mirror of
https://github.com/golang/go
synced 2024-11-19 00:44:40 -07:00
76a3b8da50
If a user is typing fast, they will quickly invalidate many snapshots. We don't want to stack up a bunch of stale type check and analysis operations, so we should propagate cancellation through the cache. Handles are long-lived, so we may cancel an operation only to restart it again later. Also, there may be multiple operations waiting on the same computation, and just because one is cancelled doesn't mean we should necessarily stop. The easiest way to support all that was to add an explicit state to each handle, and track the number of waiters. See the code for more details on Handle life cycles. As far as I can tell, the rest of gopls is prepared for this behavior. I added an explicit check to the type checking code, where I was worried it might get overly confused. But long-term it would probably be good to return an error from Get. Change-Id: I3ea6e141b52b94300a41248d3f2e039b023709d0 Reviewed-on: https://go-review.googlesource.com/c/tools/+/206879 Run-TryBot: Heschi Kreinick <heschi@google.com> Reviewed-by: Ian Cottrell <iancottrell@google.com>
260 lines
7.1 KiB
Go
260 lines
7.1 KiB
Go
// Copyright 2019 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package memoize supports memoizing the return values of functions with
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// idempotent results that are expensive to compute.
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//
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// The memoized result is returned again the next time the function is invoked.
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// To prevent excessive memory use, the return values are only remembered
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// for as long as they still have a user.
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//
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// To use this package, build a store and use it to acquire handles with the
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// Bind method.
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//
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package memoize
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import (
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"context"
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"runtime"
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"sync"
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"unsafe"
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"golang.org/x/tools/internal/xcontext"
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)
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// Store binds keys to functions, returning handles that can be used to access
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// the functions results.
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type Store struct {
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mu sync.Mutex
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// entries is the set of values stored.
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entries map[interface{}]uintptr
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}
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// Function is the type for functions that can be memoized.
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// The result must be a pointer.
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type Function func(ctx context.Context) interface{}
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type state int
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const (
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stateIdle = iota
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stateRunning
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stateCompleted
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)
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// Handle is returned from a store when a key is bound to a function.
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// It is then used to access the results of that function.
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//
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// A Handle starts out in idle state, waiting for something to demand its
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// evaluation. It then transitions into running state. While it's running,
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// waiters tracks the number of Get calls waiting for a result, and the done
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// channel is used to notify waiters of the next state transition. Once the
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// evaluation finishes, value is set, state changes to completed, and done
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// is closed, unblocking waiters. Alternatively, as Get calls are cancelled,
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// they decrement waiters. If it drops to zero, the inner context is cancelled,
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// computation is abandoned, and state resets to idle to start the process over
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// again.
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type Handle struct {
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store *Store
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key interface{}
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mu sync.Mutex
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state state
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// done is set in running state, and closed when exiting it.
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done chan struct{}
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// cancel is set in running state. It cancels computation.
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cancel context.CancelFunc
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// waiters is the number of Gets outstanding.
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waiters uint
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// the function that will be used to populate the value
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function Function
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// value is set in completed state.
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value interface{}
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}
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// Has returns true if they key is currently valid for this store.
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func (s *Store) Has(key interface{}) bool {
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s.mu.Lock()
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defer s.mu.Unlock()
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_, found := s.entries[key]
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return found
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}
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// Bind returns a handle for the given key and function.
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//
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// Each call to bind will return the same handle if it is already bound.
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// Bind will always return a valid handle, creating one if needed.
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// Each key can only have one handle at any given time.
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// The value will be held for as long as the handle is, once it has been
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// generated.
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// Bind does not cause the value to be generated.
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func (s *Store) Bind(key interface{}, function Function) *Handle {
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// panic early if the function is nil
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// it would panic later anyway, but in a way that was much harder to debug
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if function == nil {
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panic("the function passed to bind must not be nil")
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}
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// check if we already have the key
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s.mu.Lock()
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defer s.mu.Unlock()
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h := s.get(key)
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if h != nil {
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// we have a handle already, just return it
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return h
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}
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// we have not seen this key before, add a new entry
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if s.entries == nil {
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s.entries = make(map[interface{}]uintptr)
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}
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h = &Handle{
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store: s,
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key: key,
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function: function,
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}
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// now add the weak reference to the handle into the map
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s.entries[key] = uintptr(unsafe.Pointer(h))
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// add the deletion the entry when the handle is garbage collected
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runtime.SetFinalizer(h, release)
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return h
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}
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// Find returns the handle associated with a key, if it is bound.
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//
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// It cannot cause a new handle to be generated, and thus may return nil.
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func (s *Store) Find(key interface{}) *Handle {
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s.mu.Lock()
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defer s.mu.Unlock()
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return s.get(key)
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}
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// Cached returns the value associated with a key.
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//
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// It cannot cause the value to be generated.
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// It will return the cached value, if present.
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func (s *Store) Cached(key interface{}) interface{} {
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h := s.Find(key)
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if h == nil {
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return nil
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}
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return h.Cached()
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}
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//go:nocheckptr
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// nocheckptr because: https://github.com/golang/go/issues/35125#issuecomment-545671062
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func (s *Store) get(key interface{}) *Handle {
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// this must be called with the store mutex already held
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e, found := s.entries[key]
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if !found {
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return nil
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}
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return (*Handle)(unsafe.Pointer(e))
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}
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// Cached returns the value associated with a handle.
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//
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// It will never cause the value to be generated.
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// It will return the cached value, if present.
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func (h *Handle) Cached() interface{} {
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h.mu.Lock()
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defer h.mu.Unlock()
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if h.state == stateCompleted {
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return h.value
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}
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return nil
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}
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// Get returns the value associated with a handle.
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//
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// If the value is not yet ready, the underlying function will be invoked.
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// This activates the handle, and it will remember the value for as long as it exists.
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// If ctx is cancelled, Get returns nil.
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func (h *Handle) Get(ctx context.Context) interface{} {
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if ctx.Err() != nil {
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return nil
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}
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h.mu.Lock()
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switch h.state {
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case stateIdle:
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return h.run(ctx)
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case stateRunning:
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return h.wait(ctx)
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case stateCompleted:
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defer h.mu.Unlock()
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return h.value
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default:
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panic("unknown state")
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}
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}
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// run starts h.function and returns the result. h.mu must be locked.
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func (h *Handle) run(ctx context.Context) interface{} {
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childCtx, cancel := context.WithCancel(xcontext.Detach(ctx))
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h.cancel = cancel
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h.state = stateRunning
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h.done = make(chan struct{})
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go func() {
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v := h.function(childCtx)
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if childCtx.Err() != nil {
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return
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}
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h.mu.Lock()
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defer h.mu.Unlock()
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// It's theoretically possible that the handle has been cancelled out
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// of the run that started us, and then started running again since we
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// checked childCtx above. Even so, that should be harmless, since each
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// run should produce the same results.
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if h.state != stateRunning {
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return
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}
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h.value = v
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h.function = nil
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h.state = stateCompleted
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close(h.done)
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}()
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return h.wait(ctx)
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}
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// wait waits for the value to be computed, or ctx to be cancelled. h.mu must be locked.
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func (h *Handle) wait(ctx context.Context) interface{} {
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h.waiters++
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done := h.done
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h.mu.Unlock()
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select {
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case <-done:
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h.mu.Lock()
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defer h.mu.Unlock()
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if h.state == stateCompleted {
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return h.value
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}
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return nil
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case <-ctx.Done():
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h.mu.Lock()
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defer h.mu.Unlock()
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h.waiters--
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if h.waiters == 0 && h.state == stateRunning {
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h.cancel()
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close(h.done)
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h.state = stateIdle
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h.done = nil
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h.cancel = nil
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}
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return nil
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}
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}
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func release(p interface{}) {
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h := p.(*Handle)
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h.store.mu.Lock()
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defer h.store.mu.Unlock()
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// there is a small gap between the garbage collector deciding that the handle
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// is liable for collection and the finalizer being called
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// if the handle is recovered during that time, you will end up with a valid
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// handle that no longer has an entry in the map, and that no longer has a
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// finalizer associated with it, but that is okay.
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delete(h.store.entries, h.key)
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}
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