mmdebstrap/ldconfig.fakechroot

#!/usr/bin/env python3
#
# This script is in the public domain
#
# Author: Johannes Schauer Marin Rodrigues <josch@mister-muffin.de>
#
# This is command substitution for ldconfig under fakechroot:
#
#     export FAKECHROOT_CMD_SUBST=/sbin/ldconfig=/path/to/ldconfig.fakechroot
#
# Statically linked binaries cannot work with fakechroot and thus have to be
# replaced by either /bin/true or a more clever solution like this one. The
# ldconfig command supports the -r option which allows passing a chroot
# directory for ldconfig to work in. This can be used to run ldconfig without
# fakechroot but still let it create /etc/ld.so.cache inside the chroot.
#
# Since absolute symlinks are broken without fakechroot to translate them,
# we read /etc/ld.so.conf and turn all absolute symlink shared libraries into
# relative ones. At program exit, the original state is restored.


import os
import sys
import subprocess
import atexit
import glob
from pathlib import Path

symlinks = []


def restore_symlinks():
    for (link, target, atime, mtime) in symlinks:
        link.unlink()
        link.symlink_to(target)
        os.utime(link, times=None, ns=(atime, mtime), follow_symlinks=False)


atexit.register(restore_symlinks)


def get_libdirs(chroot, configs):
    res = []
    for conf in configs:
        for line in (Path(conf)).read_text().splitlines():
            line = line.strip()
            if not line:
                continue
            if line.startswith("#"):
                continue
            if line.startswith("include "):
                assert line.startswith("include /")
                res.extend(
                    get_libdirs(chroot, chroot.glob(line.removeprefix("include /")))
                )
                continue
            assert line.startswith("/"), line
            line = line.lstrip("/")
            if not (chroot / Path(line)).is_dir():
                continue
            for f in (chroot / Path(line)).iterdir():
                if not f.is_symlink():
                    continue
                linktarget = f.readlink()
                # make sure that the linktarget is an absolute path inside the
                # chroot
                if not str(linktarget).startswith("/"):
                    continue
                if chroot not in linktarget.parents:
                    continue
                # store original link so that we can restore it later
                symlinks.append(
                    (f, linktarget, f.lstat().st_atime_ns, f.lstat().st_mtime_ns)
                )
                # replace absolute symlink by relative link
                relative = os.path.relpath(linktarget, f.parent)
                f.unlink()
                f.symlink_to(relative)
    return res


def main():
    if "FAKECHROOT_BASE_ORIG" not in os.environ:
        print("FAKECHROOT_BASE_ORIG is not set", file=sys.stderr)
        print(
            "must be executed under fakechroot using FAKECHROOT_CMD_SUBST",
            file=sys.stderr,
        )
        sys.exit(1)

    chroot = Path(os.environ["FAKECHROOT_BASE_ORIG"])

    # if chrootless mode is used from within a fakechroot chroot, then
    # FAKECHROOT_BASE_ORIG will point at the outer chroot. We want to use
    # the path from DPKG_ROOT inside of that instead
    if os.environ.get("DPKG_ROOT", "") not in ["", "/"]:
        chroot /= os.environ["DPKG_ROOT"].lstrip("/")

    if not (chroot / "sbin" / "ldconfig").exists():
        sys.exit(0)

    (chroot / "var" / "cache" / "ldconfig").mkdir(
        mode=0o700, parents=True, exist_ok=True
    )

    for d in get_libdirs(chroot, [chroot / "etc" / "ld.so.conf"]):
        make_relative(d)

    # we add any additional arguments before "-r" such that any other "-r"
    # option will be overwritten by the one we set
    subprocess.check_call(
        [chroot / "sbin" / "ldconfig"] + sys.argv[1:] + ["-r", chroot]
    )


if __name__ == "__main__":
    main()