Adds optional location_prefix kwarg in to_legacy_urlsafe

[egalpin]

Fixes #3597

I encountered an issue with respect to urlsafe keys in a system that is using both google-cloud-datastore and ndb in different services. I was receiving incompatible urlsafe keys when using the output of to_legacy_urlsafe in things like ndb.Key(urlsafe=<output_from_legacy_urlsafe>) running under App Engine. I received the following error:

BadRequestError: app s~your-app-id cannot access app your-app-id's data

I then tried the suggestion as per #3597 of adding the location prefix to the project under cloud-datastore like so:

datastore.Client(project='s~your-app-id')

Unfortunately, I was met with this error (where your-app-id is a real project, with active datastore db which I can access via the Google Cloud Console):

The project s~your-app-id does not exist or it does not contain an active Cloud Datastore database.

I was able to produce compatible urlsafe keys by specifying the location prefix only when creating the urlsafe keys. This PR contains that small change.

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[egalpin]

I've signed the CLA

[googlebot]

CLAs look good, thanks!

[egalpin]

If this isn't acceptable as a change in the repo, anyone wanting to do the same should be able to use this snippet in their code:

import base64

from google.cloud.datastore import _app_engine_key_pb2
from google.cloud.datastore.key import _to_legacy_path


def to_urlsafe(key, location_prefix=None):
    """Convert to a base64 encode urlsafe string for App Engine.

    This is intended to work with the "legacy" representation of a
    datastore "Key" used within Google App Engine (a so-called
    "Reference"). The returned string can be used as the ``urlsafe``
    argument to ``ndb.Key(urlsafe=...)``. The base64 encoded values
    will have padding removed.
    .. note::
            The string returned by ``to_legacy_urlsafe`` is equivalent, but
            not identical, to the string returned by ``ndb``. The location
            prefix may need to be specified to obtain idendentical urlsafe
            keys.

    :type key: google.cloud.datastore.key.Key
    :param key: The key that you wish to convert to urlsafe

    :type location_prefix: str
    :param location_prefix: The location prefix of ones project. Often
                            this value is 's~', but may be 'dev~', 'e~', or
                            other location prefixes currently unknown.
    :rtype: bytes
    :returns: A bytestring containing the key encoded as URL-safe base64.
    """
    if location_prefix:
        project_id = location_prefix + key._project
    else:
        project_id = key._project

    reference = _app_engine_key_pb2.Reference(
        app=project_id,
        path=_to_legacy_path(key._path),  # Avoid the copy.
        name_space=key.namespace,
    )
    raw_bytes = reference.SerializeToString()
    return base64.urlsafe_b64encode(raw_bytes)

[dhermes]

Thanks for filing. I don't see this as controversial in any way, so should be easy to merge!

Do you mind adding a unit test for the newly added parameter? If not, let me know and I can lend a hand.

[dhermes]

@egalpin Thanks for the update, are you planning on sending unit test updates or shall I?

I had some strange GitHub refresh state and never saw them. My bad.

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[dhermes]

LGTM

[egalpin]

I need update author details on some commits (working with multiple GH accounts on this machine)

[dhermes]

@egalpin The CLA issue was because I pushed a commit to the PR and the CLA-bot gets confused by multi-author PRs. No need to do anything. Cheers!