CI jobs with multiple cache and cache:key:files experiencing cache miss due to new cache key prefix

Summary

CI jobs using multiple cache and cache:key:files starting to experience cache miss due to a prefix appearing in the cache key (before the hash).
This prefix corresponds to the index of the specific cache in the list of caches when multiple caches are used

Steps to reproduce

Create two CI jobs using multiple caches, at least one using files as key. Ensure the caches are in different orders. Cache keys will be different!

Example Project

https://gitlab.com/gfaugere-safti/reproduce-cache-key-file
Look at the latest pipeline: CI is green, but there is a cache miss and you can observe different cache keys

What is the current bug behavior?

The same file used has key will have different keys depending on order

What is the expected correct behavior?

Files which are the same should have the same cache key, regardless of the order of the cache declaration on the jobs

Relevant logs and/or screenshots

Checking cache for 0-a283b02686edf0a3d17ea06d6e761b72d8210907-protected...

...

Creating cache 0-a283b02686edf0a3d17ea06d6e761b72d8210907-protected...

Checking cache for 1-a283b02686edf0a3d17ea06d6e761b72d8210907-protected...
WARNING: file does not exist                       
Failed to extract cache

...

Creating cache 1-a283b02686edf0a3d17ea06d6e761b72d8210907-protected...

This bug happens on GitLab.com

Possible fixes

Caused by this fix: !104885 (merged) (thanks @kir.gilroy for the find!)
Maybe the original hash collision bug needs a different fix? And this behaviour should probably be documented (here ?) if it stays

Workarounds

Your can use the cache:key:prefix property to avoid having an auto-generated index-based prefix:

  cache:
    - key:
        prefix: ""
        files:
          - ...

Or you can make sure your cache are in the same order in all jobs which uses the same keys (seems more fragile to me)