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semaphore.go
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semaphore.go
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package rsemaphore
import (
"strings"
"sync"
"time"
"strconv"
"fmt"
"github.com/go-redis/redis"
"github.com/pkg/errors"
"github.com/rs/xid"
)
const (
DefaultPrefix = "redis_semaphore"
defaultAvailableKey = "available"
defaultGrabbedKey = "grabbed"
defaultInitializedKey = "initialized"
)
var (
ErrTimeout = errors.New("semaphore: timeout")
)
type Semaphore struct {
opts *SemaphoreOptions
keyAvailable string
keyGrabbed string
keyInitialized string
tokens []string
m sync.Mutex
}
func (s *Semaphore) init() (error error) {
defer func() {
if error != nil {
//something failed, return the state of the initialized key
s.opts.rc.GetSet(s.keyInitialized, 0).Int64()
} else {
//try and cleanup stale workers in any case
if err := s.CleanupStaleWorkers(); err != nil {
error = err
return
}
}
}()
//check if the initialized key is set, if it is, that means someone is already initializing the semaphore
set, err := s.opts.rc.SetNX(s.keyInitialized, 1, s.opts.initKeyExpiration).Result()
if err != nil {
error = errors.Wrap(err, "set initialized key")
return
}
if !set {
return
}
//if the semaphore does not appear to be initialized (never initialized or key is expired)
//check if the workers queue is available.
//It's guaranteed to be available when the semaphore is actually initialized since it's executed in a transaction
if exists, err := s.opts.rc.Exists(s.keyAvailable).Result(); err != nil {
error = errors.Wrap(err, "keyAvailable exists")
return
} else if exists == 1 {
return
}
//push the workers in a transaction
pushPipe := s.opts.rc.Pipeline()
var i int64
for i = 0; i < s.opts.size; i++ {
pushPipe.LPush(s.keyAvailable, s.generateToken())
}
pushPipe.Expire(s.keyAvailable, s.opts.keysExpiration)
pushPipe.LTrim(s.keyAvailable, 0, s.opts.size-1)
if _, err := pushPipe.Exec(); err != nil {
error = errors.Wrap(err, "pushing worker tokens")
return
}
return
}
func (s *Semaphore) generateToken() string {
return xid.New().String()
}
func (s *Semaphore) acquireToken(token string) error {
pipe := s.opts.rc.Pipeline()
pipe.HSet(s.keyGrabbed, token, time.Now().UnixNano())
pipe.Expire(s.keyGrabbed, s.opts.keysExpiration)
if _, err := pipe.Exec(); err != nil {
return errors.Wrap(err, "set get grabbed")
}
s.m.Lock()
s.tokens = append(s.tokens, token)
s.m.Unlock()
return nil
}
func (s *Semaphore) key(key string) string {
return strings.Join([]string{s.opts.prefix, key, fmt.Sprintf("%d", s.opts.size)}, "_")
}
func (s *Semaphore) Acquire(timeout time.Duration) error {
res, err := s.opts.rc.BLPop(timeout, s.keyAvailable).Result()
if err != nil {
if err == redis.Nil {
return ErrTimeout
}
return err
}
token := res[1]
return s.acquireToken(token)
}
func (s *Semaphore) Release() error {
s.m.Lock()
if len(s.tokens) == 0 {
panic("semaphore: negative release")
}
token := s.tokens[0]
s.tokens = s.tokens[1:]
s.m.Unlock()
pipe := s.opts.rc.Pipeline()
pipe.RPush(s.keyAvailable, token)
pipe.HDel(s.keyGrabbed, token)
if _, err := pipe.Exec(); err != nil {
return err
}
return nil
}
func (s *Semaphore) TryAcquire() (bool, error) {
res, err := s.opts.rc.LPop(s.keyAvailable).Result()
if err != nil {
if err == redis.Nil {
return false, nil
}
return false, err
} else if res == "" {
return false, nil
}
if err := s.acquireToken(res); err != nil {
return false, err
}
return true, nil
}
func (s *Semaphore) Available() (int64, error) {
return s.opts.rc.LLen(s.keyAvailable).Result()
}
func (s *Semaphore) CleanupStaleWorkers() error {
//cleanup stale keys, if any
grabbed, err := s.opts.rc.HGetAll(s.keyGrabbed).Result()
if err != nil {
return errors.Wrap(err, "get grabbed keys")
}
for k, v := range grabbed {
unix, err := strconv.ParseInt(v, 10, 64)
if err != nil {
return errors.Wrap(err, "parsing grabbed key timestamp")
}
t := time.Unix(0, unix)
if t.Add(s.opts.staleTimeout).Before(time.Now()) {
if err := s.opts.rc.HDel(s.keyGrabbed, k).Err(); err != nil {
return errors.Wrap(err, "deleting stale key")
}
if err := s.opts.rc.LPush(s.keyAvailable, k).Err(); err != nil {
return errors.Wrap(err, "pushing back stale key to queue")
}
}
}
return nil
}
func New(opts ...SemaphoreOption) *Semaphore {
s := &Semaphore{
opts: &SemaphoreOptions{},
}
for _, o := range opts {
o(s.opts)
}
if s.opts.size == 0 {
s.opts.size = 5
}
if s.opts.prefix == "" {
s.opts.prefix = DefaultPrefix
}
if s.opts.staleTimeout == 0 {
s.opts.staleTimeout = 10 * time.Minute
}
if s.opts.keysExpiration == 0 {
s.opts.keysExpiration = 24 * time.Hour
}
if s.opts.initKeyExpiration == 0 {
s.opts.initKeyExpiration = 30 * time.Second
}
if s.opts.initRetry == 0 {
s.opts.initRetry = 5
}
if s.opts.rc == nil {
if s.opts.ro == nil {
panic("semaphore: supply redis opts")
}
s.opts.rc = redis.NewUniversalClient(s.opts.ro)
}
s.tokens = make([]string, 0, s.opts.size)
s.keyAvailable = s.key(defaultAvailableKey)
s.keyGrabbed = s.key(defaultGrabbedKey)
s.keyInitialized = s.key(defaultInitializedKey)
var initErr error
for i := 0; i < s.opts.initRetry; i++ {
if err := s.init(); err != nil {
initErr = err
} else {
initErr = nil
break
}
}
if initErr != nil {
panic(errors.Wrap(initErr, "semaphore: init"))
}
return s
}