#!/usr/bin/env python3
"""Internal elevated writer: configuration bytes arrive only on stdin.

Use a stable directory descriptor, exclusive temporary file, exact input-length
check and same-directory rename. Never evaluate configuration as a program.
"""
import os
import secrets
import stat
import sys


def write_config(path, expected_length, source):
    parent, name = os.path.split(path)
    if not parent or not name or name in (".", "..") or expected_length < 0:
        raise ValueError("invalid destination")
    directory = os.open(parent, os.O_RDONLY | os.O_DIRECTORY | os.O_NOFOLLOW)
    temporary = ".lnd-config-" + secrets.token_hex(16)
    created = False
    try:
        try:
            info = os.stat(name, dir_fd=directory, follow_symlinks=False)
            if not stat.S_ISREG(info.st_mode) or stat.S_IMODE(info.st_mode) & 0o7000:
                raise ValueError("destination must be an ordinary regular file")
            mode = stat.S_IMODE(info.st_mode)
        except FileNotFoundError:
            info = os.fstat(directory)
            mode = 0o600
        fd = os.open(temporary, os.O_WRONLY | os.O_CREAT | os.O_EXCL | os.O_NOFOLLOW,
                     0o600, dir_fd=directory)
        created = True
        with os.fdopen(fd, "wb") as output:
            remaining = expected_length
            while remaining:
                chunk = source.read(min(65536, remaining))
                if not chunk:
                    raise ValueError("incomplete input")
                output.write(chunk)
                remaining -= len(chunk)
            if source.read(1):
                raise ValueError("excess input")
            output.flush()
            os.fchown(output.fileno(), info.st_uid, info.st_gid)
            os.fchmod(output.fileno(), mode)
            os.fsync(output.fileno())
        os.replace(temporary, name, src_dir_fd=directory, dst_dir_fd=directory)
        created = False
        os.fsync(directory)
    finally:
        if created:
            os.unlink(temporary, dir_fd=directory)
        os.close(directory)


if __name__ == "__main__":
    try:
        if len(sys.argv) != 3:
            raise ValueError("invalid arguments")
        write_config(sys.argv[1], int(sys.argv[2]), sys.stdin.buffer)
    except Exception:
        # No input contents or parser snippets in privileged process logs.
        print("LND configuration write failed", file=sys.stderr)
        sys.exit(1)