}
	}
	db.mu.Unlock()

	err := db.Close()
	if err != nil {
		t.Fatalf("error closing DB: %v", err)
	}

	var numOpen int
	if !waitCondition(t, func() bool {
		numOpen = db.numOpenConns()
		return numOpen == 0
	}) {
		t.Fatalf("%d connections still open after closing DB", numOpen)
	}
}

// numPrepares assumes that db has exactly 1 idle conn and returns
// its count of calls to Prepare