os.MkdirAll(path) doesn't return nil when the path already exists on windows

[hu13]

Right here

lumberjack/lumberjack.go

Line 209 in 4b74a4d

err := os.MkdirAll(l.dir(), 0744)

, if the path already exists, the docs https://golang.org/pkg/os/#MkdirAll of os.MkdirAll says it expects to return nil.

However, I found that this is not the case on window 10/golang 1.12.7.

Wondering if we can have a defensive check here.

[hu13]

It turned out not to be true if i ran os.MkdirAll on powershell.

Mostly, the user permission issue on my setup.