126 lines
3.0 KiB
Go
126 lines
3.0 KiB
Go
/*
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Copyright 2014 Google Inc. All rights reserved.
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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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http://www.apache.org/licenses/LICENSE-2.0
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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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*/
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package util
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import (
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"sync"
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"time"
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)
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// T stands in for any type in TimeCache
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// Should make it easy to use this as a template for an autogenerator
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// if we ever start doing that.
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type T interface{}
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type TimeCache interface {
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// Get will fetch an item from the cache if
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// it is present and recent enough.
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Get(key string) T
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}
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type timeCacheEntry struct {
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item T
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lastUpdate time.Time
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}
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type timeCache struct {
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clock Clock
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fillFunc func(string) T
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ttl time.Duration
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inFlight map[string]chan T
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inFlightLock sync.Mutex
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cache map[string]timeCacheEntry
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lock sync.RWMutex
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}
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// NewTimeCache returns a cache which calls fill to fill its entries, and
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// forgets entries after ttl has passed.
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func NewTimeCache(clock Clock, ttl time.Duration, fill func(key string) T) TimeCache {
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return &timeCache{
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clock: clock,
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fillFunc: fill,
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inFlight: map[string]chan T{},
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cache: map[string]timeCacheEntry{},
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ttl: ttl,
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}
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}
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// Get returns the value of key from the cache, if it is present
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// and recent enough; otherwise, it blocks while it gets the value.
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func (c *timeCache) Get(key string) T {
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if item, ok := c.get(key); ok {
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return item
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}
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// We need to fill the cache. Calling the function could be
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// expensive, so do it while unlocked.
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wait := c.fillOrWait(key)
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item := <-wait
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// Put it back in the channel in case there's multiple waiters
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// (this channel is non-blocking)
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wait <- item
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return item
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}
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// returns the item and true if it is found and not expired, otherwise nil and false.
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func (c *timeCache) get(key string) (T, bool) {
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c.lock.RLock()
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defer c.lock.RUnlock()
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data, ok := c.cache[key]
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now := c.clock.Now()
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if !ok || now.Sub(data.lastUpdate) > c.ttl {
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return nil, false
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}
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return data.item, true
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}
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func (c *timeCache) fillOrWait(key string) chan T {
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c.inFlightLock.Lock()
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defer c.inFlightLock.Unlock()
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// Already a call in progress?
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if current, ok := c.inFlight[key]; ok {
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return current
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}
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// We are the first, so we have to make the call.
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result := make(chan T, 1) // non-blocking
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c.inFlight[key] = result
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go func() {
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// Make potentially slow call
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data := timeCacheEntry{
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item: c.fillFunc(key),
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lastUpdate: c.clock.Now(),
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}
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result <- data.item
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// Store in cache
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c.lock.Lock()
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c.cache[key] = data
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c.lock.Unlock()
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// Remove in flight entry
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c.inFlightLock.Lock()
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delete(c.inFlight, key)
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c.inFlightLock.Unlock()
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}()
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return result
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}
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