#!/usr/bin/env python
"""Build the latest kernel but only if need be"""
import os
import shutil
import subprocess

EMERGE = os.environ['EMERGE'].split()
USEPKG = os.environ['USEPKG'].split()


def get_latest_available_kernel():
    kernel = os.environ['KERNEL']
    package_name = 'sys-kernel/{}'.format(kernel)
    popen = subprocess.Popen(
        ['portageq', 'best_visible', '/', package_name],
        stdout=subprocess.PIPE,
    )
    cpv = popen.stdout.read().decode().strip()

    return cpv.rpartition('-')[2]


def get_current_kernel():
    """what kernel does /boot/vmlinuz point to"""
    vmlinuz = '/boot/vmlinuz'

    if not os.path.islink(vmlinuz):
        return None

    filename = os.path.basename(os.path.realpath(vmlinuz))
    assert filename.startswith('vmlinuz')
    part_after_vmlinuz = filename[8:]

    # may have "-gentoo" or whatever in the name
    version = part_after_vmlinuz.rpartition('-')[0]

    return version


def install_kernel_package():
    kernel = os.environ['KERNEL']
    package_name = 'sys-kernel/{}'.format(kernel)
    cmd = EMERGE + USEPKG + ['--oneshot',  '--noreplace', package_name]

    subprocess.check_call(cmd)


def copy_kernel_config():
    filename = '/root/kernel.config'

    shutil.copy(filename, '/usr/src/linux/.config')


def build_kernel():
    makeopts = os.environ.get('MAKEOPTS', '')

    subprocess.check_call([
        'make', '-C', '/usr/src/linux', 'MAKEOPTS=' + makeopts, 'oldconfig'
    ])
    subprocess.check_call([
        'make', '-C', '/usr/src/linux', 'MAKEOPTS=' + makeopts
    ])


def remove_old_kernels():
    kernel_files = ['vmlinuz-', 'System.map-', 'config-']

    for filename in os.listdir('/boot'):
        for kernel_file in kernel_files:
            if filename.startswith(kernel_file):
                path = os.path.join('/boot', filename)
                os.unlink(path)

    if os.path.exists('/boot/vmlinuz'):
        os.unlink('/boot/vmlinuz')

    shutil.rmtree('/lib/modules', ignore_errors=True)


def install_kernel():
    makeopts = os.environ.get('MAKEOPTS', '')

    subprocess.check_call([
        'make', '-C', '/usr/src/linux', 'MAKEOPTS=' + makeopts, 'install',
        'modules_install'
    ])

    # create the symlink.  /sbin/installkernel claims this is not used on
    # "modern" distributions and doesn't create it (unless it already
    # exists).  I still do this however.
    for filename in os.listdir('/boot'):
        if filename.startswith('vmlinuz-'):
            if os.path.lexists('/boot/vmlinuz'):
                os.unlink('/boot/vmlinuz')
            os.symlink(filename, '/boot/vmlinuz')
            break


def uninstall_kernel_package():
    subprocess.check_call([
        'make', '-C', '/usr/src/linux', 'distclean'
    ])

    subprocess.check_call(EMERGE + USEPKG + ['--depclean', '--with-bdeps=n'])


def backup_kernel_config():
    shutil.copy('/usr/src/linux/.config', '/root/kernel.config')


def main():
    latest_kernel = get_latest_available_kernel()
    current_kernel = get_current_kernel()

    if current_kernel == latest_kernel:
        return

    install_kernel_package()
    copy_kernel_config()
    build_kernel()
    remove_old_kernels()
    install_kernel()
    backup_kernel_config()

    uninstall_kernel_package()


main()