SandboxKey refers to non-existing file for remote driver

[growler]

API promises that Sandbox Key (i.e. file referring to linux network namespace) should be present at the moment of Join operation. However, it seems that file is actually created after that. I.e.,

func (d *NetworkDriver) Join(req *JoinRequest) (*JoinResponse, error) {
	logf("join request %+v", req)
	s, err := os.Stat(req.SandboxKey)
	logf("sandbox key %#v %s", s, err)
       ...
}

reports

sandbox key <nil> stat /var/run/docker/netns/5e1e12ad7888: no such file or directory

Network driver process works on the same host and in the same namespace as docker itself and definitely can see /var/run/docker/netns directory.

Docker version 17.06.2-ce, build cec0b72

[ghost]

#1950

Possibly related?

[ghost]

From someone more knowledgeable on the same issue: https://news.ycombinator.com/item?id=14470231