Google CDN Middleware Prevents Blob Transfer

Context

In order to transfer blobs efficiently, the primary (old path) GCS driver needs to have a deep level of insight into the migration (new path) GCS driver. Since the base implementation of the GCS driver is hidden under quite a number of layers of wrappers, we use a series of type casts to strip the wrappers off, until we are down to the base driver.

Problem

When configured with the Google CDN, the migration driver is wrapped in an additional layer that the present code did not anticipate, preventing blob transfer from starting.