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/*
*
* Copyright 2023 gRPC authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*
*/
package grpcsync
import (
"context"
"sync"
)
// Subscriber represents an entity that is subscribed to messages published on
// a PubSub. It wraps the callback to be invoked by the PubSub when a new
// message is published.
type Subscriber interface {
// OnMessage is invoked when a new message is published. Implementations
// must not block in this method.
OnMessage(msg interface{})
}
// PubSub is a simple one-to-many publish-subscribe system that supports
// messages of arbitrary type. It guarantees that messages are delivered in
// the same order in which they were published.
//
// Publisher invokes the Publish() method to publish new messages, while
// subscribers interested in receiving these messages register a callback
// via the Subscribe() method.
//
// Once a PubSub is stopped, no more messages can be published, and
// it is guaranteed that no more subscriber callback will be invoked.
type PubSub struct {
cs *CallbackSerializer
cancel context.CancelFunc
// Access to the below fields are guarded by this mutex.
mu sync.Mutex
msg interface{}
subscribers map[Subscriber]bool
stopped bool
}
// NewPubSub returns a new PubSub instance.
func NewPubSub() *PubSub {
ctx, cancel := context.WithCancel(context.Background())
return &PubSub{
cs: NewCallbackSerializer(ctx),
cancel: cancel,
subscribers: map[Subscriber]bool{},
}
}
// Subscribe registers the provided Subscriber to the PubSub.
//
// If the PubSub contains a previously published message, the Subscriber's
// OnMessage() callback will be invoked asynchronously with the existing
// message to begin with, and subsequently for every newly published message.
//
// The caller is responsible for invoking the returned cancel function to
// unsubscribe itself from the PubSub.
func (ps *PubSub) Subscribe(sub Subscriber) (cancel func()) {
ps.mu.Lock()
defer ps.mu.Unlock()
if ps.stopped {
return func() {}
}
ps.subscribers[sub] = true
if ps.msg != nil {
msg := ps.msg
ps.cs.Schedule(func(context.Context) {
ps.mu.Lock()
defer ps.mu.Unlock()
if !ps.subscribers[sub] {
return
}
sub.OnMessage(msg)
})
}
return func() {
ps.mu.Lock()
defer ps.mu.Unlock()
delete(ps.subscribers, sub)
}
}
// Publish publishes the provided message to the PubSub, and invokes
// callbacks registered by subscribers asynchronously.
func (ps *PubSub) Publish(msg interface{}) {
ps.mu.Lock()
defer ps.mu.Unlock()
if ps.stopped {
return
}
ps.msg = msg
for sub := range ps.subscribers {
s := sub
ps.cs.Schedule(func(context.Context) {
ps.mu.Lock()
defer ps.mu.Unlock()
if !ps.subscribers[s] {
return
}
s.OnMessage(msg)
})
}
}
// Stop shuts down the PubSub and releases any resources allocated by it.
// It is guaranteed that no subscriber callbacks would be invoked once this
// method returns.
func (ps *PubSub) Stop() {
ps.mu.Lock()
defer ps.mu.Unlock()
ps.stopped = true
ps.cancel()
}
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