nsenter

The nsenter package registers a special init constructor that is called before the Go runtime has a chance to boot. This provides us the ability to setns on existing namespaces and avoid the issues that the Go runtime has with multiple threads. This constructor will be called if this package is registered, imported, in your go application.

The nsenter package will import "C" and it uses cgo package. In cgo, if the import of “C” is immediately preceded by a comment, that comment, called the preamble, is used as a header when compiling the C parts of the package. So every time we import package nsenter, the C code function nsexec() would be called. And package nsenter is now only imported in Docker execdriver, so every time before we call execdriver.Exec(), that C code would run.

nsexec() will first check the environment variable _LIBCONTAINER_INITPID which will give the process of the container that should be joined. Namespaces fd will be found from /proc/[pid]/ns and set by setns syscall.

And then get the pipe number from _LIBCONTAINER_INITPIPE, error message could be transfered through it. If tty is added, _LIBCONTAINER_CONSOLE_PATH will have value and start a console for output.

Finally, nsexec() will clone a child process , exit the parent process and let the Go runtime take over.