144 lines
4.6 KiB
Go
144 lines
4.6 KiB
Go
package cache
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import (
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"container/heap"
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"time"
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)
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// cacheEntry stores a single cache entry.
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//
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// Note that this isn't a very optimized structure currently. There are
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// a lot of improvements that can be made here in the long term.
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type cacheEntry struct {
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// Fields pertaining to the actual value
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Value interface{}
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Error error
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Index uint64
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// Metadata that is used for internal accounting
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Valid bool // True if the Value is set
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Fetching bool // True if a fetch is already active
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Waiter chan struct{} // Closed when this entry is invalidated
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// Expiry contains information about the expiration of this
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// entry. This is a pointer as its shared as a value in the
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// expiryHeap as well.
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Expiry *cacheEntryExpiry
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}
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// cacheEntryExpiry contains the expiration information for a cache
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// entry. Any modifications to this struct should be done only while
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// the Cache entriesLock is held.
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type cacheEntryExpiry struct {
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Key string // Key in the cache map
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Expires time.Time // Time when entry expires (monotonic clock)
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TTL time.Duration // TTL for this entry to extend when resetting
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HeapIndex int // Index in the heap
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}
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// Reset resets the expiration to be the ttl duration from now.
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func (e *cacheEntryExpiry) Reset() {
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e.Expires = time.Now().Add(e.TTL)
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}
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// expiryHeap is a heap implementation that stores information about
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// when entires expire. Implements container/heap.Interface.
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//
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// All operations on the heap and read/write of the heap contents require
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// the proper entriesLock to be held on Cache.
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type expiryHeap struct {
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Entries []*cacheEntryExpiry
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// NotifyCh is sent a value whenever the 0 index value of the heap
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// changes. This can be used to detect when the earliest value
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// changes.
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//
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// There is a single edge case where the heap will not automatically
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// send a notification: if heap.Fix is called manually and the index
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// changed is 0 and the change doesn't result in any moves (stays at index
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// 0), then we won't detect the change. To work around this, please
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// always call the expiryHeap.Fix method instead.
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NotifyCh chan struct{}
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}
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// Identical to heap.Fix for this heap instance but will properly handle
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// the edge case where idx == 0 and no heap modification is necessary,
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// and still notify the NotifyCh.
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//
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// This is important for cache expiry since the expiry time may have been
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// extended and if we don't send a message to the NotifyCh then we'll never
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// reset the timer and the entry will be evicted early.
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func (h *expiryHeap) Fix(entry *cacheEntryExpiry) {
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idx := entry.HeapIndex
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heap.Fix(h, idx)
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// This is the edge case we handle: if the prev (idx) and current (HeapIndex)
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// is zero, it means the head-of-line didn't change while the value
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// changed. Notify to reset our expiry worker.
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if idx == 0 && entry.HeapIndex == 0 {
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h.notify()
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}
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}
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func (h *expiryHeap) Len() int { return len(h.Entries) }
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func (h *expiryHeap) Swap(i, j int) {
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h.Entries[i], h.Entries[j] = h.Entries[j], h.Entries[i]
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h.Entries[i].HeapIndex = i
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h.Entries[j].HeapIndex = j
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// If we're moving the 0 index, update the channel since we need
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// to re-update the timer we're waiting on for the soonest expiring
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// value.
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if i == 0 || j == 0 {
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h.notify()
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}
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}
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func (h *expiryHeap) Less(i, j int) bool {
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// The usage of Before here is important (despite being obvious):
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// this function uses the monotonic time that should be available
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// on the time.Time value so the heap is immune to wall clock changes.
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return h.Entries[i].Expires.Before(h.Entries[j].Expires)
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}
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// heap.Interface, this isn't expected to be called directly.
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func (h *expiryHeap) Push(x interface{}) {
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entry := x.(*cacheEntryExpiry)
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// Set initial heap index, if we're going to the end then Swap
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// won't be called so we need to initialize
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entry.HeapIndex = len(h.Entries)
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// For the first entry, we need to trigger a channel send because
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// Swap won't be called; nothing to swap! We can call it right away
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// because all heap operations are within a lock.
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if len(h.Entries) == 0 {
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h.notify()
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}
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h.Entries = append(h.Entries, entry)
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}
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// heap.Interface, this isn't expected to be called directly.
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func (h *expiryHeap) Pop() interface{} {
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old := h.Entries
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n := len(old)
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x := old[n-1]
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h.Entries = old[0 : n-1]
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return x
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}
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func (h *expiryHeap) notify() {
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select {
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case h.NotifyCh <- struct{}{}:
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// Good
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default:
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// If the send would've blocked, we just ignore it. The reason this
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// is safe is because NotifyCh should always be a buffered channel.
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// If this blocks, it means that there is a pending message anyways
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// so the receiver will restart regardless.
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}
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}
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