SPICE - Download and cache kernel files

[subagonsouth]

Algorithm Description:

In order to be able to run processing locally, it is desirable to have some code that will download kernels to a cache location so that they can be furnished from there. There are multiple sources to download from including IMAP SDC and NAIF website.

Proposed general SPICE workflow:

• Try to furnish the latest metakernel
  • Catch any spiceypy.utils.exceptions.NotFoundError exceptions and
    1. Try to download the kernel from IMAP spice files
    2. If 1. fails, try to download the kernel from NAIF
  • Once we are able to furnish the full metakerel, processing should have everything it needs
  • A more proactive solution would be to parse the metakernel and check for existence of the kernel files
  • One issue is that all downloads will need to exactly match the metakernel directory structure. How can we accomplish this in the production EFS mount?

Questions to figure out:

• How will this work alongside kernels that are checked in to the repo as test data? For example, do we manually furnish each kernel or keep the current scheme where there is a template metakernel that gets modified with correct local paths at test run time?

Requirements:

• Files should be downloaded to where environment variable IMAP_SPICE_DIR points if files don't already exist there.
• Setup spice tests with @pytest.mark.spice to they can easily be avoided in CI jobs

Code:

Reference/reuse Libera code for doing this here: https://lasp.colorado.edu/nucleus/projects/LIBSDC/repos/libera_utils/browse/libera_utils/spice_utils.py

[subagonsouth]

It was decided that this ticket as described might be too much. The final implementation was to add test fixtures that can download files from NAIF if needed and only if the tests marked with external_kernel are run.