util/execqueue: don't hold mutex in RunSync

We don't hold q.mu while running normal ExecQueue.Add funcs, so we
shouldn't in RunSync either. Otherwise code it calls can't shut down
the queue, as seen in #18502.

Updates #18052

Co-authored-by: Nick Khyl <nickk@tailscale.com>
Change-Id: Ic5e53440411eca5e9fabac7f4a68a9f6ef026de1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/execqueue/execqueue_test.go

@@ -20,3 +20,12 @@ func TestExecQueue(t *testing.T) {
 		t.Errorf("n=%d; want 1", got)
 	}
 }
+
+// Test that RunSync doesn't hold q.mu and block Shutdown
+// as we saw in tailscale/tailscale#18502
+func TestExecQueueRunSyncLocking(t *testing.T) {
+	q := &ExecQueue{}
+	q.RunSync(t.Context(), func() {
+		q.Shutdown()
+	})
+}