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// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package cache
import (
"bytes"
"compress/gzip"
"context"
"encoding/gob"
"fmt"
"net/http"
"time"
"google.golang.org/appengine/log"
"google.golang.org/appengine/memcache"
)
// TimeKey specifies the memcache entity that keeps the logical datastore time.
var TimeKey = "cachetime"
const (
nocache = "nocache"
expiry = 10 * time.Minute
)
func newTime() uint64 { return uint64(time.Now().Unix()) << 32 }
// Now returns the current logical datastore time to use for cache lookups.
func Now(c context.Context) uint64 {
t, err := memcache.Increment(c, TimeKey, 0, newTime())
if err != nil {
log.Errorf(c, "cache.Now: %v", err)
return 0
}
return t
}
// Tick sets the current logical datastore time to a never-before-used time
// and returns that time. It should be called to invalidate the cache.
func Tick(c context.Context) uint64 {
t, err := memcache.Increment(c, TimeKey, 1, newTime())
if err != nil {
log.Errorf(c, "cache.Tick: %v", err)
return 0
}
return t
}
// Get fetches data for name at time now from memcache and unmarshals it into
// value. It reports whether it found the cache record and logs any errors to
// the admin console.
func Get(c context.Context, r *http.Request, now uint64, name string, value interface{}) bool {
if now == 0 || r.FormValue(nocache) != "" {
return false
}
key := fmt.Sprintf("%s.%d", name, now)
_, err := gzipGobCodec.Get(c, key, value)
if err == nil {
log.Debugf(c, "cache hit %q", key)
return true
}
log.Debugf(c, "cache miss %q", key)
if err != memcache.ErrCacheMiss {
log.Errorf(c, "get cache %q: %v", key, err)
}
return false
}
// Set puts value into memcache under name at time now.
// It logs any errors to the admin console.
func Set(c context.Context, r *http.Request, now uint64, name string, value interface{}) {
if now == 0 || r.FormValue(nocache) != "" {
return
}
key := fmt.Sprintf("%s.%d", name, now)
err := gzipGobCodec.Set(c, &memcache.Item{
Key: key,
Object: value,
Expiration: expiry,
})
if err != nil {
log.Errorf(c, "set cache %q: %v", key, err)
}
}
var gzipGobCodec = memcache.Codec{Marshal: marshal, Unmarshal: unmarshal}
func marshal(v interface{}) ([]byte, error) {
var b bytes.Buffer
zw := gzip.NewWriter(&b)
if err := gob.NewEncoder(zw).Encode(v); err != nil {
return nil, err
}
if err := zw.Close(); err != nil {
return nil, err
}
return b.Bytes(), nil
}
func unmarshal(b []byte, v interface{}) error {
zr, err := gzip.NewReader(bytes.NewReader(b))
if err != nil {
return err
}
return gob.NewDecoder(zr).Decode(v)
}
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