debian-forge/devices/org.osbuild.loopback

#!/usr/bin/python3
"""
Loopback device host service

This service can be used to expose a file or a subset of it as a
device node. The file is specified via the `filename`, and the
subset can be specified via `offset` and `size`.
The resulting device name is returned together with the device
node numbers (`major`, `minor`). The device is closed when the
service is shut down.
A typical use case is formatting the file or a partition in the
file with a file system or mounting a previously created file
system contained in the file.
NB: This will use the custom osbuild udev rules inhibitor to
suppress problematic udev rules. For more details see the
documentation for `osbuil.util.udev.UdevInhibitor`.
"""


import argparse
import os
import sys
from typing import Dict

from osbuild import devices, loop
from osbuild.util.udev import UdevInhibitor

SCHEMA = """
"additionalProperties": false,
"required": ["filename"],
"properties": {
  "filename": {
    "type": "string",
    "description": "File to associate with the loopback device"
  },
  "start": {
    "type": "number",
    "description": "Start of the data segment (in sectors)",
    "default": 0
  },
  "size": {
    "type": "number",
    "description": "Size limit of the data segment (in sectors)"
  },
  "sector-size": {
    "type": "number",
    "description": "Sector size (in bytes)",
    "default": 512
  },
  "lock": {
    "type": "boolean",
    "description": "Lock the device after opening it"
  }
}
"""


class LoopbackService(devices.DeviceService):

    def __init__(self, args: argparse.Namespace):
        super().__init__(args)
        self.ctl = loop.LoopControl()
        self.fd = None
        self.lo = None
        self.sector_size = None

    @staticmethod
    def setup_loop(lo: loop.Loop):
        lock = UdevInhibitor.for_device(lo.LOOP_MAJOR, lo.minor)
        lo.on_close = lambda _l: lock.release()

    def make_loop(self, fd: int, offset, sizelimit, lock):
        if not sizelimit:
            stat = os.fstat(fd)
            sizelimit = stat.st_size - offset
        else:
            sizelimit *= self.sector_size

        lo = self.ctl.loop_for_fd(fd, lock=lock,
                                  setup=self.setup_loop,
                                  offset=offset,
                                  sizelimit=sizelimit,
                                  partscan=False,
                                  autoclear=True)

        return lo

    def open(self, devpath: str, parent: str, tree: str, options: Dict):
        filename = options["filename"]
        self.sector_size = options.get("sector-size", 512)
        start = options.get("start", 0) * self.sector_size
        size = options.get("size")
        lock = options.get("lock", False)

        path = os.path.join(tree, filename.lstrip("/"))

        self.fd = os.open(path, os.O_RDWR | os.O_CLOEXEC)
        try:
            self.lo = self.make_loop(self.fd, start, size, lock)
        except Exception as error:  # pylint: disable: broad-except
            self.close()
            raise error from None

        print(f"{self.lo.devname} acquired (locked: {lock})")

        dir_fd = -1
        try:
            dir_fd = os.open(devpath, os.O_CLOEXEC | os.O_PATH)
            self.lo.mknod(dir_fd)
        finally:
            if dir_fd > -1:
                os.close(dir_fd)

        res = {
            "path": self.lo.devname,
            "node": {
                "major": self.lo.LOOP_MAJOR,
                "minor": self.lo.minor,
            }
        }

        return res

    def close(self):
        # Calling `close` is valid on closed
        # `LoopControl` and `Loop` objects
        self.ctl.close()

        if self.lo:
            # Flush the buffer cache of the loop device. This
            # seems to be required when clearing the fd of the
            # loop device (as of kernel 5.13.8) or otherwise
            # it leads to data loss.
            self.lo.flush_buf()

            # clear the fd. Since it might not immediately be
            # cleared (due to a race with udev or some other
            # process still having a reference to the loop dev)
            # we give it some time and wait for the clearing
            self.lo.clear_fd_wait(self.fd, 30)
            self.lo.close()
            self.lo = None

        if self.fd is not None:
            fd = self.fd
            self.fd = None
            try:
                os.fsync(fd)
            finally:
                os.close(fd)


def main():
    service = LoopbackService.from_args(sys.argv[1:])
    service.main()


if __name__ == '__main__':
    main()