Add SSH SFTP backend support

internal/adapters/ssh/options.go

@@ -0,0 +1,77 @@
+package ssh
+
+import (
+	"fmt"
+	"os"
+	"os/user"
+	"path"
+	"path/filepath"
+	"strconv"
+)
+
+const (
+	BackendName = "ssh"
+
+	HostKeyPolicyStrict    HostKeyPolicy = "strict"
+	HostKeyPolicyAcceptNew HostKeyPolicy = "accept-new"
+	HostKeyPolicyOff       HostKeyPolicy = "off"
+)
+
+type HostKeyPolicy string
+
+type Options struct {
+	Host          string
+	User          string
+	Port          int
+	Root          string
+	KeyFile       string
+	KnownHosts    string
+	HostKeyPolicy HostKeyPolicy
+}
+
+func (o Options) normalized() (Options, error) {
+	if o.Host == "" {
+		return Options{}, fmt.Errorf("host is required")
+	}
+	if o.User == "" {
+		current, err := user.Current()
+		if err != nil || current.Username == "" {
+			return Options{}, fmt.Errorf("user is required when current OS user cannot be determined")
+		}
+		o.User = current.Username
+	}
+	if o.Port == 0 {
+		o.Port = 22
+	}
+	if o.Port < 1 || o.Port > 65535 {
+		return Options{}, fmt.Errorf("port must be between 1 and 65535")
+	}
+	if o.Root == "" {
+		return Options{}, fmt.Errorf("path is required")
+	}
+	o.Root = path.Clean(o.Root)
+	if o.HostKeyPolicy == "" {
+		o.HostKeyPolicy = HostKeyPolicyAcceptNew
+	}
+	switch o.HostKeyPolicy {
+	case HostKeyPolicyStrict, HostKeyPolicyAcceptNew, HostKeyPolicyOff:
+	default:
+		return Options{}, fmt.Errorf("host_key_policy must be strict, accept-new, or off")
+	}
+	if o.KnownHosts == "" && o.HostKeyPolicy != HostKeyPolicyOff {
+		o.KnownHosts = defaultKnownHostsPath()
+	}
+	return o, nil
+}
+
+func (o Options) address() string {
+	return o.Host + ":" + strconv.Itoa(o.Port)
+}
+
+func defaultKnownHostsPath() string {
+	home, err := os.UserHomeDir()
+	if err != nil || home == "" {
+		return ""
+	}
+	return filepath.Join(home, ".ssh", "known_hosts")
+}