Radicale/radicale/xmlutils.py

# This file is part of Radicale Server - Calendar Server
# Copyright © 2008 Nicolas Kandel
# Copyright © 2008 Pascal Halter
# Copyright © 2008-2015 Guillaume Ayoub
#
# This library is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This library is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with Radicale.  If not, see <http://www.gnu.org/licenses/>.

"""
XML and iCal requests manager.

Note that all these functions need to receive unicode objects for full
iCal requests (PUT) and string objects with charset correctly defined
in them for XML requests (all but PUT).

"""

import posixpath
import re
import xml.etree.ElementTree as ET
from collections import OrderedDict
from datetime import datetime, timedelta, timezone
from http import client
from urllib.parse import quote, unquote, urlparse

from . import storage


MIMETYPES = {
    "VADDRESSBOOK": "text/vcard",
    "VCALENDAR": "text/calendar"}

NAMESPACES = {
    "C": "urn:ietf:params:xml:ns:caldav",
    "CR": "urn:ietf:params:xml:ns:carddav",
    "D": "DAV:",
    "CS": "http://calendarserver.org/ns/",
    "ICAL": "http://apple.com/ns/ical/",
    "ME": "http://me.com/_namespace/"}

NAMESPACES_REV = {}
for short, url in NAMESPACES.items():
    NAMESPACES_REV[url] = short
    ET.register_namespace("" if short == "D" else short, url)

CLARK_TAG_REGEX = re.compile(r" {(?P<namespace>[^}]*)}(?P<tag>.*)", re.VERBOSE)


def _pretty_xml(element, level=0):
    """Indent an ElementTree ``element`` and its children."""
    i = "\n" + level * "  "
    if len(element):
        if not element.text or not element.text.strip():
            element.text = i + "  "
        if not element.tail or not element.tail.strip():
            element.tail = i
        for sub_element in element:
            _pretty_xml(sub_element, level + 1)
        if not sub_element.tail or not sub_element.tail.strip():
            sub_element.tail = i
    else:
        if level and (not element.tail or not element.tail.strip()):
            element.tail = i
    if not level:
        return '<?xml version="1.0"?>\n%s' % ET.tostring(element, "unicode")


def _tag(short_name, local):
    """Get XML Clark notation {uri(``short_name``)}``local``."""
    return "{%s}%s" % (NAMESPACES[short_name], local)


def _tag_from_clark(name):
    """Get a human-readable variant of the XML Clark notation tag ``name``.

    For a given name using the XML Clark notation, return a human-readable
    variant of the tag name for known namespaces. Otherwise, return the name as
    is.

    """
    match = CLARK_TAG_REGEX.match(name)
    if match and match.group("namespace") in NAMESPACES_REV:
        args = {
            "ns": NAMESPACES_REV[match.group("namespace")],
            "tag": match.group("tag")}
        return "%(ns)s:%(tag)s" % args
    return name


def _response(code):
    """Return full W3C names from HTTP status codes."""
    return "HTTP/1.1 %i %s" % (code, client.responses[code])


def _href(base_prefix, href):
    """Return prefixed href."""
    return quote("%s%s" % (base_prefix, href))


def _date_to_datetime(date_):
    """Transform a date to a UTC datetime.

    If date_ is a datetime without timezone, return as UTC datetime. If date_
    is already a datetime with timezone, return as is.

    """
    if not isinstance(date_, datetime):
        date_ = datetime.combine(date_, datetime.min.time())
    if not date_.tzinfo:
        date_ = date_.replace(tzinfo=timezone.utc)
    return date_


def _comp_match(item, filter_, scope="collection"):
    """Check whether the ``item`` matches the comp ``filter_``.

    If ``scope`` is ``"collection"``, the filter is applied on the
    item's collection. Otherwise, it's applied on the item.

    See rfc4791-9.7.1.

    """
    filter_length = len(filter_)
    if scope == "collection":
        tag = item.collection.get_meta("tag")
    else:
        for component in item.components():
            if component.name in ("VTODO", "VEVENT", "VJOURNAL"):
                tag = component.name
                break
        else:
            return False
    if filter_length == 0:
        # Point #1 of rfc4791-9.7.1
        return filter_.get("name") == tag
    else:
        if filter_length == 1:
            if filter_[0].tag == _tag("C", "is-not-defined"):
                # Point #2 of rfc4791-9.7.1
                return filter_.get("name") != tag
        if filter_[0].tag == _tag("C", "time-range"):
            # Point #3 of rfc4791-9.7.1
            if not _time_range_match(item.item, filter_[0], tag):
                return False
            filter_ = filter_[1:]
        # Point #4 of rfc4791-9.7.1
        return all(
            _prop_match(item, child) if child.tag == _tag("C", "prop-filter")
            else _comp_match(item, child, scope="component")
            for child in filter_)


def _prop_match(item, filter_):
    """Check whether the ``item`` matches the prop ``filter_``.

    See rfc4791-9.7.2 and rfc6352-10.5.1.

    """
    filter_length = len(filter_)
    if item.collection.get_meta("tag") == "VCALENDAR":
        for component in item.components():
            if component.name in ("VTODO", "VEVENT", "VJOURNAL"):
                vobject_item = component
    else:
        vobject_item = item.item
    if filter_length == 0:
        # Point #1 of rfc4791-9.7.2
        return filter_.get("name").lower() in vobject_item.contents
    else:
        name = filter_.get("name").lower()
        if filter_length == 1:
            if filter_[0].tag == _tag("C", "is-not-defined"):
                # Point #2 of rfc4791-9.7.2
                return name not in vobject_item.contents
        if filter_[0].tag == _tag("C", "time-range"):
            # Point #3 of rfc4791-9.7.2
            if not _time_range_match(vobject_item, filter_[0], name):
                return False
            filter_ = filter_[1:]
        elif filter_[0].tag == _tag("C", "text-match"):
            # Point #4 of rfc4791-9.7.2
            if not _text_match(vobject_item, filter_[0], name):
                return False
            filter_ = filter_[1:]
        return all(
            _param_filter_match(vobject_item, param_filter, name)
            for param_filter in filter_)


def _time_range_match(vobject_item, filter_, child_name):
    """Check whether the ``item`` matches the time-range ``filter_``.

    See rfc4791-9.9.

    """
    start = filter_.get("start")
    end = filter_.get("end")
    if not start and not end:
        return False
    if start:
        start = datetime.strptime(start, "%Y%m%dT%H%M%SZ")
    else:
        start = datetime.min
    if end:
        end = datetime.strptime(end, "%Y%m%dT%H%M%SZ")
    else:
        end = datetime.max
    start = start.replace(tzinfo=timezone.utc)
    end = end.replace(tzinfo=timezone.utc)
    child = getattr(vobject_item, child_name.lower())

    # Comments give the lines in the tables of the specification
    if child_name == "VEVENT":
        # TODO: check if there's a timezone
        dtstart = child.dtstart.value

        if child.rruleset:
            dtstarts = child.getrruleset(addRDate=True)
        else:
            dtstarts = (dtstart,)

        dtend = getattr(child, "dtend", None)
        if dtend is not None:
            dtend = dtend.value
            original_duration = (dtend - dtstart).total_seconds()
            dtend = _date_to_datetime(dtend)

        duration = getattr(child, "duration", None)
        if duration is not None:
            original_duration = duration = duration.value

        for dtstart in dtstarts:
            dtstart_is_datetime = isinstance(dtstart, datetime)
            dtstart = _date_to_datetime(dtstart)

            if dtstart > end:
                break

            if dtend is not None:
                # Line 1
                dtend = dtstart + timedelta(seconds=original_duration)
                if start < dtend and end > dtstart:
                    return True
            elif duration is not None:
                if original_duration is None:
                    original_duration = duration.seconds
                if duration.seconds > 0:
                    # Line 2
                    if start < dtstart + duration and end > dtstart:
                        return True
                elif start <= dtstart and end > dtstart:
                    # Line 3
                    return True
            elif dtstart_is_datetime:
                # Line 4
                if start <= dtstart and end > dtstart:
                    return True
            elif start < dtstart + timedelta(days=1) and end > dtstart:
                # Line 5
                return True

    elif child_name == "VTODO":
        dtstart = getattr(child, "dtstart", None)
        duration = getattr(child, "duration", None)
        due = getattr(child, "due", None)
        completed = getattr(child, "completed", None)
        created = getattr(child, "created", None)

        if dtstart is not None:
            dtstart = _date_to_datetime(dtstart.value)
        if duration is not None:
            duration = duration.value
        if due is not None:
            due = _date_to_datetime(due.value)
            if dtstart is not None:
                original_duration = (due - dtstart).total_seconds()
        if completed is not None:
            completed = _date_to_datetime(completed.value)
            if created is not None:
                created = _date_to_datetime(created.value)
                original_duration = (completed - created).total_seconds()
        elif created is not None:
            created = _date_to_datetime(created.value)

        if child.rruleset:
            reference_dates = child.getrruleset(addRDate=True)
        else:
            if dtstart is not None:
                reference_dates = (dtstart,)
            elif due is not None:
                reference_dates = (due,)
            elif completed is not None:
                reference_dates = (completed,)
            elif created is not None:
                reference_dates = (created,)
            else:
                # Line 8
                return True

        for reference_date in reference_dates:
            reference_date = _date_to_datetime(reference_date)
            if reference_date > end:
                break

            if dtstart is not None and duration is not None:
                # Line 1
                if start <= reference_date + duration and (
                        end > reference_date or
                        end >= reference_date + duration):
                    return True
            elif dtstart is not None and due is not None:
                # Line 2
                due = reference_date + timedelta(seconds=original_duration)
                if (start < due or start <= reference_date) and (
                        end > reference_date or end >= due):
                    return True
            elif dtstart is not None:
                if start <= reference_date and end > reference_date:
                    return True
            elif due is not None:
                # Line 4
                if start < reference_date and end >= reference_date:
                    return True
            elif completed is not None and created is not None:
                # Line 5
                completed = reference_date + timedelta(
                    seconds=original_duration)
                if (start <= reference_date or start <= completed) and (
                        end >= reference_date or end >= completed):
                    return True
            elif completed is not None:
                # Line 6
                if start <= reference_date and end >= reference_date:
                    return True
            elif created is not None:
                # Line 7
                if end > reference_date:
                    return True

    elif child_name == "VJOURNAL":
        dtstart = getattr(child, "dtstart", None)

        if dtstart is not None:
            dtstart = dtstart.value
            if child.rruleset:
                dtstarts = child.getrruleset(addRDate=True)
            else:
                dtstarts = (dtstart,)

            for dtstart in dtstarts:
                dtstart_is_datetime = isinstance(dtstart, datetime)
                dtstart = _date_to_datetime(dtstart)

                if dtstart > end:
                    break

                if dtstart_is_datetime:
                    # Line 1
                    if start <= dtstart and end > dtstart:
                        return True
                elif start < dtstart + timedelta(days=1) and end > dtstart:
                    # Line 2
                    return True

    return False


def _text_match(vobject_item, filter_, child_name, attrib_name=None):
    """Check whether the ``item`` matches the text-match ``filter_``.

    See rfc4791-9.7.5.

    """
    # TODO: collations are not supported, but the default ones needed
    # for DAV servers are actually pretty useless. Texts are lowered to
    # be case-insensitive, almost as the "i;ascii-casemap" value.
    match = next(filter_.itertext()).lower()
    children = getattr(vobject_item, "%s_list" % child_name, [])
    if attrib_name:
        condition = any(
            match in attrib.lower() for child in children
            for attrib in child.params.get(attrib_name, []))
    else:
        condition = any(match in child.value.lower() for child in children)
    if filter_.get("negate-condition") == "yes":
        return not condition
    else:
        return condition


def _param_filter_match(vobject_item, filter_, parent_name):
    """Check whether the ``item`` matches the param-filter ``filter_``.

    See rfc4791-9.7.3.

    """
    name = filter_.get("name")
    children = getattr(vobject_item, "%s_list" % parent_name, [])
    condition = any(name in child.params for child in children)
    if len(filter_):
        if filter_[0].tag == _tag("C", "text-match"):
            return condition and _text_match(
                vobject_item, filter_[0], parent_name, name)
        elif filter_[0].tag == _tag("C", "is-not-defined"):
            return not condition
    else:
        return condition


def name_from_path(path, collection):
    """Return Radicale item name from ``path``."""
    path = path.strip("/") + "/"
    start = collection.path + "/"
    if not path.startswith(start):
        raise ValueError("'%s' doesn't start with '%s'" % (path, start))
    name = path[len(start):][:-1]
    if name and not storage.is_safe_path_component(name):
        raise ValueError("'%s' is not a component in collection '%s'" %
                         (path, collection.path))
    return name


def props_from_request(root, actions=("set", "remove")):
    """Return a list of properties as a dictionary."""
    result = OrderedDict()
    if root:
        if not hasattr(root, "tag"):
            root = ET.fromstring(root.encode("utf8"))
    else:
        return result

    for action in actions:
        action_element = root.find(_tag("D", action))
        if action_element is not None:
            break
    else:
        action_element = root

    prop_element = action_element.find(_tag("D", "prop"))
    if prop_element is not None:
        for prop in prop_element:
            if prop.tag == _tag("D", "resourcetype"):
                for resource_type in prop:
                    if resource_type.tag == _tag("C", "calendar"):
                        result["tag"] = "VCALENDAR"
                        break
                    elif resource_type.tag == _tag("CR", "addressbook"):
                        result["tag"] = "VADDRESSBOOK"
                        break
            elif prop.tag == _tag("C", "supported-calendar-component-set"):
                result[_tag_from_clark(prop.tag)] = ",".join(
                    supported_comp.attrib["name"]
                    for supported_comp in prop
                    if supported_comp.tag == _tag("C", "comp"))
            else:
                result[_tag_from_clark(prop.tag)] = prop.text

    return result


def delete(base_prefix, path, collection, href=None):
    """Read and answer DELETE requests.

    Read rfc4918-9.6 for info.

    """
    collection.delete(href)

    multistatus = ET.Element(_tag("D", "multistatus"))
    response = ET.Element(_tag("D", "response"))
    multistatus.append(response)

    href = ET.Element(_tag("D", "href"))
    href.text = _href(base_prefix, path)
    response.append(href)

    status = ET.Element(_tag("D", "status"))
    status.text = _response(200)
    response.append(status)

    return _pretty_xml(multistatus)


def propfind(base_prefix, path, xml_request, read_collections,
             write_collections, user):
    """Read and answer PROPFIND requests.

    Read rfc4918-9.1 for info.

    The collections parameter is a list of collections that are to be included
    in the output.

    """
    if xml_request:
        root = ET.fromstring(xml_request.encode("utf8"))
        props = [prop.tag for prop in root.find(_tag("D", "prop"))]
    else:
        props = [
            _tag("D", "getcontenttype"),
            _tag("D", "resourcetype"),
            _tag("D", "displayname"),
            _tag("D", "owner"),
            _tag("D", "getetag"),
            _tag("ICAL", "calendar-color"),
            _tag("CS", "getctag")]

    if _tag("D", "current-user-principal") in props and not user:
        # Ask for authentication
        # Returning the DAV:unauthenticated pseudo-principal as specified in
        # RFC 5397 doesn't seem to work with DAVdroid.
        return client.FORBIDDEN, None

    multistatus = ET.Element(_tag("D", "multistatus"))
    collections = []
    for collection in write_collections:
        collections.append(collection)
        response = _propfind_response(
            base_prefix, path, collection, props, user, write=True)
        multistatus.append(response)
    for collection in read_collections:
        if collection in collections:
            continue
        response = _propfind_response(
            base_prefix, path, collection, props, user, write=False)
        multistatus.append(response)

    return client.MULTI_STATUS, _pretty_xml(multistatus)


def _propfind_response(base_prefix, path, item, props, user, write=False):
    """Build and return a PROPFIND response."""
    is_collection = isinstance(item, storage.BaseCollection)
    if is_collection:
        is_leaf = item.get_meta("tag") in ("VADDRESSBOOK", "VCALENDAR")
        collection = item
    else:
        collection = item.collection

    response = ET.Element(_tag("D", "response"))

    href = ET.Element(_tag("D", "href"))
    if is_collection:
        # Some clients expect collections to end with /
        uri = "/%s/" % item.path if item.path else "/"
    else:
        uri = "/" + posixpath.join(collection.path, item.href)

    href.text = _href(base_prefix, uri)
    response.append(href)

    propstat404 = ET.Element(_tag("D", "propstat"))
    propstat200 = ET.Element(_tag("D", "propstat"))
    response.append(propstat200)

    prop200 = ET.Element(_tag("D", "prop"))
    propstat200.append(prop200)

    prop404 = ET.Element(_tag("D", "prop"))
    propstat404.append(prop404)

    for tag in props:
        element = ET.Element(tag)
        is404 = False
        if tag == _tag("D", "getetag"):
            element.text = item.etag
        elif tag == _tag("D", "getlastmodified"):
            element.text = item.last_modified
        elif tag == _tag("D", "principal-collection-set"):
            tag = ET.Element(_tag("D", "href"))
            tag.text = _href(base_prefix, "/")
            element.append(tag)
        elif (tag in (_tag("C", "calendar-user-address-set"),
                      _tag("D", "principal-URL"),
                      _tag("CR", "addressbook-home-set"),
                      _tag("C", "calendar-home-set")) and
                collection.is_principal and is_collection):
            tag = ET.Element(_tag("D", "href"))
            tag.text = _href(base_prefix, path)
            element.append(tag)
        elif tag == _tag("C", "supported-calendar-component-set"):
            human_tag = _tag_from_clark(tag)
            if is_collection and is_leaf:
                meta = item.get_meta(human_tag)
                if meta:
                    components = meta.split(",")
                else:
                    components = ("VTODO", "VEVENT", "VJOURNAL")
                for component in components:
                    comp = ET.Element(_tag("C", "comp"))
                    comp.set("name", component)
                    element.append(comp)
            else:
                is404 = True
        elif tag == _tag("D", "current-user-principal"):
            tag = ET.Element(_tag("D", "href"))
            tag.text = _href(base_prefix, ("/%s/" % user) if user else "/")
            element.append(tag)
        elif tag == _tag("D", "current-user-privilege-set"):
            privilege = ET.Element(_tag("D", "privilege"))
            if write:
                privilege.append(ET.Element(_tag("D", "all")))
                privilege.append(ET.Element(_tag("D", "write")))
                privilege.append(ET.Element(_tag("D", "write-properties")))
                privilege.append(ET.Element(_tag("D", "write-content")))
            privilege.append(ET.Element(_tag("D", "read")))
            element.append(privilege)
        elif tag == _tag("D", "supported-report-set"):
            for report_name in (
                    "principal-property-search", "sync-collection",
                    "expand-property", "principal-search-property-set"):
                supported = ET.Element(_tag("D", "supported-report"))
                report_tag = ET.Element(_tag("D", "report"))
                supported_report_tag = ET.Element(_tag("D", report_name))
                report_tag.append(supported_report_tag)
                supported.append(report_tag)
                element.append(supported)
        elif is_collection:
            if tag == _tag("D", "getcontenttype"):
                item_tag = item.get_meta("tag")
                if item_tag:
                    element.text = MIMETYPES[item_tag]
                else:
                    is404 = True
            elif tag == _tag("D", "resourcetype"):
                if item.is_principal:
                    tag = ET.Element(_tag("D", "principal"))
                    element.append(tag)
                item_tag = item.get_meta("tag")
                if is_leaf or item_tag:
                    # 2nd case happens when the collection is not stored yet,
                    # but the resource type is guessed
                    if item.get_meta("tag") == "VADDRESSBOOK":
                        tag = ET.Element(_tag("CR", "addressbook"))
                        element.append(tag)
                    elif item.get_meta("tag") == "VCALENDAR":
                        tag = ET.Element(_tag("C", "calendar"))
                        element.append(tag)
                tag = ET.Element(_tag("D", "collection"))
                element.append(tag)
            elif tag == _tag("D", "owner"):
                if is_leaf and item.owner:
                    element.text = "/%s/" % item.owner
                else:
                    is404 = True
            elif tag == _tag("D", "displayname"):
                if is_leaf:
                    element.text = item.get_meta("D:displayname") or item.path
                else:
                    is404 = True
            elif tag == _tag("CS", "getctag"):
                if is_leaf:
                    element.text = item.etag
                else:
                    is404 = True
            else:
                human_tag = _tag_from_clark(tag)
                meta = item.get_meta(human_tag)
                if meta:
                    element.text = meta
                else:
                    is404 = True
        # Not for collections
        elif tag == _tag("D", "getcontenttype"):
            name = item.name.lower()
            mimetype = "text/vcard" if name == "vcard" else "text/calendar"
            element.text = "%s; component=%s" % (mimetype, name)
        elif tag == _tag("D", "resourcetype"):
            # resourcetype must be returned empty for non-collection elements
            pass
        elif tag == _tag("D", "getcontentlength"):
            encoding = collection.configuration.get("encoding", "request")
            element.text = str(len(item.serialize().encode(encoding)))
        else:
            is404 = True

        if is404:
            prop404.append(element)
        else:
            prop200.append(element)

    status200 = ET.Element(_tag("D", "status"))
    status200.text = _response(200)
    propstat200.append(status200)

    status404 = ET.Element(_tag("D", "status"))
    status404.text = _response(404)
    propstat404.append(status404)
    if len(prop404):
        response.append(propstat404)

    return response


def _add_propstat_to(element, tag, status_number):
    """Add a PROPSTAT response structure to an element.

    The PROPSTAT answer structure is defined in rfc4918-9.1. It is added to the
    given ``element``, for the following ``tag`` with the given
    ``status_number``.

    """
    propstat = ET.Element(_tag("D", "propstat"))
    element.append(propstat)

    prop = ET.Element(_tag("D", "prop"))
    propstat.append(prop)

    clark_tag = tag if "{" in tag else _tag(*tag.split(":", 1))
    prop_tag = ET.Element(clark_tag)
    prop.append(prop_tag)

    status = ET.Element(_tag("D", "status"))
    status.text = _response(status_number)
    propstat.append(status)


def proppatch(base_prefix, path, xml_request, collection):
    """Read and answer PROPPATCH requests.

    Read rfc4918-9.2 for info.

    """
    root = ET.fromstring(xml_request.encode("utf8"))
    props_to_set = props_from_request(root, actions=("set",))
    props_to_remove = props_from_request(root, actions=("remove",))

    multistatus = ET.Element(_tag("D", "multistatus"))
    response = ET.Element(_tag("D", "response"))
    multistatus.append(response)

    href = ET.Element(_tag("D", "href"))
    href.text = _href(base_prefix, path)
    response.append(href)

    for short_name in props_to_remove:
        props_to_set[short_name] = ""
    collection.set_meta(props_to_set)

    for short_name in props_to_set:
        _add_propstat_to(response, short_name, 200)

    return _pretty_xml(multistatus)


def report(base_prefix, path, xml_request, collection):
    """Read and answer REPORT requests.

    Read rfc3253-3.6 for info.

    """
    root = ET.fromstring(xml_request.encode("utf8"))
    prop_element = root.find(_tag("D", "prop"))
    props = (
        [prop.tag for prop in prop_element]
        if prop_element is not None else [])

    if collection:
        if root.tag in (
                _tag("C", "calendar-multiget"),
                _tag("CR", "addressbook-multiget")):
            # Read rfc4791-7.9 for info
            hreferences = set()
            for href_element in root.findall(_tag("D", "href")):
                href_path = storage.sanitize_path(
                    unquote(urlparse(href_element.text).path))
                if (href_path + "/").startswith(base_prefix + "/"):
                    hreferences.add(href_path[len(base_prefix):])
                else:
                    collection.logger.info(
                        "Skipping invalid path: %s", href_path)
        else:
            hreferences = (path,)
        filters = (
            root.findall(".//%s" % _tag("C", "filter")) +
            root.findall(".//%s" % _tag("CR", "filter")))
    else:
        hreferences = filters = ()

    multistatus = ET.Element(_tag("D", "multistatus"))

    for hreference in hreferences:
        try:
            name = name_from_path(hreference, collection)
        except ValueError:
            collection.logger.info("Skipping invalid path: %s", hreference)
            response = _item_response(base_prefix, hreference,
                                      found_item=False)
            multistatus.append(response)
            continue
        if name:
            # Reference is an item
            item = collection.get(name)
            if not item:
                response = _item_response(base_prefix, hreference,
                                          found_item=False)
                multistatus.append(response)
                continue
            items = [item]
        else:
            # Reference is a collection
            items = collection.pre_filtered_list(filters)

        for item in items:
            if filters:
                match = (
                    _comp_match if collection.get_meta("tag") == "VCALENDAR"
                    else _prop_match)
                if not all(match(item, filter_[0]) for filter_ in filters
                           if filter_):
                    continue

            found_props = []
            not_found_props = []

            for tag in props:
                element = ET.Element(tag)
                if tag == _tag("D", "getetag"):
                    element.text = item.etag
                    found_props.append(element)
                elif tag == _tag("D", "getcontenttype"):
                    name = item.name.lower()
                    mimetype = (
                        "text/vcard" if name == "vcard" else "text/calendar")
                    element.text = "%s; component=%s" % (mimetype, name)
                    found_props.append(element)
                elif tag in (
                        _tag("C", "calendar-data"),
                        _tag("CR", "address-data")):
                    element.text = item.serialize()
                    found_props.append(element)
                else:
                    not_found_props.append(element)

            uri = "/" + posixpath.join(collection.path, item.href)
            multistatus.append(_item_response(
                base_prefix, uri, found_props=found_props,
                not_found_props=not_found_props, found_item=True))

    return _pretty_xml(multistatus)


def _item_response(base_prefix, href, found_props=(), not_found_props=(),
                   found_item=True):
    response = ET.Element(_tag("D", "response"))

    href_tag = ET.Element(_tag("D", "href"))
    href_tag.text = _href(base_prefix, href)
    response.append(href_tag)

    if found_item:
        for code, props in ((200, found_props), (404, not_found_props)):
            if props:
                propstat = ET.Element(_tag("D", "propstat"))
                status = ET.Element(_tag("D", "status"))
                status.text = _response(code)
                prop_tag = ET.Element(_tag("D", "prop"))
                for prop in props:
                    prop_tag.append(prop)
                propstat.append(prop_tag)
                propstat.append(status)
                response.append(propstat)
    else:
        status = ET.Element(_tag("D", "status"))
        status.text = _response(404)
        response.append(status)

    return response