fix: use fcntl file lock to prevent PID file race condition

wecker.py

@@ -5,6 +5,7 @@ import pygame
 import random
 import sys
 import os
+import fcntl
 
 from common import PID_FILE
 
@@ -16,44 +17,42 @@ logging.basicConfig(
 )
 
 
+_pid_lock_fd = None
+
+
 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())
+    """Ensures that only one instance of the script is running.
 
-            # 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
+    Uses an advisory file lock on the PID file so the check-and-write
+    sequence is atomic and race-free.
+    """
+    global _pid_lock_fd
+    _pid_lock_fd = open(PID_FILE, "a+")
+    try:
+        fcntl.flock(_pid_lock_fd, fcntl.LOCK_EX | fcntl.LOCK_NB)
+    except (BlockingIOError, OSError):
+        logging.error("Another instance of wecker is already running. Exiting.")
+        sys.exit(1)
 
-    with open(PID_FILE, 'w') as f:
-        f.write(str(os.getpid()))
+    _pid_lock_fd.seek(0)
+    _pid_lock_fd.truncate()
+    _pid_lock_fd.write(str(os.getpid()))
+    _pid_lock_fd.flush()
 
 
 def remove_pid_file():
-    """Removes the PID file."""
-    if os.path.exists(PID_FILE):
+    """Releases the PID file lock and removes the file."""
+    global _pid_lock_fd
+    if _pid_lock_fd is not None:
         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):
+            _pid_lock_fd.close()
+        except OSError:
             pass
+        _pid_lock_fd = None
+    try:
+        os.remove(PID_FILE)
+    except FileNotFoundError:
+        pass
 
 
 # GPIO Setup