Add SSH SFTP backend support

internal/adapters/ssh/auth.go

@@ -0,0 +1,61 @@
+package ssh
+
+import (
+	"fmt"
+	"io"
+	"net"
+	"os"
+
+	cryptossh "golang.org/x/crypto/ssh"
+	"golang.org/x/crypto/ssh/agent"
+)
+
+type agentDialer func(network, address string) (net.Conn, error)
+
+func authMethods(keyFile string) ([]cryptossh.AuthMethod, func(), error) {
+	return authMethodsWithAgent(os.Getenv("SSH_AUTH_SOCK"), net.Dial, keyFile)
+}
+
+func authMethodsWithAgent(agentSocket string, dial agentDialer, keyFile string) ([]cryptossh.AuthMethod, func(), error) {
+	var methods []cryptossh.AuthMethod
+	var closers []io.Closer
+	if agentSocket != "" {
+		methods = append(methods, cryptossh.PublicKeysCallback(func() ([]cryptossh.Signer, error) {
+			conn, err := dial("unix", agentSocket)
+			if err != nil {
+				return nil, err
+			}
+			closers = append(closers, conn)
+			return agent.NewClient(conn).Signers()
+		}))
+	}
+	if keyFile != "" {
+		signer, err := signerFromKeyFile(keyFile)
+		if err != nil {
+			return nil, nil, err
+		}
+		methods = append(methods, cryptossh.PublicKeys(signer))
+	}
+	if len(methods) == 0 {
+		return nil, nil, fmt.Errorf("no SSH auth methods configured; set SSH_AUTH_SOCK or ssh_key_file")
+	}
+	return methods, func() { closeAll(closers) }, nil
+}
+
+func signerFromKeyFile(path string) (cryptossh.Signer, error) {
+	key, err := os.ReadFile(path)
+	if err != nil {
+		return nil, fmt.Errorf("read ssh_key_file %q: %w", path, err)
+	}
+	signer, err := cryptossh.ParsePrivateKey(key)
+	if err != nil {
+		return nil, fmt.Errorf("parse ssh_key_file %q: %w", path, err)
+	}
+	return signer, nil
+}
+
+func closeAll(closers []io.Closer) {
+	for _, closer := range closers {
+		_ = closer.Close()
+	}
+}