func updateCacheTime(c appengine.Context, seq int64) {
const key = rootMemcacheKey
bseq := []byte(strconv.FormatInt(seq, 10))
for tries := 0; tries < 10; tries++ {
item, err := memcache.Get(c, key)
if err != nil {
c.Infof("memcache.Get %q: %v", key, err)
err = memcache.Add(c, &memcache.Item{Key: key, Value: bseq})
if err == nil {
c.Infof("memcache.Add %q %q ok", key, bseq)
return
}
c.Infof("memcache.Add %q %q: %v", key, bseq, err)
}
v, err := strconv.ParseInt(string(item.Value), 10, 64)
if err != nil {
c.Criticalf("memcache.Get %q = %q (%v)", key, item.Value, err)
return
}
if v >= seq {
return
}
item.Value = bseq
err = memcache.CompareAndSwap(c, item)
if err == nil {
c.Infof("memcache.CAS %q %d->%d ok", key, v, seq)
return
}
c.Infof("memcache.CAS %q %d->%d: %v", key, v, seq, err)
}
c.Criticalf("repeatedly failed to update root key")
}
func cacheSafeSet(c appengine.Context, item *memcache.Item) error {
var buf bytes.Buffer
err := gob.NewEncoder(&buf).Encode(item.Object)
if err != nil {
return err
}
swap := item.Value != nil
item.Value = buf.Bytes()
if swap {
err = memcache.CompareAndSwap(c, item)
switch err {
case memcache.ErrCASConflict:
// OK, cache item set by another request
return nil
case memcache.ErrNotStored:
// Item expired. Try adding below.
default:
return err
}
}
err = memcache.Add(c, item)
if err == memcache.ErrNotStored {
// OK, cache item set by another request
err = nil
}
return err
}
func (self *channelAPI) sendSlowNotifications(perma_blobref string) {
// Try 10 times then give up
for i := 0; i < 10; i++ {
item, err := memcache.Get(self.c, "notify-"+perma_blobref)
if err == memcache.ErrCacheMiss {
// Try to start a task
if item != nil {
binary.LittleEndian.PutUint64(item.Value, 1)
err = memcache.CompareAndSwap(self.c, item)
} else {
item = &memcache.Item{Key: "notify-" + perma_blobref, Value: make([]byte, 8)}
binary.LittleEndian.PutUint64(item.Value, 1)
err = memcache.Add(self.c, item)
}
if err == memcache.ErrCASConflict {
// Somebody else managed to launch the task first
return
}
// Enqueue the task
t := taskqueue.NewPOSTTask("/internal/notify", map[string][]string{"perma": {perma_blobref}})
t.Delay = 30 * 1000000
if _, err := taskqueue.Add(self.c, t, ""); err != nil {
log.Printf("ERR: " + err.String())
return
}
return
} else if err != nil {
continue
}
val := binary.LittleEndian.Uint64(item.Value)
val++
binary.LittleEndian.PutUint64(item.Value, val)
err = memcache.CompareAndSwap(self.c, item)
// The task is enqueued and has not yet sent notifications
if err == nil {
return
}
}
}
func bumpGeneration(c appengine.Context) {
if item, err := memcache.Get(c, generationKey); err == memcache.ErrCacheMiss {
initialValue, _ := json.Marshal(0)
newItem := &memcache.Item{
Key: generationKey,
Value: []byte(initialValue),
}
memcache.Set(c, newItem)
} else {
var oldValue int
json.Unmarshal(item.Value, &oldValue)
newValue := oldValue + 1
item.Value, _ = json.Marshal(newValue)
memcache.CompareAndSwap(c, item)
}
}
func channelSetup(h HandlerArgs) (interface{}, int) {
id := h.Vars["id"]
channelDescriptor := ""
status := http.StatusOK
if len(id) == config.AllowedCyphIdLength && config.AllowedCyphIds.MatchString(id) {
if item, err := memcache.Get(h.Context, id); err != memcache.ErrCacheMiss {
oldValue := item.Value
item.Value = []byte{}
if err := memcache.CompareAndSwap(h.Context, item); err != memcache.ErrCASConflict {
valueLines := strings.Split(string(oldValue), "\n")
timestamp, _ := strconv.ParseInt(valueLines[0], 10, 64)
if time.Now().Unix()-timestamp < config.NewCyphTimeout {
channelDescriptor = valueLines[1]
}
}
} else {
channelDescriptor = h.Request.FormValue("channelDescriptor")
if len(channelDescriptor) > config.MaxChannelDescriptorLength {
channelDescriptor = ""
}
if channelDescriptor != "" {
memcache.Set(h.Context, &memcache.Item{
Key: id,
Value: []byte(strconv.FormatInt(time.Now().Unix(), 10) + "\n" + channelDescriptor),
Expiration: config.MemcacheExpiration,
})
}
}
}
if channelDescriptor == "" {
status = http.StatusNotFound
}
return channelDescriptor, status
}
/*
CAS will replace expected with replacement in memcache if expected is the current value.
*/
func CAS(c TransactionContext, key string, expected, replacement interface{}) (success bool, err error) {
keyHash, err := Keyify(key)
if err != nil {
return
}
var item *memcache.Item
if item, err = memcache.Get(c, keyHash); err != nil {
if err == memcache.ErrCacheMiss {
err = nil
} else {
err = errors.Errorf("Error doing Get %#v: %v", keyHash, err)
}
return
}
var encoded []byte
if encoded, err = Codec.Marshal(expected); err != nil {
return
}
if bytes.Compare(encoded, item.Value) != 0 {
success = false
return
}
if encoded, err = Codec.Marshal(replacement); err != nil {
return
}
item.Value = encoded
if err = memcache.CompareAndSwap(c, item); err != nil {
if err == memcache.ErrCASConflict {
err = nil
} else {
marshalled, _ := Codec.Marshal(replacement)
err = errors.Errorf("Error doing CompareAndSwap %#v to %v bytes: %v", item.Key, len(marshalled), err)
}
return
}
success = true
return
}
func CompareAndSwap(c appengine.Context, item *memcache.Item) error {
return memcache.CompareAndSwap(c, item)
}
// McacheSet is a generic memcache saving function.
// It takes scalars as well as structs.
//
// Integers and strings are put into the memcache Value []byte
// structs are put into the memcache *Object* - using memcache.JSON
// Todo: types WrapString and WrapInt should be handled like string/int
//
// Scalars are tentatively saved using the CAS (compare and save) methods
func McacheSet(c appengine.Context, skey string, str_int_struct interface{}) {
var err error
var val string
tMold := reflect.TypeOf(str_int_struct)
stMold := tMold.Name() // strangely this is empty
stMold = fmt.Sprintf("%T", str_int_struct) // unlike this
if stMold != "int" &&
stMold != "string" &&
stMold != "dsu.WrapInt" &&
stMold != "dsu.WrapString" {
// struct - save it with JSON encoder
n := tMold.NumField()
_ = n
miPut := &memcache.Item{
Key: skey,
Value: []byte(tMold.Name()), // sadly - value is ignored
Object: &str_int_struct,
Expiration: 3600 * time.Second,
}
memcache.JSON.Set(c, miPut)
c.Infof("mcache set obj key %v[%s] - err %v", skey, stMold, err)
} else {
// scalar value - save it
switch chamaeleon := str_int_struct.(type) {
default:
panic(fmt.Sprintf("only string or int - instead: -%T", str_int_struct))
case nil:
val = ""
case WrapString:
val = chamaeleon.S
case string:
val = chamaeleon
case int:
val = util.Itos(chamaeleon)
case WrapInt:
val = util.Itos(chamaeleon.I)
}
/*
This is a Compare and Set (CAS) implementation of "set".
It implements optimistic locking.
We fetch the item first, then modify it, then put it back.
We rely on the hidden "casID" of the memcache item,
to detect intermittent changes by competitors.
Biggest downside is the additional roundtrip for the fetch.
Second downside: We should implement a retry after failure.
Instead I resorted to a simple "SET"
Upside: Prevention of race conditions.
But race conditions only matter if newval = f(oldval)
Otherwise last one wins should apply anyway.
*/
maxTries := 3
miCas, eget := memcache.Get(c, skey) // compare and swap
for i := 0; i <= maxTries; i++ {
if i == maxTries {
panic(fmt.Sprintf("memcache set CAS failed after %v attempts", maxTries))
}
var eput error
var putMode = ""
if eget != memcache.ErrCacheMiss {
putMode = "CAS"
miCas.Value = []byte(val)
eput = memcache.CompareAndSwap(c, miCas)
} else {
putMode = "ADD"
miCas := &memcache.Item{
Key: skey,
Value: []byte(val),
}
eput = memcache.Add(c, miCas)
}
if eput == memcache.ErrCASConflict {
c.Errorf("\t memcache CAS FAILED - concurrent update?")
// we brutally fallback to set():
miCas := &memcache.Item{
Key: skey,
Value: []byte(val),
}
eset := memcache.Set(c, miCas)
util_err.Err_log(eset)
time.Sleep(10 * time.Millisecond)
continue
}
if eput == memcache.ErrNotStored {
c.Errorf("\t memcache save FAILED - no idea why it would")
time.Sleep(10 * time.Millisecond)
continue
}
c.Infof("mcache set scalar %v[%T]=%v - mode %v - eget/eput: %v/%v",
skey, str_int_struct, val, putMode, eget, eput)
break
}
}
}