Return meaningful error when subsystem request for sftp failed

[fd0]

Hi,

I'm writing a backup program (restic) which uses the sftp library. Recently, a user reported an issue with using his NAS with restic over sftp.

It turned out that sftp was deactivated for the NAS, but restic only printed a generic error message:

subsystem request failed on channel 0

I'd like to be able to catch this error and replace it with a more meaningful message for my users.

Unfortunately, the original error (from ssh/session) is created like this:

err = errors.New("ssh: subsystem request failed")

and the sftp library just passes it up the stack (here:

if err := s.RequestSubsystem("sftp"); err != nil {
    return nil, err
}

So in order to distinguish this error from others, the only way is to match against the string. That's ugly.

Would you accept a PR which returns a custom wrapper error type when RequestSubsystem returns an error? Something along the lines:

type subsystemRequestFailed string

func (err subsystemRequestFailed) Error() string {
   return string(err)
}

func (err subsystemRequestFailed) SubsystemRequestFailed() bool {
   return true
}

[...]
// NewClient creates a new SFTP client on conn, using zero or more option
// functions.
func NewClient(conn *ssh.Client, opts ...func(*Client) error) (*Client, error) {
    s, err := conn.NewSession()
    if err != nil {
        return nil, err
    }
    if err := s.RequestSubsystem("sftp"); err != nil {
        return nil, subsystemRequestFailed(err.Error())
    }
    pw, err := s.StdinPipe()
    if err != nil {
        return nil, err
    }
    pr, err := s.StdoutPipe()
    if err != nil {
        return nil, err
    }

    return NewClientPipe(pr, pw, opts...)
}

[...]

Then the caller can test the behavior of the error by checking if it implements the method SubsystemRequestFailed().

Does that sound good? Any better idea for the method name?

[fd0]

Hm, digging deeper into this I recognize that the error which showed up for the top-level code is not created in RequestSubsystem, the error string is slightly different: subsystem request failed on channel 0 vs ssh: subsystem request failed

Since I can't find the string anywhere in the Go code, I suspect that the error is printed by my ssh client program. I therefore close this issue, because this has nothing to do with the sftp package.

I'm sorry for everybody who read my journey here ;)

[fd0]

Solution to this issue: The error message is indeed printed by the ssh program I started. The error is printed for server which do not offer the sftp subsystem. Mystery solved.