mod: use log file as stderr for ssh command

internal/serve/serve.go

@@ -4,7 +4,6 @@ import (
 	"context"
 	"errors"
 	"fmt"
-	"io"
 	"net"
 	"os"
 	"os/signal"
@@ -57,10 +56,14 @@ func setupSshServer(host string, port string, host_key_path string, users map[st
 			func(next ssh.Handler) ssh.Handler {
 				return func(s ssh.Session) {
 					cmd := wish.Command(s, name, args...)
+					file, err := os.OpenFile("log", os.O_APPEND|os.O_WRONLY|os.O_CREATE, 0600)
+					if err != nil {
+						wish.Fatalln(s, err)
+					}
+					cmd.SetStderr(file)
 					if err := cmd.Run(); err != nil {
 						wish.Fatalln(s, err)
 					}
-					io.Copy(os.Stderr, s.Stderr())
 					next(s)
 				}
 			},