import contextlib
import errno
import urllib.error
import urllib.request

import debian.deb822

from dedup.compression import decompress

def fetchiter(cursor):
    rows = cursor.fetchmany()
    while rows:
        yield from rows
        rows = cursor.fetchmany()

def open_compressed_mirror_url(url, extensions=(".xz", ".gz", "")):
    """Fetch the given url. Try appending each of the given compression
    schemes and move on in case it doesn't exist. Decompress the resulting
    stream on the fly.
    @returns: a file-like with the decompressed contents
    """
    for ext in extensions:
        try:
            handle = urllib.request.urlopen(url + ext)
        except urllib.error.HTTPError as error:
            if error.code != 404:
                raise
        except urllib.error.URLError as error:
            if not hasattr(error.reason, "errno"):
                raise
            if error.reason.errno != errno.ENOENT:
                raise
        else:
            return decompress(handle, ext)
    raise OSError(errno.ENOENT, "No such file or directory")

def iterate_packages(mirror, architecture, distribution="sid", section="main"):
    """Download the relevant binary package list and generate
    debian.deb822.Packages objects per listed package."""
    url = "%s/dists/%s/%s/binary-%s/Packages" % \
            (mirror, distribution, section, architecture)
    with contextlib.closing(open_compressed_mirror_url(url)) as pkglist:
        yield from debian.deb822.Packages.iter_paragraphs(pkglist)