66bdb695ae
If the callback passed to Once panics it was leaving the waiter in place that would never be completed. Move writing the value and signalling the waiter to defer. Test: TestOncePerPanic Change-Id: Icc4d3b779a79914fcd881d61d38dffcc2f591c39
114 lines
3.8 KiB
Go
114 lines
3.8 KiB
Go
// Copyright 2016 Google Inc. All rights reserved.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package android
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import (
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"fmt"
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"sync"
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)
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type OncePer struct {
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values sync.Map
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}
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type onceValueWaiter chan bool
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func (once *OncePer) maybeWaitFor(key OnceKey, value interface{}) interface{} {
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if wait, isWaiter := value.(onceValueWaiter); isWaiter {
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// The entry in the map is a placeholder waiter because something else is constructing the value
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// wait until the waiter is signalled, then load the real value.
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<-wait
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value, _ = once.values.Load(key)
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if _, isWaiter := value.(onceValueWaiter); isWaiter {
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panic(fmt.Errorf("Once() waiter completed but key is still not valid"))
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}
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}
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return value
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}
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// Once computes a value the first time it is called with a given key per OncePer, and returns the
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// value without recomputing when called with the same key. key must be hashable. If value panics
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// the panic will be propagated but the next call to Once with the same key will return nil.
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func (once *OncePer) Once(key OnceKey, value func() interface{}) interface{} {
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// Fast path: check if the key is already in the map
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if v, ok := once.values.Load(key); ok {
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return once.maybeWaitFor(key, v)
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}
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// Slow path: create a OnceValueWrapper and attempt to insert it
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waiter := make(onceValueWaiter)
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if v, loaded := once.values.LoadOrStore(key, waiter); loaded {
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// Got a value, something else inserted its own waiter or a constructed value
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return once.maybeWaitFor(key, v)
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}
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// The waiter is inserted, call the value constructor, store it, and signal the waiter. Use defer in case
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// the function panics.
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var v interface{}
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defer func() {
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once.values.Store(key, v)
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close(waiter)
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}()
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v = value()
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return v
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}
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// Get returns the value previously computed with Once for a given key. If Once has not been called for the given
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// key Get will panic.
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func (once *OncePer) Get(key OnceKey) interface{} {
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v, ok := once.values.Load(key)
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if !ok {
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panic(fmt.Errorf("Get() called before Once()"))
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}
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return once.maybeWaitFor(key, v)
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}
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// OnceStringSlice is the same as Once, but returns the value cast to a []string
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func (once *OncePer) OnceStringSlice(key OnceKey, value func() []string) []string {
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return once.Once(key, func() interface{} { return value() }).([]string)
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}
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// OnceStringSlice is the same as Once, but returns two values cast to []string
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func (once *OncePer) Once2StringSlice(key OnceKey, value func() ([]string, []string)) ([]string, []string) {
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type twoStringSlice [2][]string
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s := once.Once(key, func() interface{} {
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var s twoStringSlice
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s[0], s[1] = value()
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return s
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}).(twoStringSlice)
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return s[0], s[1]
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}
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// OnceKey is an opaque type to be used as the key in calls to Once.
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type OnceKey struct {
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key interface{}
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}
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// NewOnceKey returns an opaque OnceKey object for the provided key. Two calls to NewOnceKey with the same key string
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// DO NOT produce the same OnceKey object.
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func NewOnceKey(key string) OnceKey {
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return OnceKey{&key}
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}
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// NewCustomOnceKey returns an opaque OnceKey object for the provided key. The key can be any type that is valid as the
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// key in a map, i.e. comparable. Two calls to NewCustomOnceKey with key values that compare equal will return OnceKey
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// objects that access the same value stored with Once.
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func NewCustomOnceKey(key interface{}) OnceKey {
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return OnceKey{key}
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}
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