Generating Cloud Storage Signed URL from Google Cloud Function without using explicit key file

I'd like to create a pre-signed upload URL to a storage bucket, and would like to avoid an explicit reference to a json key.

Currently, I'm attempting to do this with the Default App Engine Service Account

I'm attempting to follow along with this answer but am getting this error:

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.

My Cloud Function code looks like this:

from google.cloud import storage
import datetime
import google.auth

def generate_upload_url(blob_name, additional_metadata: dict = {}):
    credentials, project_id = google.auth.default()
    # Perform a refresh request to get the access token of the current credentials (Else, it's None)
    from google.auth.transport import requests

    r = requests.Request()
    credentials.refresh(r)

    client = storage.Client()
    bucket = client.get_bucket("my_bucket")
    blob = bucket.blob(blob_name)
    
    service_account_email = credentials.service_account_email
    print(f"attempting to create signed url for {service_account_email}")
    url = blob.generate_signed_url(
        version="v4",
        service_account_email=service_account_email,
        access_token=credentials.token,
        # This URL is valid for 120 minutes
        expiration=datetime.timedelta(minutes=120),
        # Allow PUT requests using this URL.
        method="PUT",
        content_type="application/octet-stream",
        
    )
    return url


def get_upload_url(request):
    blob_name = get_param(request, "blob_name")
    url = generate_upload_url(blob_name)
    return url

When you use version v4 of signed URL, the first line of the method calls ensure_signed_credentialsmethod that check if the current service account can generate a signature in standalone mode (so with a private key). And so, that's break the current behavior.

In the comment of the function, it's clearly describe that a service account JSON file is required

        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.

So, use v2 version instead.