Artifacts API: ETag caching is broken, same ETag value returned always

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Summary

GitLab documentation suggests using the ETag and If-None-Match headers to avoid unnecessary load on the server when checking for changes: https://docs.gitlab.com/ee/development/polling.html

A regression was introduced many months ago, this caching approach no longer works when using the single artifact download API: /projects/:id/jobs/artifacts/:ref_name/raw/*artifact_path?job=name

Steps to reproduce

I have created a repository that reproduces this issue on public the GitLab.com instance.

  1. Fork repository https://gitlab.com/intgr/ci-artifacts-test
  2. Edit file artifact.txt, commit and wait for the CI to pass
  3. Send a request at the artifact API while saving headers (change repo name intgr%2Fci-artifacts-test as needed) 
    GET https://gitlab.com/api/v4/projects/intgr%2Fci-artifacts-test/jobs/artifacts/main/raw/output/artifact.txt?job=compile
    Observe that the returned ETag header is Etag: W/"12ae32cb1ec02d01eda3581b127c1fee"
  4. Edt artifact.txt, commit, wait CI again
  5. Send a request with If-None-Match header with the previous ETag 
    GET https://gitlab.com/api/v4/projects/intgr%2Fci-artifacts-test/jobs/artifacts/main/raw/output/artifact.txt?job=compile
If-None-Match: W/"12ae32cb1ec02d01eda3581b127c1fee"
    GitLab responds with 304 Not Modified response code, as if the artifact had NOT changed!
  6. If you retry the request from step # 3, note that the ETag header still has the same value. But the HTTP body content has actually changed!

Output of checks

This bug happens on GitLab.com

Edited by 🤖 GitLab Bot 🤖