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