crane digest: fallback to GET when HEAD fails

[imjasonh]

Before

$ crane digest registry.access.redhat.com/ubi8/ubi-minimal
2021/03/20 10:19:50 No matching credentials were found for "registry.access.redhat.com/ubi8/ubi-minimal", falling back on anonymous
2021/03/20 10:19:51 computing digest: too many parts in hash: 

My, what a very unhelpful error message that is! 🤔

After

$ go run ./cmd/crane digest registry.access.redhat.com/ubi8/ubi-minimal
2021/03/20 10:20:10 No matching credentials were found for "registry.access.redhat.com/ubi8/ubi-minimal", falling back on anonymous
2021/03/20 10:20:10 HEAD request failed, falling back on GET: response did not include Docker-Content-Digest header
2021/03/20 10:20:10 No matching credentials were found for "registry.access.redhat.com/ubi8/ubi-minimal", falling back on anonymous
sha256:fdfb0770bff33e0f97d78583efd68b546a19d0a4b0ac23eef25ef261bca3e975

This should also make crane digest work against registries that don't support HEAD at all, or that don't include all the necessary descriptor information in response headers, though neglecting Docker-Content-Digest is probably by far the most common omission.

If this lgty in general I can add some tests.

[codecov-io]

Codecov Report

Merging #971 (5e8022c) into main (4a92f6c) will increase coverage by 0.04%.
The diff coverage is 94.11%.

❗ Current head 5e8022c differs from pull request most recent head def730a. Consider uploading reports for the commit def730a to get more accurate results

@@            Coverage Diff             @@
##             main     #971      +/-   ##
==========================================
+ Coverage   75.19%   75.23%   +0.04%     
==========================================
  Files         107      107              
  Lines        4837     4850      +13     
==========================================
+ Hits         3637     3649      +12     
- Misses        669      670       +1     
  Partials      531      531              

Impacted Files	Coverage Δ
pkg/crane/digest.go	68.18% <80.00%> (+1.51%)	⬆️
pkg/v1/remote/descriptor.go	75.14% <100.00%> (+1.36%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4a92f6c...def730a. Read the comment docs.

[jonjohnsonjr]

LGTM in general. One thing I'm slightly worried about is the headManifest errors not making sense out of context. WDYT about prepending them with HEAD <url>: ?

[imjasonh]

LGTM in general. One thing I'm slightly worried about is the headManifest errors not making sense out of context. WDYT about prepending them with HEAD <url>: ?

Good call, done.

Added tests.

[jonjohnsonjr]

One final nit is that the reason this error was particularly confusing is here:

go-containerregistry/pkg/v1/hash.go

Line 90 in 4a92f6c

return fmt.Errorf("too many parts in hash: %s", unquoted)

In particular, "too many" is definitely wrong for NewHash("").

Can we make that slightly better?

[imjasonh]

Can we make that slightly better?

Updated to return fmt.Errorf("cannot parse hash: %q", unquoted)