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Fix race condition: subscriber misses publisher-connected notification (No such feed) #1254

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Fix race condition: subscriber misses publisher-connected notification (No such feed) #1254

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21 changes: 21 additions & 0 deletions async/notifier.go
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Original file line number Diff line number Diff line change
Expand Up @@ -24,8 +24,11 @@ package async
import (
"context"
"sync"
"time"
)

const notifiedKeyTTL = 30 * time.Second

type rootWaiter struct {
key string
ch chan struct{}
Expand Down Expand Up @@ -56,6 +59,9 @@ type Notifier struct {
waiters map[string]*rootWaiter
// +checklocks:Mutex
waiterMap map[string]map[*Waiter]bool
// +checklocks:Mutex
// Sticky: remembers recent notifications so late waiters don't miss them.
notifiedAt map[string]time.Time
}

type ReleaseFunc func()
Expand All @@ -64,6 +70,15 @@ func (n *Notifier) NewWaiter(key string) (*Waiter, ReleaseFunc) {
n.Lock()
defer n.Unlock()

// If this key was notified recently, return an already-closed channel
// so the caller retries immediately without waiting.
if t, ok := n.notifiedAt[key]; ok && time.Since(t) < notifiedKeyTTL {
ch := make(chan struct{})
close(ch)
w := &Waiter{key: key, ch: ch}
return w, func() {}
}

waiter, found := n.waiters[key]
if !found {
waiter = &rootWaiter{
Expand Down Expand Up @@ -103,6 +118,7 @@ func (n *Notifier) Reset() {
}
n.waiters = nil
n.waiterMap = nil
n.notifiedAt = nil
}

func (n *Notifier) release(w *Waiter) {
Expand All @@ -126,6 +142,11 @@ func (n *Notifier) Notify(key string) {
n.Lock()
defer n.Unlock()

if n.notifiedAt == nil {
n.notifiedAt = make(map[string]time.Time)
}
n.notifiedAt[key] = time.Now()

if w, found := n.waiters[key]; found {
w.notify()
delete(n.waiters, w.key)
Expand Down
5 changes: 3 additions & 2 deletions sfu/janus/subscriber.go
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Original file line number Diff line number Diff line change
Expand Up @@ -169,8 +169,6 @@ func (p *janusSubscriber) joinRoom(ctx context.Context, stream *streamSelection,
return
}

waiter, stop := p.mcu.newPublisherConnectedWaiter(p.publisher, p.streamType)
defer stop()

loggedNotPublishingYet := false
retry:
Expand Down Expand Up @@ -253,11 +251,14 @@ retry:
sfuinternal.StatsWaitingForPublisherTotal.WithLabelValues(string(p.streamType)).Inc()
}

waiter, stop := p.mcu.newPublisherConnectedWaiter(p.publisher, p.streamType)
if err := waiter.Wait(ctx); err != nil {
stop()
p.Close(context.Background())
callback(err, nil)
return
}
stop()
p.logger.Printf("Retry subscribing %s from %s", p.streamType, p.publisher)
goto retry
default:
Expand Down