import os

def sanitize_filename(filename):
    """
    Sanitizes a filename to prevent directory traversal and other malicious paths.
    Removes path separators and '..'.
    """
    # Remove any directory separators
    filename = filename.replace('/', '').replace('\\', '')
    # Remove any .. sequences
    filename = filename.replace('..', '')
    # Further restrict characters if needed, e.g., to only alphanumeric, dots, hyphens, underscores
    # import re
    # filename = re.sub(r'[^a-zA-Z0-9_.-]', '', filename)
    return filename

def get_safe_filepath(base_dir, user_input_filename):
    """
    Combines a base directory with a sanitized filename to create a safe path.
    Ensures the resulting path is always within the specified base directory.
    """
    # Resolve base_dir to its real path to prevent symlink tricks
    base_dir = os.path.realpath(base_dir)
    
    # Sanitize the user-provided filename
    sanitized_name = sanitize_filename(user_input_filename)
    
    # Construct the full path
    full_path = os.path.join(base_dir, sanitized_name)
    
    # Ensure the resulting path is still within the base directory
    # This is a critical double-check after joining
    resolved_full_path = os.path.realpath(full_path)
    
    if not resolved_full_path.startswith(base_dir):
        raise ValueError("Attempted directory traversal detected!")
        
    return resolved_full_path

# Example usage:
# UPLOAD_DIR = '/var/www/uploads'
# user_filename = '../../../etc/passwd.jpg'
# try:
#     safe_path = get_safe_filepath(UPLOAD_DIR, user_filename)
#     print(f"Safe path: {safe_path}")
# except ValueError as e:
#     print(f"Error: {e}")

# user_filename_good = 'my_image.png'
# safe_path_good = get_safe_filepath(UPLOAD_DIR, user_filename_good)
# print(f"Good path: {safe_path_good}")