Add a placeholder constant for ignoring cancelled subscription errors from libp2p pubsub. (#6496)

* Add a placeholder constant for ignoring cancelled subscription errors
from libp2p pubsub.

This change temporarily resolves
https://github.com/prysmaticlabs/prysm/issues/6449.

This change should be revisited after
https://github.com/libp2p/go-libp2p-pubsub/pull/356 merges and the prysm
libp2p pubsub dependency is updated.
* Merge branch 'master' into issue-6449
* Merge branch 'master' into issue-6449
* Merge branch 'master' into issue-6449

beacon-chain/sync/subscriber.go

@@ -2,6 +2,7 @@ package sync
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"reflect"
 	"runtime/debug"
@@ -25,6 +26,11 @@ import (
 
 const pubsubMessageTimeout = 30 * time.Second
 
+var (
+	// TODO(6449): Replace this with pubsub.ErrSubscriptionCancelled.
+	errSubscriptionCancelled = errors.New("subscription cancelled by calling sub.Cancel()")
+)
+
 // subHandler represents handler for a given subscription.
 type subHandler func(context.Context, proto.Message) error
 
@@ -147,7 +153,9 @@ func (s *Service) subscribeWithBase(base proto.Message, topic string, validator
 			msg, err := sub.Next(s.ctx)
 			if err != nil {
 				// This should only happen when the context is cancelled or subscription is cancelled.
-				log.WithError(err).Warn("Subscription next failed")
+				if err != errSubscriptionCancelled { // Only log a warning on unexpected errors.
+					log.WithError(err).Warn("Subscription next failed")
+				}
 				return
 			}