generate_signed_post_policy_v4() should not call ensure_signed_credentials() when `service_account_email` and `access_token` are specified

[jasonkit]

I am calling generate_signed_post_policy_v4() on pod running on GKE using Application default credentials to avoid using service account key.

In latest code, even though generate_signed_post_policy_v4() support passing in service_account_email and access_token and use them to generate signature, it still unconditional call ensure_signed_credentials()

python-storage/google/cloud/storage/client.py

Lines 1726 to 1727 in 02a972d

	credentials = self._credentials if credentials is None else credentials
	ensure_signed_credentials(credentials)

If we are Application default credentials and getting the credential from GKE metadata server, it will not pass the ensure_signed_credentials() checking and returning
AttributeError: you need a private key to sign credentials.the credentials you are currently using <class 'google.auth.compute_engine.credentials.Credentials'> just contains a token. see https://googleapis.dev/python/google-api-core/latest/auth.html#setting-up-a-service-account for more details.

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generate_signed_post_policy_v4() should have similar handling as in generate_signed_url_v4()

python-storage/google/cloud/storage/_signing.py

Lines 541 to 547 in e3cfc47

	# If you are on Google Compute Engine, you can't generate a signed URL.
	# See https://github.com/googleapis/google-cloud-python/issues/922
	client_email = service_account_email
	if not access_token or not service_account_email:
	ensure_signed_credentials(credentials)
	client_email = credentials.signer_email

My current workaround is to implement class an pass it to generate_signed_post_policy_v4() as credentials to by-pass the checking of ensure_signed_credentials()

from google.auth.credentials import Signing

class _SigningCredential(Signing):
    def __init__(self, service_account_email: str):
        self._signer_email = service_account_email

    @property
    def signer_email(self):
        return self._signer_email

    @property
    def signer(self):
        raise NotImplementedError('Not in use')

    def sign_bytes(self, message):
        raise NotImplementedError('Not in use')

Environment details

• google-cloud-storage version: 2.17.0