134 lines
3.7 KiB
Go
134 lines
3.7 KiB
Go
package stream
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import (
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"context"
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"errors"
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"sync/atomic"
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)
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const (
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// subscriptionStateOpen is the default state of a subscription. An open
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// subscription may receive new events.
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subscriptionStateOpen uint32 = 0
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// subscriptionStateClosed indicates that the subscription was closed, possibly
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// as a result of a change to an ACL token, and will not receive new events.
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// The subscriber must issue a new Subscribe request.
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subscriptionStateClosed uint32 = 1
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)
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// ErrSubscriptionClosed is a error signalling the subscription has been
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// closed. The client should Unsubscribe, then re-Subscribe.
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var ErrSubscriptionClosed = errors.New("subscription closed by server, client must reset state and resubscribe")
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// Subscription provides events on a Topic. Events may be filtered by Key.
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// Events are returned by Next(), and may start with a Snapshot of events.
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type Subscription struct {
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// state is accessed atomically 0 means open, 1 means closed with reload
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state uint32
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// req is the requests that we are responding to
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req SubscribeRequest
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// currentItem stores the current snapshot or topic buffer item we are on. It
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// is mutated by calls to Next.
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currentItem *bufferItem
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// forceClosed is closed when forceClose is called. It is used by
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// EventPublisher to cancel Next().
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forceClosed chan struct{}
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// unsub is a function set by EventPublisher that is called to free resources
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// when the subscription is no longer needed.
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// It must be safe to call the function from multiple goroutines and the function
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// must be idempotent.
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unsub func()
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}
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// SubscribeRequest identifies the types of events the subscriber would like to
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// receiver. Topic and Token are required.
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type SubscribeRequest struct {
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Topic Topic
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Key string
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Token string
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Index uint64
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}
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// newSubscription return a new subscription. The caller is responsible for
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// calling Unsubscribe when it is done with the subscription, to free resources.
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func newSubscription(req SubscribeRequest, item *bufferItem, unsub func()) *Subscription {
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return &Subscription{
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forceClosed: make(chan struct{}),
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req: req,
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currentItem: item,
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unsub: unsub,
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}
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}
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// Next returns the next Event to deliver. It must only be called from a
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// single goroutine concurrently as it mutates the Subscription.
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func (s *Subscription) Next(ctx context.Context) (Event, error) {
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if atomic.LoadUint32(&s.state) == subscriptionStateClosed {
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return Event{}, ErrSubscriptionClosed
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}
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for {
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next, err := s.currentItem.Next(ctx, s.forceClosed)
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switch {
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case err != nil && atomic.LoadUint32(&s.state) == subscriptionStateClosed:
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return Event{}, ErrSubscriptionClosed
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case err != nil:
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return Event{}, err
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}
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s.currentItem = next
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if len(next.Events) == 0 {
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continue
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}
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event, ok := filterByKey(s.req, next.Events)
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if !ok {
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continue
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}
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return event, nil
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}
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}
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func newEventFromBatch(req SubscribeRequest, events []Event) Event {
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first := events[0]
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if len(events) == 1 {
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return first
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}
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return Event{
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Topic: req.Topic,
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Key: req.Key,
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Index: first.Index,
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Payload: events,
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}
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}
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func filterByKey(req SubscribeRequest, events []Event) (Event, bool) {
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event := newEventFromBatch(req, events)
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if req.Key == "" {
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return event, true
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}
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fn := func(e Event) bool {
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return req.Key == e.Key
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}
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return event.Filter(fn)
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}
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// Close the subscription. Subscribers will receive an error when they call Next,
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// and will need to perform a new Subscribe request.
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// It is safe to call from any goroutine.
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func (s *Subscription) forceClose() {
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swapped := atomic.CompareAndSwapUint32(&s.state, subscriptionStateOpen, subscriptionStateClosed)
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if swapped {
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close(s.forceClosed)
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}
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}
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// Unsubscribe the subscription, freeing resources.
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func (s *Subscription) Unsubscribe() {
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s.unsub()
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}
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