feat: «Здесь» from a machine's file pane opens the terminal in that pane's folder (the agent starts the shell there; an older agent, or a folder gone, at home)

internal/agent/terminal.go

@@ -4,6 +4,8 @@ import (
 	"fmt"
 	"os"
 	"os/exec"
+	"path/filepath"
+	"strings"
 	"sync"
 	"syscall"
 	"time"
@@ -41,8 +43,12 @@ func CloseTerminals() {
 // StartPTY launches a login shell attached to a new PTY. With user empty (or
 // equal to the agent's own user) it is the agent's shell - root by default,
 // since the agent runs as a root systemd service. Any other user is entered via
-// `su -l`, which only a root agent can do without a password.
-func StartPTY(user string) (*PTY, error) {
+// `su -l`, which only a root agent can do without a password. dir, when it
+// is a folder, is where the shell starts instead of home.
+func StartPTY(user, dir string) (*PTY, error) {
+	if dir != "" && (!filepath.IsAbs(dir) || strings.ContainsAny(dir, "\x00\r\n") || !isDir(dir)) {
+		dir = ""
+	}
 	var cmd *exec.Cmd
 	env := append(os.Environ(), "TERM=xterm-256color")
 	if user == "" || user == currentUser() {
@@ -54,6 +60,9 @@ func StartPTY(user string) (*PTY, error) {
 			cmd.Dir = home
 			env = append(env, "HOME="+home)
 		}
+		if dir != "" {
+			cmd.Dir = dir
+		}
 	} else {
 		if !validUserName(user) {
 			return nil, fmt.Errorf("недопустимое имя пользователя %q", user)
@@ -69,6 +78,11 @@ func StartPTY(user string) (*PTY, error) {
 			return nil, fmt.Errorf("su не найден: %w", err)
 		}
 		cmd = exec.Command(su, "-l", user)
+		// su -l always goes home; to start elsewhere, its shell changes
+		// directory first and becomes the login shell (sh and csh alike).
+		if dir != "" {
+			cmd = exec.Command(su, "-l", user, "-c", "cd "+shQuote(dir)+"; exec \"$SHELL\" -l")
+		}
 		// su -l goes to the user's home; starting there too keeps the shell
 		// out of "/" should it not.
 		if home := homeDir(user); isDir(home) {
@@ -126,3 +140,9 @@ func (p *PTY) Close() error {
 	})
 	return err
 }
+
+// shQuote puts s in single quotes for sh and csh alike, closing and
+// reopening them around each quote inside.
+func shQuote(s string) string {
+	return "'" + strings.ReplaceAll(s, "'", `'\''`) + "'"
+}