How do I create a Google Cloud Storage resumable upload URL with Google Client Library for Java on App Engine?

[Stella Laurenzo via StackOverflow]

It took some digging, but I came up with the following which does the right thing. Some official documentation on how to do this would have been nice, especially because the endpoint for actually triggering the resumable upload is different from what the docs call out. What is here came from using the gsutil tool to sign requests and then working out what was being done. The under-documented additional thing is that the code which POSTs to this URL to get a resumable session URL must include the "x-goog-resumable: start" header to trigger the upload. From there, everything is the same as the docs for performing a resumable upload to GCS.

import base64
import datetime
import time
import urllib

from google.appengine.api import app_identity

SIGNED_URL_EXPIRATION = datetime.timedelta(days=7)

def SignResumableUploadUrl(gcs_resource_path):
  """Generates a signed resumable upload URL.

  Note that documentation on this ability is sketchy. The canonical source
  is derived from running the gsutil program to generate a RESUMABLE URL
  with the "-m RESUMABLE" argument. Run "gsutil help signurl" for info and
  the following for an example:
    gsutil -m RESUMABLE -d 10m keyfile gs://bucket/file/name

  Note that this generates a URL different from the standard mechanism for
  deriving a resumable start URL and the initiator needs to add the header:
    x-goog-resumable:start

  Args:
    gcs_resource_path: The path of the GCS resource, including bucket name.

  Returns:
    A full signed URL.
  """
  method = "POST"
  expiration = datetime.datetime.utcnow() + SIGNED_URL_EXPIRATION
  expiration = int(time.mktime(expiration.timetuple()))
  signature_string = "\n".join([
      method,
      "",  # content md5
      "",  # content type
      str(expiration),
      "x-goog-resumable:start",
      gcs_resource_path
  ])
  _, signature_bytes = app_identity.sign_blob(signature_string)
  signature = base64.b64encode(signature_bytes)

  query_params = {
      "GoogleAccessId": app_identity.get_service_account_name(),
      "Expires": str(expiration),
      "Signature": signature,
  }

  return "{endpoint}{resource}?{querystring}".format(
      endpoint="https://storage.googleapis.com",
      resource=gcs_resource_path,
      querystring=urllib.urlencode(query_params))

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