Classroom_Desktop/scripts/classroom-desktop-layout

#!/usr/bin/env python3

"""Match the classroom's panel to the machine it is on.

Two of the bar's launchers depend on the machine rather than on the
release, and this edits the marked line for each of them in the installed
layout, and nothing else.

    the disc player   a room with no optical drive should not be offered
                      one. The machine is asked; nothing is remembered.

    Wonder Ink        WONDERINK_ENABLED in device.conf. One future device
                      is a laptop that will not have Wonder Ink at all,
                      and a device that is not a classroom should not
                      carry a lesson application it cannot use.

The two are deliberately answered differently. A drive is something a
machine can be asked about, and asking beats remembering to edit a file
when a computer moves between rooms. A licence is not, so it is a
setting.

Run before the display manager by ekklesia-classroom-layout.service, and
again by udev when a drive is plugged in or pulled out. That ordering is
the whole point: the panel is built from this layout the moment a session
starts, and a room that is told about its hardware afterwards keeps the
wrong bar until the next login. It was being left to the provisioning
service, which runs at boot alongside the session rather than before it.

It writes the *layout*, not the running panel. Editing
plasma-org.kde.plasma.desktop-appletsrc underneath a live plasmashell was
tried and it destroyed the panel: the shell owns that file and rewrites
it, so an edit races with it. The layout is a file the package owns and
the shell only reads, and classroom-desktop-reset makes the shell read it
again at the next login.

"""

import glob
import os
import re
import sys

sys.path.insert(0, "/usr/local/lib/ekklesia-classroom")
sys.path.insert(
    0,
    os.path.join(os.path.dirname(os.path.abspath(__file__)), "..", "lib"),
)

from classroom_config import enabled, read_device_config  # noqa: E402

LAYOUT = (
    "/usr/share/plasma/look-and-feel/org.ekklesia.classroom.desktop"
    "/contents/layouts/org.kde.plasma.desktop-layout.js"
)

DVD_MARKER = "/* CLASSROOM-DVD-LAUNCHER */"
DVD_ENTRY = "ekklesia-dvd.desktop"

WONDERINK_MARKER = "/* CLASSROOM-WONDERINK-LAUNCHER */"
WONDERINK_ENTRY = "ekklesia-wonderink.desktop"

# The kernel names optical drives sr0, sr1, ... regardless of bus. Both
# views are checked: /dev is what anything opening the drive will use, and
# /sys/block is the kernel's own list, which cannot be left behind by a
# device node somebody or something created by hand.
DRIVE_GLOB = "/dev/sr[0-9]*"
SYSFS_GLOB = "/sys/block/sr[0-9]*"


def drives():
    return sorted(glob.glob(DRIVE_GLOB)), sorted(glob.glob(SYSFS_GLOB))


def has_optical_drive():
    nodes, blocks = drives()

    # Said out loud every time. When a room shows a disc player it has no
    # drive for, or hides one it has, this line in the journal is the
    # difference between an answer and an afternoon.
    print("classroom-desktop-layout: /dev "
          f"[{', '.join(nodes) or 'none'}], /sys/block "
          f"[{', '.join(blocks) or 'none'}]")

    return bool(nodes and blocks)


def rewrite(text, marker, entry, wanted):
    """Comment one marked launcher in or out, idempotently."""
    line = re.compile(
        r"^(\s*)(//\s*)?(" + re.escape(marker)
        + r'\s*"applications:' + re.escape(entry) + r'")',
        re.M,
    )

    def swap(match):
        indent, _, body = match.groups()
        return f"{indent}{'' if wanted else '// '}{body}"

    return line.sub(swap, text)


def wonderink_wanted():
    """WONDERINK_ENABLED, defaulting to true.

    A classroom that has never heard of the setting is a classroom, so the
    absent case is the one every existing room is in and it has to mean
    yes. Said out loud for the same reason the drive is.
    """
    config = read_device_config()
    wanted = enabled(config, "WONDERINK_ENABLED", default=True)
    print(f"classroom-desktop-layout: WONDERINK_ENABLED "
          f"[{config.get('WONDERINK_ENABLED', 'unset')}]")
    return wanted


def main():
    path = sys.argv[1] if len(sys.argv) > 1 else LAYOUT

    try:
        with open(path, "r", encoding="utf-8") as handle:
            before = handle.read()
    except OSError as error:
        print(f"classroom-desktop-layout: {error}", file=sys.stderr)
        return 0

    if DVD_MARKER not in before:
        print("classroom-desktop-layout: no DVD marker in the layout",
              file=sys.stderr)
        return 0

    after = rewrite(before, DVD_MARKER, DVD_ENTRY, has_optical_drive())

    if WONDERINK_MARKER in before:
        after = rewrite(
            after, WONDERINK_MARKER, WONDERINK_ENTRY, wonderink_wanted()
        )
    else:
        print("classroom-desktop-layout: no Wonder Ink marker in the layout",
              file=sys.stderr)

    if after == before:
        print("classroom-desktop-layout: the layout already matches "
              "this machine")
        return 0

    # Replace the file rather than edit in place, so a full disk or a
    # crash leaves the old layout intact rather than half of a new one.
    temporary = f"{path}.new"

    try:
        with open(temporary, "w", encoding="utf-8") as handle:
            handle.write(after)
        os.replace(temporary, path)
    except OSError as error:
        print(f"classroom-desktop-layout: {error}", file=sys.stderr)
        return 1

    print("classroom-desktop-layout: the layout now matches this machine")
    return 0


if __name__ == "__main__":
    sys.exit(main())