Generate a V4 signed URL to provide query-string auth'n to a resource.

    .. note::

        Assumes ``credentials`` implements the
        :class:`google.auth.credentials.Signing` interface. Also assumes
        ``credentials`` has a ``service_account_email`` property which
        identifies the credentials.

    .. note::

        If you are on Google Compute Engine, you can't generate a signed URL.
        Follow `Issue 922`_ for updates on this. If you'd like to be able to
        generate a signed URL from GCE, you can use a standard service account
        from a JSON file rather than a GCE service account.

    See headers `reference`_ for more details on optional arguments.

    .. _Issue 922: https://github.com/GoogleCloudPlatform/\
                   google-cloud-python/issues/922
    .. _reference: https://cloud.google.com/storage/docs/reference-headers

    :type credentials: :class:`google.auth.credentials.Signing`
    :param credentials: Credentials object with an associated private key to
                        sign text.

    :type resource: str
    :param resource: A pointer to a specific resource
                     (typically, ``/bucket-name/path/to/blob.txt``).

    :type expiration: Union[Integer, datetime.datetime, datetime.timedelta]
    :param expiration: Point in time when the signed URL should expire.

    :type api_access_endpoint: str
    :param api_access_endpoint: Optional URI base. Defaults to
                                "https://storage.googleapis.com/"

    :type method: str
    :param method: The HTTP verb that will be used when requesting the URL.
                   Defaults to ``'GET'``. If method is ``'RESUMABLE'`` then the
                   signature will additionally contain the `x-goog-resumable`
                   header, and the method changed to POST. See the signed URL
                   docs regarding this flow:
                   https://cloud.google.com/storage/docs/access-control/signed-urls


    :type content_md5: str
    :param content_md5: (Optional) The MD5 hash of the object referenced by
                        ``resource``.

    :type content_type: str
    :param content_type: (Optional) The content type of the object referenced
                         by ``resource``.

    :type response_type: str
    :param response_type: (Optional) Content type of responses to requests for
                          the signed URL. Used to over-ride the content type of
                          the underlying resource.

    :type response_disposition: str
    :param response_disposition: (Optional) Content disposition of responses to
                                 requests for the signed URL.

    :type generation: str
    :param generation: (Optional) A value that indicates which generation of
                       the resource to fetch.

    :type headers: dict
    :param headers:
        (Optional) Additional HTTP headers to be included as part of the
        signed URLs.  See:
        https://cloud.google.com/storage/docs/xml-api/reference-headers
        Requests using the signed URL *must* pass the specified header
        (name and value) with each request for the URL.

    :type query_parameters: dict
    :param query_parameters:
        (Optional) Additional query paramtersto be included as part of the
        signed URLs.  See:
        https://cloud.google.com/storage/docs/xml-api/reference-headers#query

    :raises: :exc:`TypeError` when expiration is not a valid type.
    :raises: :exc:`AttributeError` if credentials is not an instance
            of :class:`google.auth.credentials.Signing`.

    :rtype: str
    :returns: A signed URL you can use to access the resource
              until expiration.

def generate_signed_url_v4(
    credentials,
    resource,
    expiration,
    api_access_endpoint=DEFAULT_ENDPOINT,
    method="GET",
    content_md5=None,
    content_type=None,
    response_type=None,
    response_disposition=None,
    generation=None,
    headers=None,
    query_parameters=None,
    _request_timestamp=None,  # for testing only
):
    """Generate a V4 signed URL to provide query-string auth'n to a resource.

    .. note::

        Assumes ``credentials`` implements the
        :class:`google.auth.credentials.Signing` interface. Also assumes
        ``credentials`` has a ``service_account_email`` property which
        identifies the credentials.

    .. note::

        If you are on Google Compute Engine, you can't generate a signed URL.
        Follow `Issue 922`_ for updates on this. If you'd like to be able to
        generate a signed URL from GCE, you can use a standard service account
        from a JSON file rather than a GCE service account.

    See headers `reference`_ for more details on optional arguments.

    .. _Issue 922: https://github.com/GoogleCloudPlatform/\
                   google-cloud-python/issues/922
    .. _reference: https://cloud.google.com/storage/docs/reference-headers

    :type credentials: :class:`google.auth.credentials.Signing`
    :param credentials: Credentials object with an associated private key to
                        sign text.

    :type resource: str
    :param resource: A pointer to a specific resource
                     (typically, ``/bucket-name/path/to/blob.txt``).

    :type expiration: Union[Integer, datetime.datetime, datetime.timedelta]
    :param expiration: Point in time when the signed URL should expire.

    :type api_access_endpoint: str
    :param api_access_endpoint: Optional URI base. Defaults to
                                "https://storage.googleapis.com/"

    :type method: str
    :param method: The HTTP verb that will be used when requesting the URL.
                   Defaults to ``'GET'``. If method is ``'RESUMABLE'`` then the
                   signature will additionally contain the `x-goog-resumable`
                   header, and the method changed to POST. See the signed URL
                   docs regarding this flow:
                   https://cloud.google.com/storage/docs/access-control/signed-urls


    :type content_md5: str
    :param content_md5: (Optional) The MD5 hash of the object referenced by
                        ``resource``.

    :type content_type: str
    :param content_type: (Optional) The content type of the object referenced
                         by ``resource``.

    :type response_type: str
    :param response_type: (Optional) Content type of responses to requests for
                          the signed URL. Used to over-ride the content type of
                          the underlying resource.

    :type response_disposition: str
    :param response_disposition: (Optional) Content disposition of responses to
                                 requests for the signed URL.

    :type generation: str
    :param generation: (Optional) A value that indicates which generation of
                       the resource to fetch.

    :type headers: dict
    :param headers:
        (Optional) Additional HTTP headers to be included as part of the
        signed URLs.  See:
        https://cloud.google.com/storage/docs/xml-api/reference-headers
        Requests using the signed URL *must* pass the specified header
        (name and value) with each request for the URL.

    :type query_parameters: dict
    :param query_parameters:
        (Optional) Additional query paramtersto be included as part of the
        signed URLs.  See:
        https://cloud.google.com/storage/docs/xml-api/reference-headers#query

    :raises: :exc:`TypeError` when expiration is not a valid type.
    :raises: :exc:`AttributeError` if credentials is not an instance
            of :class:`google.auth.credentials.Signing`.

    :rtype: str
    :returns: A signed URL you can use to access the resource
              until expiration.
    """
    ensure_signed_credentials(credentials)
    expiration_seconds = get_expiration_seconds_v4(expiration)

    if _request_timestamp is None:
        now = NOW()
        request_timestamp = now.strftime("%Y%m%dT%H%M%SZ")
        datestamp = now.date().strftime("%Y%m%d")
    else:
        request_timestamp = _request_timestamp
        datestamp = _request_timestamp[:8]

    client_email = credentials.signer_email
    credential_scope = "{}/auto/storage/goog4_request".format(datestamp)
    credential = "{}/{}".format(client_email, credential_scope)

    if headers is None:
        headers = {}

    if content_type is not None:
        headers["Content-Type"] = content_type

    if content_md5 is not None:
        headers["Content-MD5"] = content_md5

    header_names = [key.lower() for key in headers]
    if "host" not in header_names:
        headers["Host"] = "storage.googleapis.com"

    if method.upper() == "RESUMABLE":
        method = "POST"
        headers["x-goog-resumable"] = "start"

    canonical_headers, ordered_headers = get_canonical_headers(headers)
    canonical_header_string = (
        "\n".join(canonical_headers) + "\n"
    )  # Yes, Virginia, the extra newline is part of the spec.
    signed_headers = ";".join([key for key, _ in ordered_headers])

    if query_parameters is None:
        query_parameters = {}
    else:
        query_parameters = {key: value or "" for key, value in query_parameters.items()}

    query_parameters["X-Goog-Algorithm"] = "GOOG4-RSA-SHA256"
    query_parameters["X-Goog-Credential"] = credential
    query_parameters["X-Goog-Date"] = request_timestamp
    query_parameters["X-Goog-Expires"] = expiration_seconds
    query_parameters["X-Goog-SignedHeaders"] = signed_headers

    if response_type is not None:
        query_parameters["response-content-type"] = response_type

    if response_disposition is not None:
        query_parameters["response-content-disposition"] = response_disposition

    if generation is not None:
        query_parameters["generation"] = generation

    ordered_query_parameters = sorted(query_parameters.items())
    canonical_query_string = six.moves.urllib.parse.urlencode(ordered_query_parameters)

    canonical_elements = [
        method,
        resource,
        canonical_query_string,
        canonical_header_string,
        signed_headers,
        "UNSIGNED-PAYLOAD",
    ]
    canonical_request = "\n".join(canonical_elements)

    canonical_request_hash = hashlib.sha256(
        canonical_request.encode("ascii")
    ).hexdigest()

    string_elements = [
        "GOOG4-RSA-SHA256",
        request_timestamp,
        credential_scope,
        canonical_request_hash,
    ]
    string_to_sign = "\n".join(string_elements)

    signature_bytes = credentials.sign_bytes(string_to_sign.encode("ascii"))
    signature = binascii.hexlify(signature_bytes).decode("ascii")

    return "{}{}?{}&X-Goog-Signature={}".format(
        api_access_endpoint, resource, canonical_query_string, signature
    )