safe_json_file.py

# Because it turns out writing files is hard
# Process can crash halfway through, and a collision can overwrite the file
# mid operation.
# https://stackoverflow.com/questions/7645338/how-to-do-atomic-file-replacement
# http://www.weirdnet.nl/apple/rename.html
import os
import hjson
import filelock

def safe_json_read(filename, timeout = None):
    """
    Turns out reading a json file safely, atomically,
    and without corrupting the file is a bit tricky.
    """
    with filelock.FileLock(f"{filename}.lck", timeout = timeout):
        # Read the file
        with open(filename) as f:
            return hjson.load(f)

def safe_json_write(filename, data, *json_args, **json_kwargs):
    """
    Turns out writing a json file safely, atomically,
    and without corrupting the file is a bit tricky.
    """
    with filelock.FileLock(f"{filename}.lck"):
        # Write to a temp file
        with open(f"{filename}.tmp", "w") as f:
            hjson.dump(data, f, *json_args, **json_kwargs)

        # Replace the original file
        os.replace(f"{filename}.tmp", filename)