Implement single instance enforcement using PID file

wecker.py

@@ -4,6 +4,7 @@ import logging
 import pygame
 import random
 import sys
+import os
 
 # Configure logging
 logging.basicConfig(
@@ -12,6 +13,49 @@ logging.basicConfig(
     handlers=[logging.FileHandler("wecker.log"), logging.StreamHandler()],
 )
 
+PID_FILE = "wecker.pid"
+
+
+def ensure_single_instance():
+    """Ensures that only one instance of the script is running."""
+    if os.path.exists(PID_FILE):
+        try:
+            with open(PID_FILE, 'r') as f:
+                old_pid = int(f.read().strip())
+
+            # Check if the process is still running
+            os.kill(old_pid, 0)
+            # If os.kill succeeds, the process is running.
+            logging.error(f"Another instance of wecker (PID {old_pid}) is already running. Exiting.")
+            sys.exit(1)
+        except (ValueError, ProcessLookupError):
+            # ValueError: PID file is not an integer
+            # ProcessLookupError: PID doesn't exist
+            pass
+        except PermissionError:
+            # Process exists but we don't have permission to signal it.
+            logging.error(f"Another instance of wecker (PID {old_pid}) is already running. Exiting.")
+            sys.exit(1)
+        except OSError:
+            # Other OS errors, might mean process is dead
+            pass
+
+    with open(PID_FILE, 'w') as f:
+        f.write(str(os.getpid()))
+
+
+def remove_pid_file():
+    """Removes the PID file."""
+    if os.path.exists(PID_FILE):
+        try:
+            with open(PID_FILE, 'r') as f:
+                current_pid = int(f.read().strip())
+            if current_pid == os.getpid():
+                os.remove(PID_FILE)
+        except (ValueError, OSError):
+            pass
+
+
 # GPIO Setup
 BUTTON_PIN = 17
 LED_PIN = 27
@@ -181,9 +225,11 @@ def run_alarm(test_mode=False):
 
 
 if __name__ == "__main__":
+    ensure_single_instance()
     try:
         run_alarm()
     finally:
         pygame.mixer.quit()
         GPIO.cleanup()
+        remove_pid_file()
         logging.info("Program finished.")