From f57b4eb46dd35e4360320672c703f9be408e8bc4 Mon Sep 17 00:00:00 2001
From: Anthony Romano <anthony.romano@coreos.com>
Date: Fri, 3 Jun 2016 11:08:00 -0700
Subject: [PATCH] mvcc: don't cancel watcher if stream is already closed

Close() already cancels all the watchers but doesn't bother to clear out
the bookkeeping maps so Cancel() may try to cancel twice.

Fixes #5533
---
 mvcc/watcher.go | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mvcc/watcher.go b/mvcc/watcher.go
index 5d2f8f71f..9353f8356 100644
--- a/mvcc/watcher.go
+++ b/mvcc/watcher.go
@@ -117,13 +117,18 @@ func (ws *watchStream) Chan() <-chan WatchResponse {
 }
 
 func (ws *watchStream) Cancel(id WatchID) error {
+	ws.mu.Lock()
 	cancel, ok := ws.cancels[id]
+	ok = ok && !ws.closed
+	if ok {
+		delete(ws.cancels, id)
+		delete(ws.watchers, id)
+	}
+	ws.mu.Unlock()
 	if !ok {
 		return ErrWatcherNotExist
 	}
 	cancel()
-	delete(ws.cancels, id)
-	delete(ws.watchers, id)
 	return nil
 }