remote: Collect ETag in response, optimistically send that back in future updates

[imjasonh]

When pulling a manifest, if the response includes an ETag header, hold on to it, and send that back in an If-Match request header when putting that manifest back.

This should prevent some race conditions during read-modify-write cycles, if the registry respects the If-Match header, since the resource's etag will have changed when the other client updated it.

TODO: Add test.

• https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag#avoiding_mid-air_collisions
• Document Conditional Requests opencontainers/distribution-spec#251

[codecov-commenter]

Codecov Report

Merging #1386 (8eed56d) into main (12aeccc) will decrease coverage by 0.06%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main    #1386      +/-   ##
==========================================
- Coverage   74.19%   74.12%   -0.07%     
==========================================
  Files         113      113              
  Lines        8458     8474      +16     
==========================================
+ Hits         6275     6281       +6     
- Misses       1575     1584       +9     
- Partials      608      609       +1     

Impacted Files	Coverage Δ
pkg/v1/mutate/image.go	67.37% <0.00%> (-1.86%)	⬇️
pkg/v1/partial/with.go	64.00% <ø> (ø)
pkg/v1/remote/write.go	64.51% <0.00%> (-0.32%)	⬇️
pkg/v1/remote/image.go	79.33% <50.00%> (-0.67%)	⬇️
pkg/v1/remote/descriptor.go	72.79% <69.23%> (+0.40%)	⬆️
pkg/v1/remote/index.go	71.27% <80.00%> (-0.08%)	⬇️

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[jonjohnsonjr]

Maybe it's time to have formal unwrapping?

I was thinking something more along the lines of #1387 for the partial stuff, but it might be better to have an actual unwrapping API (borrowed from https://go.dev/blog/go1.13-errors) so we can check "does this (or something it wraps) implement ETag?" instead of just returning an empty string.

[github-actions]

This Pull Request is stale because it has been open for 90 days with
no activity. It will automatically close after 30 more days of
inactivity. Keep fresh with the 'lifecycle/frozen' label.

[oofnish]

Hello, I know this PR is currently closed, but I'm interested in picking up and running with the ETag support here. Is it just tests that are missing for this, or would the "Formal unwrapping" approach be a requirement for a MVP here?