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Add more explanation for nsenter 

Now the README in nsenter is a little confused
and hard for new commings to understand.

Signed-off-by: Sun Jianbo <wonderflow@zju.edu.cn>
This commit is contained in:
jianbosun 2015-04-11 20:38:48 +08:00
parent 883cbf7d28
commit 317686c673
1 changed files with 23 additions and 4 deletions
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27
nsenter/README.md
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@ -1,6 +1,25 @@
## 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 is only called if this package is
registered, imported, in your go application and the argv 0 is `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](https://golang.org/cmd/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.