Add --pidfile support with signal-aware cleanup to cloudflared access tcp

Add --pidfile flag to the access tcp subcommand (also applies to rdp,
ssh, and smb aliases). This writes the process ID to a file after
startup and removes it on exit.

Signal handling (SIGTERM/SIGINT) ensures the PID file is cleaned up
when the process is killed, not just on graceful shutdown.

Includes unit tests for writePidFile and removePidFile.

Closes #723

cmd/cloudflared/access/carrier.go

@@ -5,8 +5,12 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"os"
+	"os/signal"
 	"strings"
+	"syscall"
 
+	"github.com/mitchellh/go-homedir"
 	"github.com/pkg/errors"
 	"github.com/rs/zerolog"
 	"github.com/urfave/cli/v2"
@@ -69,6 +73,26 @@ func ssh(c *cli.Context) error {
 	}
 	log := logger.CreateSSHLoggerFromContext(c, outputTerminal)
 
+	if c.IsSet(sshPidFileFlag) {
+		pidFile := c.String(sshPidFileFlag)
+		writePidFile(pidFile, log)
+		defer removePidFile(pidFile, log)
+
+		// Trap SIGTERM/SIGINT to clean up the PID file before exiting.
+		// Without this, signals kill the process before defers can run.
+		sigCh := make(chan os.Signal, 1)
+		signal.Notify(sigCh, syscall.SIGTERM, syscall.SIGINT)
+		go func() {
+			<-sigCh
+			removePidFile(pidFile, log)
+			signal.Reset(syscall.SIGTERM, syscall.SIGINT)
+			// Re-raise so the process exits with the default signal behavior
+			if p, err := os.FindProcess(os.Getpid()); err == nil {
+				_ = p.Signal(syscall.SIGTERM)
+			}
+		}()
+	}
+
 	// get the hostname from the cmdline and error out if its not provided
 	rawHostName := c.String(sshHostnameFlag)
 	url, err := parseURL(rawHostName)
@@ -145,3 +169,32 @@ func ssh(c *cli.Context) error {
 	}
 	return carrier.StartClient(wsConn, s, options)
 }
+
+// writePidFile writes the current process ID to a given file path.
+// It expands ~ in paths using go-homedir.
+func writePidFile(pidPathname string, log *zerolog.Logger) {
+	expandedPath, err := homedir.Expand(pidPathname)
+	if err != nil {
+		log.Err(err).Str("pidPath", pidPathname).Msg("Unable to expand the path, try to use absolute path in --pidfile")
+		return
+	}
+	file, err := os.Create(expandedPath)
+	if err != nil {
+		log.Err(err).Str("pidPath", expandedPath).Msg("Unable to write pid")
+		return
+	}
+	defer file.Close()
+	fmt.Fprintf(file, "%d", os.Getpid())
+}
+
+// removePidFile removes the PID file at the given path.
+// Errors are logged but do not cause a failure.
+func removePidFile(pidPathname string, log *zerolog.Logger) {
+	expandedPath, err := homedir.Expand(pidPathname)
+	if err != nil {
+		return
+	}
+	if err := os.Remove(expandedPath); err != nil && !os.IsNotExist(err) {
+		log.Err(err).Str("pidPath", expandedPath).Msg("Unable to remove pid file")
+	}
+}

cmd/cloudflared/access/carrier_test.go

@@ -0,0 +1,53 @@
+package access
+
+import (
+	"os"
+	"path/filepath"
+	"strconv"
+	"testing"
+
+	"github.com/rs/zerolog"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestWritePidFile(t *testing.T) {
+	log := zerolog.Nop()
+
+	t.Run("writes current PID to file", func(t *testing.T) {
+		pidFile := filepath.Join(t.TempDir(), "test.pid")
+
+		writePidFile(pidFile, &log)
+
+		content, err := os.ReadFile(pidFile)
+		require.NoError(t, err)
+
+		pid, err := strconv.Atoi(string(content))
+		require.NoError(t, err)
+		assert.Equal(t, os.Getpid(), pid)
+	})
+
+	t.Run("handles invalid path gracefully", func(t *testing.T) {
+		// Should not panic on a path that can't be created
+		writePidFile("/nonexistent/directory/test.pid", &log)
+	})
+}
+
+func TestRemovePidFile(t *testing.T) {
+	log := zerolog.Nop()
+
+	t.Run("removes existing pid file", func(t *testing.T) {
+		pidFile := filepath.Join(t.TempDir(), "test.pid")
+
+		writePidFile(pidFile, &log)
+		assert.FileExists(t, pidFile)
+
+		removePidFile(pidFile, &log)
+		assert.NoFileExists(t, pidFile)
+	})
+
+	t.Run("handles missing file gracefully", func(t *testing.T) {
+		// Should not panic when removing a file that doesn't exist
+		removePidFile("/tmp/nonexistent-cloudflared-test.pid", &log)
+	})
+}

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_ACCESS_PIDFILE"},
+						},
 					},
 				},
 				{