feat: add --pidfile support for access subcommand

Add --pidfile flag to the 'access tcp' (ssh/rdp/smb) subcommand, matching
the existing behavior of 'cloudflared tunnel --pidfile'.

When set, the process writes its PID to the specified file on startup and
removes it on exit, enabling daemon management with systemd, launchd, or
custom init scripts.

Closes cloudflare/cloudflared#723

cmd/cloudflared/access/carrier.go

@@ -5,8 +5,10 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"os"
 	"strings"
 
+	"github.com/mitchellh/go-homedir"
 	"github.com/pkg/errors"
 	"github.com/rs/zerolog"
 	"github.com/urfave/cli/v2"
@@ -69,6 +71,21 @@ func ssh(c *cli.Context) error {
 	}
 	log := logger.CreateSSHLoggerFromContext(c, outputTerminal)
 
+	if pidFile := c.String(sshPidFileFlag); pidFile != "" {
+		expandedPidFile, err := homedir.Expand(pidFile)
+		if err != nil {
+			log.Err(err).Msg("unable to expand pidfile path")
+		} else if err := writePidFile(expandedPidFile, log); err != nil {
+			log.Err(err).Msg("failed to write pidfile")
+		} else {
+			defer func() {
+				if err := os.Remove(expandedPidFile); err != nil {
+					log.Err(err).Msg("failed to remove pidfile")
+				}
+			}()
+		}
+	}
+
 	// get the hostname from the cmdline and error out if its not provided
 	rawHostName := c.String(sshHostnameFlag)
 	url, err := parseURL(rawHostName)
@@ -145,3 +162,17 @@ func ssh(c *cli.Context) error {
 	}
 	return carrier.StartClient(wsConn, s, options)
 }
+
+// writePidFile writes the current process ID to the given path.
+func writePidFile(path string, log *zerolog.Logger) error {
+	file, err := os.Create(path)
+	if err != nil {
+		return fmt.Errorf("unable to create pidfile %q: %w", path, err)
+	}
+	defer file.Close()
+	if _, err := fmt.Fprintf(file, "%d", os.Getpid()); err != nil {
+		return fmt.Errorf("unable to write pid to %q: %w", path, err)
+	}
+	log.Info().Str("pidfile", path).Msg("wrote pidfile")
+	return nil
+}

cmd/cloudflared/access/cmd.go

@@ -38,6 +38,7 @@ const (
 	sshGenCertFlag     = "short-lived-cert"
 	sshConnectTo       = "connect-to"
 	sshDebugStream     = "debug-stream"
+	sshPidFileFlag     = "pidfile"
 	sshConfigTemplate  = `
 Add to your {{.Home}}/.ssh/config:
 
@@ -204,6 +205,11 @@ func Commands() []*cli.Command {
 							Hidden: true,
 							Usage:  "Writes up-to the max provided stream payloads to the logger as debug statements.",
 						},
+						&cli.StringFlag{
+							Name:    sshPidFileFlag,
+							Usage:   "Write the application's PID to this file after startup.",
+							EnvVars: []string{"TUNNEL_PIDFILE"},
+						},
 					},
 				},
 				{