pip/src/pip/_internal/index.py

"""Routines related to PyPI, indexes"""
from __future__ import absolute_import

import cgi
import itertools
import logging
import mimetypes
import os
import posixpath
import re
import sys
from collections import namedtuple

from pip._vendor import html5lib, requests, six
from pip._vendor.distlib.compat import unescape
from pip._vendor.packaging import specifiers
from pip._vendor.packaging.utils import canonicalize_name
from pip._vendor.packaging.version import parse as parse_version
from pip._vendor.requests.exceptions import RetryError, SSLError
from pip._vendor.six.moves.urllib import parse as urllib_parse
from pip._vendor.six.moves.urllib import request as urllib_request

from pip._internal.download import HAS_TLS, is_url, path_to_url, url_to_path
from pip._internal.exceptions import (
    BestVersionAlreadyInstalled, DistributionNotFound, InvalidWheelFilename,
    UnsupportedWheel,
)
from pip._internal.models.candidate import InstallationCandidate
from pip._internal.models.format_control import FormatControl
from pip._internal.models.index import PyPI
from pip._internal.models.link import Link
from pip._internal.pep425tags import get_supported
from pip._internal.utils.compat import ipaddress
from pip._internal.utils.deprecation import deprecated
from pip._internal.utils.logging import indent_log
from pip._internal.utils.misc import (
    ARCHIVE_EXTENSIONS, SUPPORTED_EXTENSIONS, normalize_path,
    remove_auth_from_url,
)
from pip._internal.utils.packaging import check_requires_python
from pip._internal.wheel import Wheel, wheel_ext

__all__ = ['FormatControl', 'PackageFinder']


SECURE_ORIGINS = [
    # protocol, hostname, port
    # Taken from Chrome's list of secure origins (See: http://bit.ly/1qrySKC)
    ("https", "*", "*"),
    ("*", "localhost", "*"),
    ("*", "127.0.0.0/8", "*"),
    ("*", "::1/128", "*"),
    ("file", "*", None),
    # ssh is always secure.
    ("ssh", "*", "*"),
]


logger = logging.getLogger(__name__)


def _match_vcs_scheme(url):
    """Look for VCS schemes in the URL.

    Returns the matched VCS scheme, or None if there's no match.
    """
    from pip._internal.vcs import VcsSupport
    for scheme in VcsSupport.schemes:
        if url.lower().startswith(scheme) and url[len(scheme)] in '+:':
            return scheme
    return None


def _is_url_like_archive(url):
    """Return whether the URL looks like an archive.
    """
    filename = Link(url).filename
    for bad_ext in ARCHIVE_EXTENSIONS:
        if filename.endswith(bad_ext):
            return True
    return False


class _NotHTML(Exception):
    def __init__(self, content_type, request_desc):
        super(_NotHTML, self).__init__(content_type, request_desc)
        self.content_type = content_type
        self.request_desc = request_desc


def _ensure_html_header(response):
    """Check the Content-Type header to ensure the response contains HTML.

    Raises `_NotHTML` if the content type is not text/html.
    """
    content_type = response.headers.get("Content-Type", "")
    if not content_type.lower().startswith("text/html"):
        raise _NotHTML(content_type, response.request.method)


class _NotHTTP(Exception):
    pass


def _ensure_html_response(url, session):
    """Send a HEAD request to the URL, and ensure the response contains HTML.

    Raises `_NotHTTP` if the URL is not available for a HEAD request, or
    `_NotHTML` if the content type is not text/html.
    """
    scheme, netloc, path, query, fragment = urllib_parse.urlsplit(url)
    if scheme not in {'http', 'https'}:
        raise _NotHTTP()

    resp = session.head(url, allow_redirects=True)
    resp.raise_for_status()

    _ensure_html_header(resp)


def _get_html_response(url, session):
    """Access an HTML page with GET, and return the response.

    This consists of three parts:

    1. If the URL looks suspiciously like an archive, send a HEAD first to
       check the Content-Type is HTML, to avoid downloading a large file.
       Raise `_NotHTTP` if the content type cannot be determined, or
       `_NotHTML` if it is not HTML.
    2. Actually perform the request. Raise HTTP exceptions on network failures.
    3. Check the Content-Type header to make sure we got HTML, and raise
       `_NotHTML` otherwise.
    """
    if _is_url_like_archive(url):
        _ensure_html_response(url, session=session)

    logger.debug('Getting page %s', url)

    resp = session.get(
        url,
        headers={
            "Accept": "text/html",
            # We don't want to blindly returned cached data for
            # /simple/, because authors generally expecting that
            # twine upload && pip install will function, but if
            # they've done a pip install in the last ~10 minutes
            # it won't. Thus by setting this to zero we will not
            # blindly use any cached data, however the benefit of
            # using max-age=0 instead of no-cache, is that we will
            # still support conditional requests, so we will still
            # minimize traffic sent in cases where the page hasn't
            # changed at all, we will just always incur the round
            # trip for the conditional GET now instead of only
            # once per 10 minutes.
            # For more information, please see pypa/pip#5670.
            "Cache-Control": "max-age=0",
        },
    )
    resp.raise_for_status()

    # The check for archives above only works if the url ends with
    # something that looks like an archive. However that is not a
    # requirement of an url. Unless we issue a HEAD request on every
    # url we cannot know ahead of time for sure if something is HTML
    # or not. However we can check after we've downloaded it.
    _ensure_html_header(resp)

    return resp


def _handle_get_page_fail(link, reason, url, meth=None):
    if meth is None:
        meth = logger.debug
    meth("Could not fetch URL %s: %s - skipping", link, reason)


def _get_html_page(link, session=None):
    if session is None:
        raise TypeError(
            "_get_html_page() missing 1 required keyword argument: 'session'"
        )

    url = link.url.split('#', 1)[0]

    # Check for VCS schemes that do not support lookup as web pages.
    vcs_scheme = _match_vcs_scheme(url)
    if vcs_scheme:
        logger.debug('Cannot look at %s URL %s', vcs_scheme, link)
        return None

    # Tack index.html onto file:// URLs that point to directories
    scheme, _, path, _, _, _ = urllib_parse.urlparse(url)
    if (scheme == 'file' and os.path.isdir(urllib_request.url2pathname(path))):
        # add trailing slash if not present so urljoin doesn't trim
        # final segment
        if not url.endswith('/'):
            url += '/'
        url = urllib_parse.urljoin(url, 'index.html')
        logger.debug(' file: URL is directory, getting %s', url)

    try:
        resp = _get_html_response(url, session=session)
    except _NotHTTP as exc:
        logger.debug(
            'Skipping page %s because it looks like an archive, and cannot '
            'be checked by HEAD.', link,
        )
    except _NotHTML as exc:
        logger.debug(
            'Skipping page %s because the %s request got Content-Type: %s',
            link, exc.request_desc, exc.content_type,
        )
    except requests.HTTPError as exc:
        _handle_get_page_fail(link, exc, url)
    except RetryError as exc:
        _handle_get_page_fail(link, exc, url)
    except SSLError as exc:
        reason = "There was a problem confirming the ssl certificate: "
        reason += str(exc)
        _handle_get_page_fail(link, reason, url, meth=logger.info)
    except requests.ConnectionError as exc:
        _handle_get_page_fail(link, "connection error: %s" % exc, url)
    except requests.Timeout:
        _handle_get_page_fail(link, "timed out", url)
    else:
        return HTMLPage(resp.content, resp.url, resp.headers)


class PackageFinder(object):
    """This finds packages.

    This is meant to match easy_install's technique for looking for
    packages, by reading pages and looking for appropriate links.
    """

    def __init__(self, find_links, index_urls, allow_all_prereleases=False,
                 trusted_hosts=None, process_dependency_links=False,
                 session=None, format_control=None, platform=None,
                 versions=None, abi=None, implementation=None,
                 prefer_binary=False):
        """Create a PackageFinder.

        :param format_control: A FormatControl object or None. Used to control
            the selection of source packages / binary packages when consulting
            the index and links.
        :param platform: A string or None. If None, searches for packages
            that are supported by the current system. Otherwise, will find
            packages that can be built on the platform passed in. These
            packages will only be downloaded for distribution: they will
            not be built locally.
        :param versions: A list of strings or None. This is passed directly
            to pep425tags.py in the get_supported() method.
        :param abi: A string or None. This is passed directly
            to pep425tags.py in the get_supported() method.
        :param implementation: A string or None. This is passed directly
            to pep425tags.py in the get_supported() method.
        """
        if session is None:
            raise TypeError(
                "PackageFinder() missing 1 required keyword argument: "
                "'session'"
            )

        # Build find_links. If an argument starts with ~, it may be
        # a local file relative to a home directory. So try normalizing
        # it and if it exists, use the normalized version.
        # This is deliberately conservative - it might be fine just to
        # blindly normalize anything starting with a ~...
        self.find_links = []
        for link in find_links:
            if link.startswith('~'):
                new_link = normalize_path(link)
                if os.path.exists(new_link):
                    link = new_link
            self.find_links.append(link)

        self.index_urls = index_urls
        self.dependency_links = []

        # These are boring links that have already been logged somehow:
        self.logged_links = set()

        self.format_control = format_control or FormatControl(set(), set())

        # Domains that we won't emit warnings for when not using HTTPS
        self.secure_origins = [
            ("*", host, "*")
            for host in (trusted_hosts if trusted_hosts else [])
        ]

        # Do we want to allow _all_ pre-releases?
        self.allow_all_prereleases = allow_all_prereleases

        # Do we process dependency links?
        self.process_dependency_links = process_dependency_links

        # The Session we'll use to make requests
        self.session = session

        # The valid tags to check potential found wheel candidates against
        self.valid_tags = get_supported(
            versions=versions,
            platform=platform,
            abi=abi,
            impl=implementation,
        )

        # Do we prefer old, but valid, binary dist over new source dist
        self.prefer_binary = prefer_binary

        # If we don't have TLS enabled, then WARN if anyplace we're looking
        # relies on TLS.
        if not HAS_TLS:
            for link in itertools.chain(self.index_urls, self.find_links):
                parsed = urllib_parse.urlparse(link)
                if parsed.scheme == "https":
                    logger.warning(
                        "pip is configured with locations that require "
                        "TLS/SSL, however the ssl module in Python is not "
                        "available."
                    )
                    break

    def get_formatted_locations(self):
        lines = []
        if self.index_urls and self.index_urls != [PyPI.simple_url]:
            lines.append(
                "Looking in indexes: {}".format(", ".join(
                    remove_auth_from_url(url) for url in self.index_urls))
            )
        if self.find_links:
            lines.append(
                "Looking in links: {}".format(", ".join(self.find_links))
            )
        return "\n".join(lines)

    def add_dependency_links(self, links):
        # FIXME: this shouldn't be global list this, it should only
        # apply to requirements of the package that specifies the
        # dependency_links value
        # FIXME: also, we should track comes_from (i.e., use Link)
        if self.process_dependency_links:
            deprecated(
                "Dependency Links processing has been deprecated and will be "
                "removed in a future release.",
                replacement="PEP 508 URL dependencies",
                gone_in="19.0",
                issue=4187,
            )
            self.dependency_links.extend(links)

    @staticmethod
    def _sort_locations(locations, expand_dir=False):
        """
        Sort locations into "files" (archives) and "urls", and return
        a pair of lists (files,urls)
        """
        files = []
        urls = []

        # puts the url for the given file path into the appropriate list
        def sort_path(path):
            url = path_to_url(path)
            if mimetypes.guess_type(url, strict=False)[0] == 'text/html':
                urls.append(url)
            else:
                files.append(url)

        for url in locations:

            is_local_path = os.path.exists(url)
            is_file_url = url.startswith('file:')

            if is_local_path or is_file_url:
                if is_local_path:
                    path = url
                else:
                    path = url_to_path(url)
                if os.path.isdir(path):
                    if expand_dir:
                        path = os.path.realpath(path)
                        for item in os.listdir(path):
                            sort_path(os.path.join(path, item))
                    elif is_file_url:
                        urls.append(url)
                    else:
                        logger.warning(
                            "Path '{0}' is ignored: "
                            "it is a directory.".format(path),
                        )
                elif os.path.isfile(path):
                    sort_path(path)
                else:
                    logger.warning(
                        "Url '%s' is ignored: it is neither a file "
                        "nor a directory.", url,
                    )
            elif is_url(url):
                # Only add url with clear scheme
                urls.append(url)
            else:
                logger.warning(
                    "Url '%s' is ignored. It is either a non-existing "
                    "path or lacks a specific scheme.", url,
                )

        return files, urls

    def _candidate_sort_key(self, candidate):
        """
        Function used to generate link sort key for link tuples.
        The greater the return value, the more preferred it is.
        If not finding wheels, then sorted by version only.
        If finding wheels, then the sort order is by version, then:
          1. existing installs
          2. wheels ordered via Wheel.support_index_min(self.valid_tags)
          3. source archives
        If prefer_binary was set, then all wheels are sorted above sources.
        Note: it was considered to embed this logic into the Link
              comparison operators, but then different sdist links
              with the same version, would have to be considered equal
        """
        support_num = len(self.valid_tags)
        build_tag = tuple()
        binary_preference = 0
        if candidate.location.is_wheel:
            # can raise InvalidWheelFilename
            wheel = Wheel(candidate.location.filename)
            if not wheel.supported(self.valid_tags):
                raise UnsupportedWheel(
                    "%s is not a supported wheel for this platform. It "
                    "can't be sorted." % wheel.filename
                )
            if self.prefer_binary:
                binary_preference = 1
            pri = -(wheel.support_index_min(self.valid_tags))
            if wheel.build_tag is not None:
                match = re.match(r'^(\d+)(.*)$', wheel.build_tag)
                build_tag_groups = match.groups()
                build_tag = (int(build_tag_groups[0]), build_tag_groups[1])
        else:  # sdist
            pri = -(support_num)
        return (binary_preference, candidate.version, build_tag, pri)

    def _validate_secure_origin(self, logger, location):
        # Determine if this url used a secure transport mechanism
        parsed = urllib_parse.urlparse(str(location))
        origin = (parsed.scheme, parsed.hostname, parsed.port)

        # The protocol to use to see if the protocol matches.
        # Don't count the repository type as part of the protocol: in
        # cases such as "git+ssh", only use "ssh". (I.e., Only verify against
        # the last scheme.)
        protocol = origin[0].rsplit('+', 1)[-1]

        # Determine if our origin is a secure origin by looking through our
        # hardcoded list of secure origins, as well as any additional ones
        # configured on this PackageFinder instance.
        for secure_origin in (SECURE_ORIGINS + self.secure_origins):
            if protocol != secure_origin[0] and secure_origin[0] != "*":
                continue

            try:
                # We need to do this decode dance to ensure that we have a
                # unicode object, even on Python 2.x.
                addr = ipaddress.ip_address(
                    origin[1]
                    if (
                        isinstance(origin[1], six.text_type) or
                        origin[1] is None
                    )
                    else origin[1].decode("utf8")
                )
                network = ipaddress.ip_network(
                    secure_origin[1]
                    if isinstance(secure_origin[1], six.text_type)
                    else secure_origin[1].decode("utf8")
                )
            except ValueError:
                # We don't have both a valid address or a valid network, so
                # we'll check this origin against hostnames.
                if (origin[1] and
                        origin[1].lower() != secure_origin[1].lower() and
                        secure_origin[1] != "*"):
                    continue
            else:
                # We have a valid address and network, so see if the address
                # is contained within the network.
                if addr not in network:
                    continue

            # Check to see if the port patches
            if (origin[2] != secure_origin[2] and
                    secure_origin[2] != "*" and
                    secure_origin[2] is not None):
                continue

            # If we've gotten here, then this origin matches the current
            # secure origin and we should return True
            return True

        # If we've gotten to this point, then the origin isn't secure and we
        # will not accept it as a valid location to search. We will however
        # log a warning that we are ignoring it.
        logger.warning(
            "The repository located at %s is not a trusted or secure host and "
            "is being ignored. If this repository is available via HTTPS we "
            "recommend you use HTTPS instead, otherwise you may silence "
            "this warning and allow it anyway with '--trusted-host %s'.",
            parsed.hostname,
            parsed.hostname,
        )

        return False

    def _get_index_urls_locations(self, project_name):
        """Returns the locations found via self.index_urls

        Checks the url_name on the main (first in the list) index and
        use this url_name to produce all locations
        """

        def mkurl_pypi_url(url):
            loc = posixpath.join(
                url,
                urllib_parse.quote(canonicalize_name(project_name)))
            # For maximum compatibility with easy_install, ensure the path
            # ends in a trailing slash.  Although this isn't in the spec
            # (and PyPI can handle it without the slash) some other index
            # implementations might break if they relied on easy_install's
            # behavior.
            if not loc.endswith('/'):
                loc = loc + '/'
            return loc

        return [mkurl_pypi_url(url) for url in self.index_urls]

    def find_all_candidates(self, project_name):
        """Find all available InstallationCandidate for project_name

        This checks index_urls, find_links and dependency_links.
        All versions found are returned as an InstallationCandidate list.

        See _link_package_versions for details on which files are accepted
        """
        index_locations = self._get_index_urls_locations(project_name)
        index_file_loc, index_url_loc = self._sort_locations(index_locations)
        fl_file_loc, fl_url_loc = self._sort_locations(
            self.find_links, expand_dir=True,
        )
        dep_file_loc, dep_url_loc = self._sort_locations(self.dependency_links)

        file_locations = (Link(url) for url in itertools.chain(
            index_file_loc, fl_file_loc, dep_file_loc,
        ))

        # We trust every url that the user has given us whether it was given
        #   via --index-url or --find-links
        # We explicitly do not trust links that came from dependency_links
        # We want to filter out any thing which does not have a secure origin.
        url_locations = [
            link for link in itertools.chain(
                (Link(url) for url in index_url_loc),
                (Link(url) for url in fl_url_loc),
                (Link(url) for url in dep_url_loc),
            )
            if self._validate_secure_origin(logger, link)
        ]

        logger.debug('%d location(s) to search for versions of %s:',
                     len(url_locations), project_name)

        for location in url_locations:
            logger.debug('* %s', location)

        canonical_name = canonicalize_name(project_name)
        formats = self.format_control.get_allowed_formats(canonical_name)
        search = Search(project_name, canonical_name, formats)
        find_links_versions = self._package_versions(
            # We trust every directly linked archive in find_links
            (Link(url, '-f') for url in self.find_links),
            search
        )

        page_versions = []
        for page in self._get_pages(url_locations, project_name):
            logger.debug('Analyzing links from page %s', page.url)
            with indent_log():
                page_versions.extend(
                    self._package_versions(page.iter_links(), search)
                )

        dependency_versions = self._package_versions(
            (Link(url) for url in self.dependency_links), search
        )
        if dependency_versions:
            logger.debug(
                'dependency_links found: %s',
                ', '.join([
                    version.location.url for version in dependency_versions
                ])
            )

        file_versions = self._package_versions(file_locations, search)
        if file_versions:
            file_versions.sort(reverse=True)
            logger.debug(
                'Local files found: %s',
                ', '.join([
                    url_to_path(candidate.location.url)
                    for candidate in file_versions
                ])
            )

        # This is an intentional priority ordering
        return (
            file_versions + find_links_versions + page_versions +
            dependency_versions
        )

    def find_requirement(self, req, upgrade):
        """Try to find a Link matching req

        Expects req, an InstallRequirement and upgrade, a boolean
        Returns a Link if found,
        Raises DistributionNotFound or BestVersionAlreadyInstalled otherwise
        """
        all_candidates = self.find_all_candidates(req.name)

        # Filter out anything which doesn't match our specifier
        compatible_versions = set(
            req.specifier.filter(
                # We turn the version object into a str here because otherwise
                # when we're debundled but setuptools isn't, Python will see
                # packaging.version.Version and
                # pkg_resources._vendor.packaging.version.Version as different
                # types. This way we'll use a str as a common data interchange
                # format. If we stop using the pkg_resources provided specifier
                # and start using our own, we can drop the cast to str().
                [str(c.version) for c in all_candidates],
                prereleases=(
                    self.allow_all_prereleases
                    if self.allow_all_prereleases else None
                ),
            )
        )
        applicable_candidates = [
            # Again, converting to str to deal with debundling.
            c for c in all_candidates if str(c.version) in compatible_versions
        ]

        if applicable_candidates:
            best_candidate = max(applicable_candidates,
                                 key=self._candidate_sort_key)
        else:
            best_candidate = None

        if req.satisfied_by is not None:
            installed_version = parse_version(req.satisfied_by.version)
        else:
            installed_version = None

        if installed_version is None and best_candidate is None:
            logger.critical(
                'Could not find a version that satisfies the requirement %s '
                '(from versions: %s)',
                req,
                ', '.join(
                    sorted(
                        {str(c.version) for c in all_candidates},
                        key=parse_version,
                    )
                )
            )

            raise DistributionNotFound(
                'No matching distribution found for %s' % req
            )

        best_installed = False
        if installed_version and (
                best_candidate is None or
                best_candidate.version <= installed_version):
            best_installed = True

        if not upgrade and installed_version is not None:
            if best_installed:
                logger.debug(
                    'Existing installed version (%s) is most up-to-date and '
                    'satisfies requirement',
                    installed_version,
                )
            else:
                logger.debug(
                    'Existing installed version (%s) satisfies requirement '
                    '(most up-to-date version is %s)',
                    installed_version,
                    best_candidate.version,
                )
            return None

        if best_installed:
            # We have an existing version, and its the best version
            logger.debug(
                'Installed version (%s) is most up-to-date (past versions: '
                '%s)',
                installed_version,
                ', '.join(sorted(compatible_versions, key=parse_version)) or
                "none",
            )
            raise BestVersionAlreadyInstalled

        logger.debug(
            'Using version %s (newest of versions: %s)',
            best_candidate.version,
            ', '.join(sorted(compatible_versions, key=parse_version))
        )
        return best_candidate.location

    def _get_pages(self, locations, project_name):
        """
        Yields (page, page_url) from the given locations, skipping
        locations that have errors.
        """
        seen = set()
        for location in locations:
            if location in seen:
                continue
            seen.add(location)

            page = _get_html_page(location, session=self.session)
            if page is None:
                continue

            yield page

    _py_version_re = re.compile(r'-py([123]\.?[0-9]?)$')

    def _sort_links(self, links):
        """
        Returns elements of links in order, non-egg links first, egg links
        second, while eliminating duplicates
        """
        eggs, no_eggs = [], []
        seen = set()
        for link in links:
            if link not in seen:
                seen.add(link)
                if link.egg_fragment:
                    eggs.append(link)
                else:
                    no_eggs.append(link)
        return no_eggs + eggs

    def _package_versions(self, links, search):
        result = []
        for link in self._sort_links(links):
            v = self._link_package_versions(link, search)
            if v is not None:
                result.append(v)
        return result

    def _log_skipped_link(self, link, reason):
        if link not in self.logged_links:
            logger.debug('Skipping link %s; %s', link, reason)
            self.logged_links.add(link)

    def _link_package_versions(self, link, search):
        """Return an InstallationCandidate or None"""
        version = None
        if link.egg_fragment:
            egg_info = link.egg_fragment
            ext = link.ext
        else:
            egg_info, ext = link.splitext()
            if not ext:
                self._log_skipped_link(link, 'not a file')
                return
            if ext not in SUPPORTED_EXTENSIONS:
                self._log_skipped_link(
                    link, 'unsupported archive format: %s' % ext,
                )
                return
            if "binary" not in search.formats and ext == wheel_ext:
                self._log_skipped_link(
                    link, 'No binaries permitted for %s' % search.supplied,
                )
                return
            if "macosx10" in link.path and ext == '.zip':
                self._log_skipped_link(link, 'macosx10 one')
                return
            if ext == wheel_ext:
                try:
                    wheel = Wheel(link.filename)
                except InvalidWheelFilename:
                    self._log_skipped_link(link, 'invalid wheel filename')
                    return
                if canonicalize_name(wheel.name) != search.canonical:
                    self._log_skipped_link(
                        link, 'wrong project name (not %s)' % search.supplied)
                    return

                if not wheel.supported(self.valid_tags):
                    self._log_skipped_link(
                        link, 'it is not compatible with this Python')
                    return

                version = wheel.version

        # This should be up by the search.ok_binary check, but see issue 2700.
        if "source" not in search.formats and ext != wheel_ext:
            self._log_skipped_link(
                link, 'No sources permitted for %s' % search.supplied,
            )
            return

        if not version:
            version = _egg_info_matches(egg_info, search.canonical, link)
        if version is None:
            self._log_skipped_link(
                link, 'Missing project version for %s' % search.supplied)
            return

        match = self._py_version_re.search(version)
        if match:
            version = version[:match.start()]
            py_version = match.group(1)
            if py_version != sys.version[:3]:
                self._log_skipped_link(
                    link, 'Python version is incorrect')
                return
        try:
            support_this_python = check_requires_python(link.requires_python)
        except specifiers.InvalidSpecifier:
            logger.debug("Package %s has an invalid Requires-Python entry: %s",
                         link.filename, link.requires_python)
            support_this_python = True

        if not support_this_python:
            logger.debug("The package %s is incompatible with the python"
                         "version in use. Acceptable python versions are:%s",
                         link, link.requires_python)
            return
        logger.debug('Found link %s, version: %s', link, version)

        return InstallationCandidate(search.supplied, version, link)


def _egg_info_matches(egg_info, canonical_name, link):
    """Pull the version part out of a string.

    :param egg_info: The string to parse. E.g. foo-2.1
    :param canonical_name: The canonicalized name of the package this
        belongs to.
    :param link: The link the string came from, for logging on failure.
    """
    if not canonicalize_name(egg_info).startswith(canonical_name):
        return None
    # Project name and version must be separated by a dash.
    if egg_info[len(canonical_name)] != "-":
        return None
    return egg_info[(len(canonical_name) + 1):]


def _determine_base_url(document, page_url):
    """Determine the HTML document's base URL.

    This looks for a ``<base>`` tag in the HTML document. If present, its href
    attribute denotes the base URL of anchor tags in the document. If there is
    no such tag (or if it does not have a valid href attribute), the HTML
    file's URL is used as the base URL.

    :param document: An HTML document representation. The current
        implementation expects the result of ``html5lib.parse()``.
    :param page_url: The URL of the HTML document.
    """
    for base in document.findall(".//base"):
        href = base.get("href")
        if href is not None:
            return href
    return page_url


def _get_encoding_from_headers(headers):
    """Determine if we have any encoding information in our headers.
    """
    if headers and "Content-Type" in headers:
        content_type, params = cgi.parse_header(headers["Content-Type"])
        if "charset" in params:
            return params['charset']
    return None


_CLEAN_LINK_RE = re.compile(r'[^a-z0-9$&+,/:;=?@.#%_\\|-]', re.I)


def _clean_link(url):
    """Makes sure a link is fully encoded.  That is, if a ' ' shows up in
    the link, it will be rewritten to %20 (while not over-quoting
    % or other characters)."""
    return _CLEAN_LINK_RE.sub(lambda match: '%%%2x' % ord(match.group(0)), url)


class HTMLPage(object):
    """Represents one page, along with its URL"""

    def __init__(self, content, url, headers=None):
        self.content = content
        self.url = url
        self.headers = headers

    def __str__(self):
        return self.url

    def iter_links(self):
        """Yields all links in the page"""
        document = html5lib.parse(
            self.content,
            transport_encoding=_get_encoding_from_headers(self.headers),
            namespaceHTMLElements=False,
        )
        base_url = _determine_base_url(document, self.url)
        for anchor in document.findall(".//a"):
            if anchor.get("href"):
                href = anchor.get("href")
                url = _clean_link(urllib_parse.urljoin(base_url, href))
                pyrequire = anchor.get('data-requires-python')
                pyrequire = unescape(pyrequire) if pyrequire else None
                yield Link(url, self.url, requires_python=pyrequire)


Search = namedtuple('Search', 'supplied canonical formats')
"""Capture key aspects of a search.

:attribute supplied: The user supplied package.
:attribute canonical: The canonical package name.
:attribute formats: The formats allowed for this package. Should be a set
    with 'binary' or 'source' or both in it.
"""