fix(update): use build tags for platform-specific watcher code

Split platform-specific functions into separate files:
- cmd_unix.go: Unix implementation using Setpgid and signal 0
- cmd_windows.go: Windows implementation using CREATE_NEW_PROCESS_GROUP
  and OpenProcess for PID checking

Fixes Windows cross-compilation error where Setpgid field doesn't exist.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

pkg/updater/cmd.go

@@ -3,11 +3,7 @@ package updater
 import (
 	"fmt"
 	"os"
-	"os/exec"
 	"runtime"
-	"strconv"
-	"syscall"
-	"time"
 
 	"github.com/host-uk/core/pkg/cli"
 	"github.com/spf13/cobra"
@@ -223,79 +219,3 @@ func handleDevTagUpdate(currentVersion string) error {
 	return nil
 }
 
-// spawnWatcher spawns a background process that watches for the current process
-// to exit, then restarts the binary with --version to confirm the update.
-func spawnWatcher() error {
-	executable, err := os.Executable()
-	if err != nil {
-		return err
-	}
-
-	pid := os.Getpid()
-
-	// Spawn: core update --watch-pid=<pid>
-	cmd := exec.Command(executable, "update", "--watch-pid", strconv.Itoa(pid))
-	cmd.Stdout = os.Stdout
-	cmd.Stderr = os.Stderr
-
-	// Detach from parent process group
-	cmd.SysProcAttr = &syscall.SysProcAttr{
-		Setpgid: true,
-	}
-
-	return cmd.Start()
-}
-
-// watchAndRestart waits for the given PID to exit, then restarts the binary.
-func watchAndRestart(pid int) error {
-	// Wait for the parent process to die
-	for {
-		if !isProcessRunning(pid) {
-			break
-		}
-		time.Sleep(100 * time.Millisecond)
-	}
-
-	// Small delay to ensure file handle is released
-	time.Sleep(200 * time.Millisecond)
-
-	// Get executable path
-	executable, err := os.Executable()
-	if err != nil {
-		return err
-	}
-
-	// On Unix, use exec to replace this process
-	if runtime.GOOS != "windows" {
-		return syscall.Exec(executable, []string{executable, "--version"}, os.Environ())
-	}
-
-	// On Windows, spawn new process and exit
-	cmd := exec.Command(executable, "--version")
-	cmd.Stdout = os.Stdout
-	cmd.Stderr = os.Stderr
-	if err := cmd.Start(); err != nil {
-		return err
-	}
-
-	os.Exit(0)
-	return nil
-}
-
-// isProcessRunning checks if a process with the given PID is still running.
-func isProcessRunning(pid int) bool {
-	process, err := os.FindProcess(pid)
-	if err != nil {
-		return false
-	}
-
-	// On Unix, FindProcess always succeeds, so we need to send signal 0
-	// to check if the process actually exists
-	if runtime.GOOS != "windows" {
-		err = process.Signal(syscall.Signal(0))
-		return err == nil
-	}
-
-	// On Windows, FindProcess returns an error if process doesn't exist
-	return true
-}

pkg/updater/cmd_unix.go

@@ -0,0 +1,70 @@
+//go:build !windows
+
+package updater
+
+import (
+	"os"
+	"os/exec"
+	"strconv"
+	"syscall"
+	"time"
+)
+
+// spawnWatcher spawns a background process that watches for the current process
+// to exit, then restarts the binary with --version to confirm the update.
+func spawnWatcher() error {
+	executable, err := os.Executable()
+	if err != nil {
+		return err
+	}
+
+	pid := os.Getpid()
+
+	// Spawn: core update --watch-pid=<pid>
+	cmd := exec.Command(executable, "update", "--watch-pid", strconv.Itoa(pid))
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+
+	// Detach from parent process group
+	cmd.SysProcAttr = &syscall.SysProcAttr{
+		Setpgid: true,
+	}
+
+	return cmd.Start()
+}
+
+// watchAndRestart waits for the given PID to exit, then restarts the binary.
+func watchAndRestart(pid int) error {
+	// Wait for the parent process to die
+	for {
+		if !isProcessRunning(pid) {
+			break
+		}
+		time.Sleep(100 * time.Millisecond)
+	}
+
+	// Small delay to ensure file handle is released
+	time.Sleep(200 * time.Millisecond)
+
+	// Get executable path
+	executable, err := os.Executable()
+	if err != nil {
+		return err
+	}
+
+	// Use exec to replace this process
+	return syscall.Exec(executable, []string{executable, "--version"}, os.Environ())
+}
+
+// isProcessRunning checks if a process with the given PID is still running.
+func isProcessRunning(pid int) bool {
+	process, err := os.FindProcess(pid)
+	if err != nil {
+		return false
+	}
+
+	// On Unix, FindProcess always succeeds, so we need to send signal 0
+	// to check if the process actually exists
+	err = process.Signal(syscall.Signal(0))
+	return err == nil
+}

pkg/updater/cmd_windows.go

@@ -0,0 +1,76 @@
+//go:build windows
+
+package updater
+
+import (
+	"os"
+	"os/exec"
+	"strconv"
+	"syscall"
+	"time"
+)
+
+// spawnWatcher spawns a background process that watches for the current process
+// to exit, then restarts the binary with --version to confirm the update.
+func spawnWatcher() error {
+	executable, err := os.Executable()
+	if err != nil {
+		return err
+	}
+
+	pid := os.Getpid()
+
+	// Spawn: core update --watch-pid=<pid>
+	cmd := exec.Command(executable, "update", "--watch-pid", strconv.Itoa(pid))
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+
+	// On Windows, use CREATE_NEW_PROCESS_GROUP to detach
+	cmd.SysProcAttr = &syscall.SysProcAttr{
+		CreationFlags: syscall.CREATE_NEW_PROCESS_GROUP,
+	}
+
+	return cmd.Start()
+}
+
+// watchAndRestart waits for the given PID to exit, then restarts the binary.
+func watchAndRestart(pid int) error {
+	// Wait for the parent process to die
+	for {
+		if !isProcessRunning(pid) {
+			break
+		}
+		time.Sleep(100 * time.Millisecond)
+	}
+
+	// Small delay to ensure file handle is released
+	time.Sleep(500 * time.Millisecond)
+
+	// Get executable path
+	executable, err := os.Executable()
+	if err != nil {
+		return err
+	}
+
+	// On Windows, spawn new process and exit
+	cmd := exec.Command(executable, "--version")
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	if err := cmd.Start(); err != nil {
+		return err
+	}
+
+	os.Exit(0)
+	return nil
+}
+
+// isProcessRunning checks if a process with the given PID is still running.
+func isProcessRunning(pid int) bool {
+	// On Windows, try to open the process with query rights
+	handle, err := syscall.OpenProcess(syscall.PROCESS_QUERY_INFORMATION, false, uint32(pid))
+	if err != nil {
+		return false
+	}
+	syscall.CloseHandle(handle)
+	return true
+}