adds file locking

bin/vcmp

@@ -13,6 +13,31 @@ video_extensions: list[str] = ["mp4", "mkv", "avi", "wmv", "mov", "m4v", "ts", "
 
 total_files: int = 0
 
+def acquire_file_lock(filepath: pathlib.Path) -> bool:
+    lock_file = filepath.with_suffix(filepath.suffix + '.lock')
+    try:
+        # Atomic file creation
+        lock_fd = os.open(str(lock_file), os.O_CREAT | os.O_EXCL | os.O_WRONLY)
+        with os.fdopen(lock_fd, 'w') as f:
+            f.write(f"{os.getpid()}\n{time.time()}\n")
+        return True
+    except FileExistsError:
+        try:
+            lock_age = time.time() - lock_file.stat().st_mtime
+            if lock_age > 86400:  # 24 hours
+                lock_file.unlink()
+                return acquire_file_lock(filepath)
+        except FileNotFoundError:
+            return acquire_file_lock(filepath)
+        return False
+
+def release_file_lock(filepath: pathlib.Path) -> None:
+    lock_file = filepath.with_suffix(filepath.suffix + '.lock')
+    try:
+        lock_file.unlink()
+    except FileNotFoundError:
+        pass
+
 def setup_logging(verbose: bool) -> None:
     """Configure logging to both file and stdout."""
     log_level = logging.DEBUG if verbose else logging.INFO
@@ -150,6 +175,10 @@ def process_single_file(
         nice: int,
         file_index: int
 ) -> bool:  # Return success/failure status
+    if not acquire_file_lock(filepath):
+        logging.info(f"Skipping {filepath}: already being processed")
+        return False
+
     temp_path = temp_dir / filepath.with_suffix(".mp4").name
     clean_directory(temp_dir) # Should be empty if mktemp was used, but clear it regardless.
     logging.info(f"Processing {filepath} to {target_path}")
@@ -163,11 +192,13 @@ def process_single_file(
         logging.warning("Process interrupted by user")
         if temp_path.exists():
             temp_path.unlink()
+        release_file_lock(filepath)
         raise  # Re-raise KeyboardInterrupt to exit the program
     except subprocess.CalledProcessError as e:
         logging.error(f"FFmpeg error processing {filepath}: {e}")
         if temp_path.exists():
             temp_path.unlink()
+        release_file_lock(filepath)
         return False
     else:
         # File successfully converted to av1, move to destination
@@ -178,6 +209,7 @@ def process_single_file(
         if remove_source:
             logging.info(f"Removing source file {filepath}")
             filepath.unlink()
+        release_file_lock(filepath)
         return True
 
 def process_directory(