implemented embed client context

[awskii]

Hi.

Sometimes we need to store some metadata with client context, but there are no right way to do it. So i think it would be useful for any user of the library.

[fclairamb]

I don't think it's a good move:

• It can be implemented on the client driver and you can now override the client's logger and add the additional parameter.
• It makes the API more complex: for every call we add to the API we have to update all the existing implementations. By goal is to do the opposite: Avoid to implement things that might not be necessary to everyone and can be implemented at another level.
• The standard context is usually performed for long synchronous calls. Here it would be used to store the FTP session context. It doesn't look like a good usage of it.

In other words if you take the sample implementation, you could have:

type ClientDriver struct {
	BaseDir string                 // Base directory from which to server file
	Context map[string]interface{} // Context
}
[...]
func (driver *MainDriver) AuthUser(cc server.ClientContext, user, pass string) (server.ClientHandlingDriver, error) {

	for _, act := range driver.config.Users {
		if act.User == user && act.Pass == pass {
			// If we are authenticated, we can return a client driver containing *our* basedir
			baseDir := driver.BaseDir + string(os.PathSeparator) + act.Dir
			os.MkdirAll(baseDir, 0777)
			return &ClientDriver{
				BaseDir: baseDir,
				Context: make(map[string]interface{}),
			}, nil
		}
	}

	return nil, fmt.Errorf("could not authenticate you")
}

[awskii]

You can't pass ClientHandlingDriver into UserLeft method, for example. Assume, i want to drop some cache if user has left, and the only way i have - use ClientContext. But i can't store some user data e.g. UUID in that context.

Correct me if I'm wrong.