Forked from golang.org/x/sync/singleflight at the x/sync repo's commit 67f06af15bc961c363a7260195bcd53487529a21 Updates golang/go#53427 Change-Id: Iec2b47b7777940017bb9b3db9bd7d93ba4a2e394 Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>