`reading plugin stderr` error message in logs

[skriss]

I'm seeing this log statement a lot in logs:

time="2019-04-24T17:40:27Z" level=error msg="reading plugin stderr" cmd=/velero controller=backup-sync error="read |0: file already closed" logSource="pkg/plugin/clientmgmt/logrus_adapter.go:89" pluginName=velero

It doesn't seem to actually be causing issues but we need to figure out what's going on.

[skriss]

@carlisia can you look into this? we need to resolve this before 1.0

[skriss]

Was able to get a consistent repro and a POC of a fix for this today -- see hashicorp/go-plugin#116.