137 lines
3.7 KiB
Go
137 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 should 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 set of events 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 nil, 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 nil, ErrSubscriptionClosed
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case err != nil:
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return nil, err
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}
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s.currentItem = next
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events := filter(s.req.Key, next.Events)
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if len(events) == 0 {
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continue
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}
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return events, nil
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}
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}
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// filter events to only those that match the key exactly.
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func filter(key string, events []Event) []Event {
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if key == "" || len(events) == 0 {
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return events
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}
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var count int
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for _, e := range events {
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if key == e.Key {
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count++
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}
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}
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// Only allocate a new slice if some events need to be filtered out
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switch count {
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case 0:
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return nil
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case len(events):
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return events
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}
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result := make([]Event, 0, count)
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for _, e := range events {
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if key == e.Key {
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result = append(result, e)
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}
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}
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return result
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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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