open-consul/agent/consul/stream/subscription.go

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package stream
import (
"context"
"errors"
"sync/atomic"
)
const (
// subscriptionStateOpen is the default state of a subscription. An open
// subscription may receive new events.
subscriptionStateOpen uint32 = 0
// subscriptionStateClosed indicates that the subscription was closed, possibly
// as a result of a change to an ACL token, and will not receive new events.
// The subscriber must issue a new Subscribe request.
subscriptionStateClosed uint32 = 1
)
var (
// ErrSubscriptionReload is a error signalling reload event should be sent to
// the client and the server should close.
ErrSubscriptionReload = errors.New("subscription closed by server, client should retry")
)
// Subscription holds state about a single Subscribe call. Subscribe clients
// access their next event by calling Next(). This may initially include the
// snapshot events to catch them up if they are new or behind.
type Subscription struct {
// state is accessed atomically 0 means open, 1 means closed with reload
state uint32
// req is the requests that we are responding to
req *SubscribeRequest
// currentItem stores the current snapshot or topic buffer item we are on. It
// is mutated by calls to Next.
currentItem *BufferItem
// ctx is the Subscription context that wraps the context of the streaming RPC
// handler call.
ctx context.Context
// cancelFn stores the context cancel function that will wake up the
// in-progress Next call on a server-initiated state change e.g. Reload.
cancelFn func()
// Unsubscribe is a function set by EventPublisher that is called to
// free resources when the subscription is no longer needed.
Unsubscribe func()
}
// SubscribeRequest identifies the types of events the subscriber would like to
// receiver. Topic and Token are required.
type SubscribeRequest struct {
Topic Topic
Key string
Token string
Index uint64
}
// NewSubscription return a new subscription. The caller is responsible for
// calling Unsubscribe when it is done with the subscription, to free resources.
func NewSubscription(ctx context.Context, req *SubscribeRequest, item *BufferItem) *Subscription {
subCtx, cancel := context.WithCancel(ctx)
return &Subscription{
ctx: subCtx,
cancelFn: cancel,
req: req,
currentItem: item,
}
}
// Next returns the next set of events to deliver. It must only be called from a
// single goroutine concurrently as it mutates the Subscription.
func (s *Subscription) Next() ([]Event, error) {
if atomic.LoadUint32(&s.state) == subscriptionStateClosed {
return nil, ErrSubscriptionReload
}
for {
next, err := s.currentItem.Next(s.ctx)
if err != nil {
// Check we didn't return because of a state change cancelling the context
if atomic.LoadUint32(&s.state) == subscriptionStateClosed {
return nil, ErrSubscriptionReload
}
return nil, err
}
// Advance our cursor for next loop or next call
s.currentItem = next
// Assume happy path where all events (or none) are relevant.
allMatch := true
// If there is a specific key, see if we need to filter any events
if s.req.Key != "" {
for _, e := range next.Events {
if s.req.Key != e.Key {
allMatch = false
break
}
}
}
// Only if we need to filter events should we bother allocating a new slice
// as this is a hot loop.
events := next.Events
if !allMatch {
events = make([]Event, 0, len(next.Events))
for _, e := range next.Events {
// Only return it if the key matches.
if s.req.Key == "" || s.req.Key == e.Key {
events = append(events, e)
}
}
}
if len(events) > 0 {
return events, nil
}
// Keep looping until we find some events we are interested in.
}
}
// Close the subscription. Subscribers will receive an error when they call Next,
// and will need to perform a new Subscribe request.
// It is safe to call from any goroutine.
func (s *Subscription) Close() {
swapped := atomic.CompareAndSwapUint32(&s.state, subscriptionStateOpen, subscriptionStateClosed)
if swapped {
s.cancelFn()
}
}