add NewSSHSessionForExternalClient for managing ssh client externally (#125)

* add NewSSHSessionForExternalClient for transport_ssh

This enables user to initiate and manage (including closing)
of ssh client externally.
This enables transport session to reuse external ssh client for
multiple sessions.

* Update netconf/transport_ssh.go

refactor: rename externClient to managedSession on TransportSSH

Co-authored-by: Brandon Bennett <[email protected]>

* fixup! Update netconf/transport_ssh.go

Signed-off-by: thebinary <[email protected]>

* rename NewSSHSessionForExternalClient to NewSSHClientSession

Signed-off-by: thebinary <[email protected]>

* refactor: remove redundant clientToTransport fn

Signed-off-by: thebinary <[email protected]>

---------

Signed-off-by: thebinary <[email protected]>
Co-authored-by: Brandon Bennett <[email protected]>

Author: thebinary

netconf/transport_ssh.go

@@ -34,6 +34,9 @@ type TransportSSH struct {
 	TransportBasicIO
 	sshClient  *ssh.Client
 	sshSession *ssh.Session
+
+	// SSH Client connection is managed externally
+	managedSession bool
 }
 
 // Close closes an existing SSH session and socket if they exist.
@@ -48,13 +51,15 @@ func (t *TransportSSH) Close() error {
 		if err := t.sshSession.Close(); err != nil {
 			// If we receive an error when trying to close the session, then
 			// lets try to close the socket, otherwise it will be left open
-			t.sshClient.Close()
+			if !t.managedSession {
+				t.sshClient.Close()
+			}
 			return err
 		}
 	}
 
 	// Close the socket
-	if t.sshClient != nil {
+	if !t.managedSession && t.sshClient != nil {
 		return t.sshClient.Close()
 	}
 	return fmt.Errorf("No connection to close")
@@ -117,6 +122,22 @@ func NewSSHSession(conn net.Conn, config *ssh.ClientConfig) (*Session, error) {
 	return NewSession(t), nil
 }
 
+// NewSSHClientSession creates a new NETCONF session using an existing ssh.Client
+// initiated and managed externally.
+func NewSSHClientSession(client *ssh.Client) (*Session, error) {
+	t := &TransportSSH{
+		sshClient:      client,
+		managedSession: true,
+	}
+
+	err := t.setupSession()
+	if err != nil {
+		return nil, err
+	}
+
+	return NewSession(t), nil
+}
+
 // DialSSH creates a new NETCONF session using a SSH Transport.
 // See TransportSSH.Dial for arguments.
 func DialSSH(target string, config *ssh.ClientConfig) (*Session, error) {