blob: da3e365b34c7276f5a37b0105e8d7f6222293b97 [file] [log] [blame]
// Copyright 2019 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package memoize supports memoizing the return values of functions with
// idempotent results that are expensive to compute.
// The memoized result is returned again the next time the function is invoked.
// To prevent excessive memory use, the return values are only remembered
// for as long as they still have a user.
// To use this package, build a store and use it to acquire handles with the
// Bind method.
package memoize
import (
// Store binds keys to functions, returning handles that can be used to access
// the functions results.
type Store struct {
mu sync.Mutex
// entries is the set of values stored.
entries map[interface{}]uintptr
// Function is the type for functions that can be memoized.
// The result must be a pointer.
type Function func(ctx context.Context) interface{}
// Handle is returned from a store when a key is bound to a function.
// It is then used to access the results of that function.
type Handle struct {
mu sync.Mutex
store *Store
key interface{}
// the function that will be used to populate the value
function Function
// the lazily poplulated value
value interface{}
// wait is used to block until the value is ready
// will only be non nil if the generator is already running
wait chan interface{}
// the cancel function for the context being used by the generator
// it can be used to abort the generator if the handle is garbage
// collected.
cancel context.CancelFunc
// Has returns true if they key is currently valid for this store.
func (s *Store) Has(key interface{}) bool {
_, found := s.entries[key]
return found
// Bind returns a handle for the given key and function.
// Each call to bind will return the same handle if it is already bound.
// Bind will always return a valid handle, creating one if needed.
// Each key can only have one handle at any given time.
// The value will be held for as long as the handle is, once it has been
// generated.
// Bind does not cause the value to be generated.
func (s *Store) Bind(key interface{}, function Function) *Handle {
// panic early if the function is nil
// it would panic later anyway, but in a way that was much harder to debug
if function == nil {
panic("the function passed to bind must not be nil")
// check if we already have the key
h := s.get(key)
if h != nil {
// we have a handle already, just return it
return h
// we have not seen this key before, add a new entry
if s.entries == nil {
s.entries = make(map[interface{}]uintptr)
h = &Handle{
store: s,
key: key,
function: function,
// now add the weak reference to the handle into the map
s.entries[key] = uintptr(unsafe.Pointer(h))
// add the deletion the entry when the handle is garbage collected
runtime.SetFinalizer(h, release)
return h
// Find returns the handle associated with a key, if it is bound.
// It cannot cause a new handle to be generated, and thus may return nil.
func (s *Store) Find(key interface{}) *Handle {
return s.get(key)
// Cached returns the value associated with a key.
// It cannot cause the value to be generated.
// It will return the cached value, if present.
func (s *Store) Cached(key interface{}) interface{} {
h := s.Find(key)
if h == nil {
return nil
return h.Cached()
// nocheckptr because:
func (s *Store) get(key interface{}) *Handle {
// this must be called with the store mutex already held
e, found := s.entries[key]
if !found {
return nil
return (*Handle)(unsafe.Pointer(e))
// Cached returns the value associated with a handle.
// It will never cause the value to be generated.
// It will return the cached value, if present.
func (h *Handle) Cached() interface{} {
return h.value
// Get returns the value associated with a handle.
// If the value is not yet ready, the underlying function will be invoked.
// This activates the handle, and it will remember the value for as long as it exists.
func (h *Handle) Get(ctx context.Context) interface{} {
if h.function == nil {
return h.value
// value not ready yet
select {
case h.value = <
// successfully got the value
h.function = nil
h.cancel = nil
return h.value
case <-ctx.Done():
// cancelled outer context, leave the generator running
// for someone else to pick up later
return nil
// run starts the generator if necessary and returns the value channel.
func (h *Handle) run(ctx context.Context) chan interface{} {
if h.wait != nil {
// generator already running
return h.wait
// we use a length one "postbox" so the go routine can quit even if
// nobody wants the result yet
h.wait = make(chan interface{}, 1)
ctx, cancel := context.WithCancel(xcontext.Detach(ctx))
h.cancel = cancel
go func() {
// in here the handle lock is not held
// we post the value back to the first caller that waits for it
h.wait <- h.function(ctx)
return h.wait
func release(p interface{}) {
h := p.(*Handle)
// there is a small gap between the garbage collector deciding that the handle
// is liable for collection and the finalizer being called
// if the handle is recovered during that time, you will end up with a valid
// handle that no longer has an entry in the map, and that no longer has a
// finalizer associated with it, but that is okay.
delete(, h.key)