## Problem
Currently, our backward compatibility tests only look one release back.
That means, for example, that when we switch on image layer compression
by default, we'll test reading of uncompressed layers for one release,
and then stop doing it. When we make an index_part.json format change,
we'll test against the old format for a week, then stop (unless we write
separate unit tests for each old format).
The reality in the field is that data in old formats will continue to
exist for weeks/months/years. When we make major format changes, we
should retain examples of the old format data, and continuously verify
that the latest code can still read them.
This test uses contents from a new path in the public S3 bucket,
`compatibility-data-snapshots/`. It is populated by hand. The first
important artifact is one from before we switch on compression, so that
we will keep testing reads of uncompressed data. We will generate more
artifacts ahead of other key changes, like when we update remote storage
format for archival timelines.
Closes: https://github.com/neondatabase/cloud/issues/15576
John Spray
0a667bc8ef