConn, dns, wgdev, and tundev
while a queued linkChange was still using them, panicking during
shutdown. Add ExecQueue.ShutdownAndWait, which discards queued
functions that have not started and waits for the in-flight one, and
use it in Close with a bounded context before tearing anything down.
The eventbus client is closed first and is the queue's only producer,
so no new work can arrive after the drain.

Updates #17641

Change-Id: I0350bcb59c1ee4b0dcac88cf66b93828466c8c98
Signed-off-by: Adel-Ayoub <adelayoub.maaziz@gmail.com>

util/execqueue/execqueue.go

@@ -88,6 +88,36 @@ func (q *ExecQueue) Shutdown() {
 	}
 }
 
+// ShutdownAndWait signals the queue to stop, discards any queued
+// functions that have not started, and waits for the currently
+// executing function, if any, to complete or ctx to expire.
+//
+// It must not be called while holding a lock that a queued function
+// may acquire, or it will not return until ctx expires.
+func (q *ExecQueue) ShutdownAndWait(ctx context.Context) error {
+	q.mu.Lock()
+	q.closed = true
+	if q.cancel != nil {
+		q.cancel()
+	}
+	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()
+	}
+}
+
 func (q *ExecQueue) initCtxLocked() {
 	if q.ctx == nil {
 		q.ctx, q.cancel = context.WithCancel(context.Background())