import contextlib
import os
import yaml
import sys

from glob import glob


@contextlib.contextmanager
def pushd(newDir):
    previousDir = os.getcwd()
    os.chdir(newDir)
    try:
        yield

    finally:
        os.chdir(previousDir)


class DependencyException(Exception):
    pass


class BackupException(Exception):
    pass


def deptree(sources, deps=None):
    if deps is None:
        deps = []

    deferred = []
    for name in sources.keys():
        source = sources[name]
        if "depends" in source and not source["depends"]:
            deferred.append(name)

        else:
            deps.append(name)

    while deferred:
        name = deferred.pop()
        depends = sources[name]["depends"]
        # todo - detect dependency loop
        if name in depends:
            raise DependencyException('Source {} depends upon itself'.format(name))

        elif set(depends).issubset(set(deps)):
            deps.append(name)

        elif not set(depends).issubset(set(deps)):
            missing = ', '.join(set(depends).difference(set(deps)))
            raise DependencyException(
                    'Source {0} has missing dependencies: {1}'.format(name, missing))

        else:
            deferred.append(name)

    return deps


def status(name, value=None):
    if not hasattr(status, "_handle"):
        status._handle = {}

    if value is None:
        return status._handle[name] if name in status._handle else None

    status._handle[name] = value


def backup(name):
    source = main.sources[name]
    failed = [x for x in source["depends"] if not status(x)]
    if failed:
        raise BackupException(
                "Unable to backup {0} due to ncomplete backups: {1}".format(
                    name, failed))

    # TODO - handle explicit failures with false
    # status(name, False)
    status(name, True)


def main(args):
    with pushd('etc/backup.d'):
        with open("backup.yml") as f:
            main.config = yaml.load(f)

        sources = {}
        for source in main.config['sources']:
            source = os.path.realpath(source)
            for path in glob('{}/*.yml'.format(source)):
                path = os.path.realpath(path)
                with pushd(os.path.dirname(path)), open(path) as f:
                    data = yaml.load(f)
                    if "depends" not in data:
                        data["depends"] = []

                    for i in range(0, len(data["depends"])):
                        data["depends"][i] = os.path.realpath(
                                '{}.yml'.format(data["depends"][i]))

                    sources[path] = data

    main.sources = sources
    main.deptree = deptree(sources)
    errors = []
    for name in main.deptree:
        try:
            backup(name)

        except BackupException as ex:
            print(ex)
            errors.append(ex)

    if errors:
        raise BackupException("At least one backup failed")


if __name__ == '__main__':
    try:
        main(sys.argv[1:])

    except DependencyException as ex:
        print(ex)
        sys.exit(1)

    except BackupException as ex:
        print(ex)
        sys.exit(1)

    except Exception:
        from traceback import format_exc
        msg = "Error encountered:\n" + format_exc().strip()
        print(msg)
        sys.exit(1)