Patch `libbackend.so` of Turing Complete's bleeding edge build to use `xterm` rather than `gnome-terminal`. `argparse` version

patch-backend.py

#!/usr/bin/env python3

from __future__ import annotations

import argparse
import hashlib
import sys

from pathlib import Path

from typing import TYPE_CHECKING
if TYPE_CHECKING:
    from typing import Literal


LIBBACKEND_FILENAME = 'libbackend.so'

OLD = b'gnome-terminal -- bash -ic "./si'
NEW = b'setsid -f xterm -e bash -c "./si'


def hexdigest(data: bytes) -> str:
    return hashlib.md5(data, usedforsecurity=False).hexdigest()


def patch_backend(file: Path, action: Literal['do', 'undo', 'view']) -> None:

    orig_content = file.read_bytes()
    count_old = orig_content.count(OLD)
    count_new = orig_content.count(NEW)
    old_content = orig_content.replace(NEW, OLD)
    new_content = orig_content.replace(OLD, NEW)

    if action == 'view':
        print(f"Found {count_old} occurrences of the original byte sequence and {count_new} of the replacement.")
        print(f"  md5sum with original byte sequence:      {hexdigest(old_content)}")
        print(f"  md5sum with current file content:        {hexdigest(orig_content)}")
        print(f"  md5sum with replacement byte sequence:   {hexdigest(new_content)}")
        return

    if count_old + count_new == 0:
        raise ValueError("Found neither original nor replacement byte sequence in file.")

    if count_old + count_new > 1:
        raise ValueError("Found more than one occurrence of original or replacement byte sequence in file.")

    byte_offset = orig_content.find(OLD) if count_old else orig_content.find(NEW)
    if byte_offset < 1024 or (byte_offset & 0xF) != 0:
        raise ValueError(f"Found byte sequence at suspicious offset {byte_offset} / {byte_offset:#x}.")

    match action:
        case 'do':
            if count_old == 0:
                print("Original byte sequence does not occur in file.", file=sys.stderr)
                print(f"  md5sum:   {hexdigest(orig_content)}", file=sys.stderr)

            else:
                print(f"  md5sum was:      {hexdigest(orig_content)}", file=sys.stderr)
                file.write_bytes(new_content)
                print(f"  md5sum now is:   {hexdigest(new_content)}", file=sys.stderr)

        case 'undo':
            if count_new == 0:
                print("Replacement byte sequence does not occur in file.", file=sys.stderr)
                print(f"  md5sum:   {hexdigest(orig_content)}", file=sys.stderr)

            else:
                print(f"  md5sum was:      {hexdigest(orig_content)}", file=sys.stderr)
                file.write_bytes(old_content)
                print(f"  md5sum now is:   {hexdigest(old_content)}", file=sys.stderr)

        case _:
            raise NotImplementedError(f"unimplemented action {action!r}")


def cli() -> None:
    """Replace debug terminal app in Turing Complete's libbackend.so."""

    parser = argparse.ArgumentParser(description=cli.__doc__)
    parser.add_argument('file', nargs='?', type=str, help='alternative file to patch', default=None)
    parser.add_argument('--do',         dest='action', action='store_const', const='do',   help='replace gnome-terminal with xterm (default action)', default='do')
    parser.add_argument('--undo', '-u', dest='action', action='store_const', const='undo', help='go back to using gnome-terminal')
    parser.add_argument('--view', '-n', dest='action', action='store_const', const='view', help='count the occurrences and view the checksums')

    args = parser.parse_args()

    try:
        patch_backend(
            file = Path(args.file) if args.file is not None else Path(sys.argv[0]).parent / LIBBACKEND_FILENAME,
            action = args.action,
        )

    except Exception as e:
        raise SystemExit(e)


if __name__ == '__main__':
    cli()