type depSet map[any]bool // set of true bools

// The finalCloser interface is used by (*DB).addDep and related
// dependency reference counting.
type finalCloser interface {
	// finalClose is called when the reference count of an object
	// goes to zero. (*DB).mu is not held while calling it.
	finalClose() error
}

// addDep notes that x now depends on dep, and x's finalClose won't be

	for fc := range db.dep {
		db.dumpDep(t, 0, fc, map[finalCloser]bool{})
	}
}

func (db *DB) dumpDep(t *testing.T, depth int, dep finalCloser, seen map[finalCloser]bool) {
	seen[dep] = true
	indent := strings.Repeat("  ", depth)
	ds := db.dep[dep]
	for k := range ds {
		t.Logf("%s%T (%p) waiting for -> %T (%p)", indent, dep, dep, k, k)
		if fc, ok := k.(finalCloser); ok {
			if !seen[fc] {