From 54f20b5a2aa2a2b7565a287a0879c6d48e251767 Mon Sep 17 00:00:00 2001
From: Preston Van Loon <preston@prysmaticlabs.com>
Date: Mon, 6 Jul 2020 10:58:44 -0700
Subject: [PATCH] Define a public error ErrSubscriptionCancelled.

---
 pubsub.go | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/pubsub.go b/pubsub.go
index ac3652f..61b12ea 100644
--- a/pubsub.go
+++ b/pubsub.go
@@ -28,6 +28,10 @@ const DefaultMaxMessageSize = 1 << 20
 
 var (
 	TimeCacheDuration = 120 * time.Second
+
+	// ErrSubscriptionCancelled may be returned when a subscription Next() is called after the
+	// subscription has been cancelled.
+	ErrSubscriptionCancelled = errors.New("subscription cancelled by calling sub.Cancel()")
 )
 
 var log = logging.Logger("pubsub")
@@ -629,7 +633,7 @@ func (p *PubSub) handleRemoveSubscription(sub *Subscription) {
 		return
 	}
 
-	sub.err = fmt.Errorf("subscription cancelled by calling sub.Cancel()")
+	sub.err = ErrSubscriptionCancelled
 	sub.close()
 	delete(subs, sub)