The evergreen function "upload release" uploads the release tarball. The function "fetch source" fetches that uploaded tarball.

Currently "upload release" uploads to:

mongo-c-driver/${branch_name}/mongo-c-driver-${CURRENT_VERSION}.tar.gz

(Note, it starts with mongo-c-driver)

And "fetch source" fetches from:

https://s3.amazonaws.com/mciuploads/${project}/${branch_name}/mongo-c-driver-${CURRENT_VERSION}.tar.gz

(Note it starts with ${project})

This works for the mongo-c-driver evergreen project, since ${project} = mongo-c-driver, but not the projects based on release branches, like mongo-c-driver-r1.14.0.