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ring.go
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ring.go
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package storage
import (
"errors"
"runtime"
"sync/atomic"
)
var (
// ErrDisposed is returned when an operation is performed on a disposed
// queue.
ErrDisposed = errors.New(`queue: disposed`)
// ErrTimeout is returned when an applicable queue operation times out.
ErrTimeout = errors.New(`queue: poll timed out`)
// ErrEmptyQueue is returned when an non-applicable queue operation was called
// due to the queue's empty item state
ErrEmptyQueue = errors.New(`queue: empty queue`)
)
// roundUp takes a uint64 greater than 0 and rounds it up to the next
// power of 2.
func roundUp(v uint64) uint64 {
v--
v |= v >> 1
v |= v >> 2
v |= v >> 4
v |= v >> 8
v |= v >> 16
v |= v >> 32
v++
return v
}
type node struct {
position uint64
data interface{}
}
type nodes []*node
// RingBuffer is a MPMC buffer that achieves threadsafety with CAS operations
// only. A put on full or get on empty call will block until an item
// is put or retrieved. Calling Dispose on the RingBuffer will unblock
// any blocked threads with an error. This buffer is similar to the buffer
// described here: http://www.1024cores.net/home/lock-free-algorithms/queues/bounded-mpmc-queue
// with some minor additions.
type RingBuffer struct {
_padding0 [8]uint64
queue uint64
_padding1 [8]uint64
dequeue uint64
_padding2 [8]uint64
mask, disposed uint64
_padding3 [8]uint64
nodes nodes
}
func (rb *RingBuffer) init(size uint64) {
size = roundUp(size)
rb.nodes = make(nodes, size)
for i := uint64(0); i < size; i++ {
rb.nodes[i] = &node{position: i}
}
rb.mask = size - 1 // so we don't have to do this with every put/get operation
}
// Put adds the provided item to the queue. If the queue is full, this
// call will block until an item is added to the queue or Dispose is called
// on the queue. An error will be returned if the queue is disposed.
func (rb *RingBuffer) Put(item interface{}) error {
_, err := rb.put(item, false)
return err
}
// Offer adds the provided item to the queue if there is space. If the queue
// is full, this call will return false. An error will be returned if the
// queue is disposed.
func (rb *RingBuffer) Offer(item interface{}) (bool, error) {
return rb.put(item, true)
}
func (rb *RingBuffer) put(item interface{}, offer bool) (bool, error) {
var n *node
pos := atomic.LoadUint64(&rb.queue)
L:
for {
if atomic.LoadUint64(&rb.disposed) == 1 {
return false, ErrDisposed
}
n = rb.nodes[pos&rb.mask]
seq := atomic.LoadUint64(&n.position)
switch dif := seq - pos; {
case dif == 0:
if atomic.CompareAndSwapUint64(&rb.queue, pos, pos+1) {
break L
}
case dif < 0:
panic(`Ring buffer in a compromised state during a put operation.`)
default:
pos = atomic.LoadUint64(&rb.queue)
}
if offer {
return false, nil
}
runtime.Gosched() // free up the cpu before the next iteration
}
n.data = item
atomic.StoreUint64(&n.position, pos+1)
return true, nil
}
// Poll will return the next item in the queue. This call will block
// if the queue is empty. This call will unblock when an item is added
// to the queue, Dispose is called on the queue, or the timeout is reached. An
// error will be returned if the queue is disposed or a timeout occurs. A
// non-positive timeout will block indefinitely.
func (rb *RingBuffer) Poll(block bool) (interface{}, error) {
var (
n *node
pos = atomic.LoadUint64(&rb.dequeue)
)
L:
for {
if atomic.LoadUint64(&rb.disposed) == 1 {
return nil, ErrDisposed
}
n = rb.nodes[pos&rb.mask]
seq := atomic.LoadUint64(&n.position)
switch dif := seq - (pos + 1); {
case dif == 0:
if atomic.CompareAndSwapUint64(&rb.dequeue, pos, pos+1) {
break L
}
case dif < 0:
panic(`Ring buffer in compromised state during a get operation.`)
default:
pos = atomic.LoadUint64(&rb.dequeue)
}
if !block {
return nil, nil
}
runtime.Gosched() // free up the cpu before the next iteration
}
data := n.data
n.data = nil
atomic.StoreUint64(&n.position, pos+rb.mask+1)
return data, nil
}
// Len returns the number of items in the queue.
func (rb *RingBuffer) Len() uint64 {
return atomic.LoadUint64(&rb.queue) - atomic.LoadUint64(&rb.dequeue)
}
// Cap returns the capacity of this ring buffer.
func (rb *RingBuffer) Cap() uint64 {
return uint64(len(rb.nodes))
}
// Dispose will dispose of this queue and free any blocked threads
// in the Put and/or Get methods. Calling those methods on a disposed
// queue will return an error.
func (rb *RingBuffer) Dispose() {
atomic.CompareAndSwapUint64(&rb.disposed, 0, 1)
}
// IsDisposed will return a bool indicating if this queue has been
// disposed.
func (rb *RingBuffer) IsDisposed() bool {
return atomic.LoadUint64(&rb.disposed) == 1
}
// NewRingBuffer will allocate, initialize, and return a ring buffer
// with the specified size.
func NewRingBuffer(size uint64) *RingBuffer {
rb := &RingBuffer{}
rb.init(size)
return rb
}