#!/usr/bin/python3

import contextlib
import json
import math
import os
import subprocess
import sys
import tempfile

def tree_size(tree):
    size = 0
    for root, dirs, files in os.walk(tree):
        for entry in files + dirs:
            path = os.path.join(root, entry)
            size += os.stat(path, follow_symlinks=False).st_size
    return size

@contextlib.contextmanager
def mount(source, dest, *options):
    os.makedirs(dest, 0o755, True)
    subprocess.run(["mount", *options, source, dest], check=True)
    try:
        yield
    finally:
        subprocess.run(["umount", "-R", dest], check=True)

@contextlib.contextmanager
def mount_api(dest):
    with mount("/dev", f"{dest}/dev", "-o", "rbind"), \
         mount("/proc", f"{dest}/proc", "-o", "rbind"), \
         mount("/sys", f"{dest}/sys", "-o", "rbind"), \
         mount("none", f"{dest}/run", "-t", "tmpfs"):
        yield

@contextlib.contextmanager
def loop_device(image, size, offset=0):
    r = subprocess.run(["losetup", "--partscan", "--show", "--find", "--sizelimit", str(size), "--offset", str(offset),
                        image], stdout=subprocess.PIPE, encoding="utf-8", check=True)
    loop = r.stdout.strip()
    try:
        yield loop
    finally:
        subprocess.run(["losetup", "-d", loop], check=True)

def main(tree, output_dir, options):
    filename = options["filename"]
    root_fs_uuid = options["root_fs_uuid"]

    # Create a working directory on a tmpfs, maybe we should implicitly
    # always do this.
    with tempfile.TemporaryDirectory() as workdir:
        image = f"{workdir}/image.raw"
        mountpoint = f"{workdir}/mnt"

        # Create an empty image file of the right size
        size = int(math.ceil(tree_size(tree) * 1.2 / 512) * 512)
        subprocess.run(["truncate", "--size", str(size), image], check=True)

        # Set up the partition table of the image
        partition_table = "label: mbr\nlabel-id: {partition_table_id}\nbootable, type=83"
        subprocess.run(["sfdisk", "-q", image], input=partition_table, encoding='utf-8', check=True)
        r = subprocess.run(["sfdisk", "--json", image], stdout=subprocess.PIPE, encoding='utf-8', check=True)
        partition_table = json.loads(r.stdout)
        partition = partition_table["partitiontable"]["partitions"][0]
        partition_offset = partition["start"] * 512
        partition_size = partition["size"] * 512

        # Populate the first partition of the image with an ext4 fs and fill it with the contents of the
        # tree we are operating on.
        subprocess.run(["mkfs.ext4", "-d", tree, "-U", root_fs_uuid, "-E", f"offset={partition_offset}", image,
                        f"{int(partition_size / 1024)}k"], input="y", encoding='utf-8', check=True)

        # Mount the created image as a loopback device
        with loop_device(image, partition_offset) as loop_block, \
            loop_device(image, partition_size, partition_offset) as loop_part, \
            mount(loop_part, mountpoint):
            # Install grub2 into the boot sector of the image, and copy the grub2 imagise into /boot/grub2
            with mount_api(mountpoint):
                subprocess.run(["chroot", mountpoint, "grub2-install", "--no-floppy", "--modules=part_msdos", "--target=i386-pc", loop_block],
                               check=True)

        subprocess.run(["qemu-img", "convert", "-O", "qcow2", "-c", image, f"{output_dir}/{filename}"], check=True)

if __name__ == '__main__':
    args = json.load(sys.stdin)
    ret = main(args["tree"], args["output_dir"], args["options"])
    sys.exit(ret)