feat: publish standalone worker

Separate worker packaging and service lifecycle from the control plane.

internal/util/unix/pidfile.go

@@ -0,0 +1,31 @@
+// Package unix provides functionality.
+package unix
+
+import (
+	"fmt"
+	"os"
+	"strconv"
+	"syscall"
+)
+
+// WritePidFile provides functionality.
+// Write a pid file, but first make sure it doesn't exist with a running pid.
+func WritePidFile(pidFile string) error {
+	// Read in the pid file as a slice of bytes.
+	if piddata, err := os.ReadFile(pidFile); err == nil {
+		// Convert the file contents to an integer.
+		if pid, err := strconv.Atoi(string(piddata)); err == nil {
+			// Look for the pid in the process list.
+			if process, err := os.FindProcess(pid); err == nil {
+				// Send the process a signal zero kill.
+				if err := process.Signal(syscall.Signal(0)); err == nil {
+					// We only get an error if the pid isn't running, or it's not ours.
+					return fmt.Errorf("pid already running: %d", pid)
+				}
+			}
+		}
+	}
+	// If we get here, then the pidfile didn't exist,
+	// or the pid in it doesn't belong to the user running this app.
+	return os.WriteFile(pidFile, []byte(fmt.Sprintf("%d", os.Getpid())), 0o664)
+}