Add --fast flag to crane validate

[jonjohnsonjr]

Fixes #838

Add validate.Fast option
Add Exists to remote layers
Add Exists to layout
Add Exists to partial

Simplify some of our typechecking logic with unwrap().

Stop double-wrapping layoutImage layers.

[codecov-commenter]

Codecov Report

Merging #1013 (836d7bd) into main (13fba64) will decrease coverage by 0.11%.
The diff coverage is 75.60%.

❗ Current head 836d7bd differs from pull request most recent head 17a8dbf. Consider uploading reports for the commit 17a8dbf to get more accurate results

@@            Coverage Diff             @@
##             main    #1013      +/-   ##
==========================================
- Coverage   75.33%   75.22%   -0.12%     
==========================================
  Files         107      107              
  Lines        5036     5053      +17     
==========================================
+ Hits         3794     3801       +7     
- Misses        701      706       +5     
- Partials      541      546       +5     

Impacted Files	Coverage Δ
pkg/v1/remote/descriptor.go	73.36% <45.45%> (-1.78%)	⬇️
pkg/v1/layout/image.go	73.33% <75.00%> (-2.28%)	⬇️
pkg/v1/partial/with.go	61.90% <88.88%> (-2.16%)	⬇️
pkg/v1/remote/image.go	79.72% <100.00%> (+0.27%)	⬆️
pkg/v1/remote/layer.go	65.51% <100.00%> (+2.55%)	⬆️
pkg/v1/remote/mount.go	76.92% <100.00%> (+0.92%)	⬆️

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[jonjohnsonjr]

I'm not thrilled about this still, but the unwrap thing makes me happy.

I almost want to implement this across the board -- something like:

type Unwrapper interface {
  Unwrap() interface{}
}

This is kind of like go 1.13 error stuff -- maybe we'd want partial.Is and partial.As as well?

We could have MountableLayer implement this instead of adding pass-through methods like we're doing now:

go-containerregistry/pkg/v1/remote/mount.go

Lines 31 to 35 in 6a2e261

	// Descriptor retains the original descriptor from an image manifest.
	// See partial.Descriptor.
	func (ml *MountableLayer) Descriptor() (*v1.Descriptor, error) {
	return partial.Descriptor(ml.Layer)
	}

We're doing weird stuff to get partial.Descriptor to work all over the place:

$ rg -g '!*_test.go' "Descriptor\(\)"
v1/tarball/layer.go
45:func (l *layer) Descriptor() (*v1.Descriptor, error) {

v1/tarball/image.go
251:func (fl *foreignUncompressedLayer) Descriptor() (*v1.Descriptor, error) {

v1/remote/mount.go
33:func (ml *MountableLayer) Descriptor() (*v1.Descriptor, error) {
93:func (mi *mountableImage) Descriptor() (*v1.Descriptor, error) {

v1/remote/index.go
110:func (r *remoteIndex) Descriptor() (*v1.Descriptor, error) {

v1/remote/image.go
118:func (r *remoteImage) Descriptor() (*v1.Descriptor, error) {
220:func (rl *remoteImageLayer) Descriptor() (*v1.Descriptor, error) {

v1/partial/with.go
284:    Descriptor() (*v1.Descriptor, error)
301:            return wd.Descriptor()
306:            return wd.Descriptor()

This might be a nice interim until we can rip out the partial stuff altogether?

[imjasonh]

If the method returns (bool, error) I would expect there to be some possibility of returning false, nil -- otherwise, the bool is just equivalent err != nil, and doesn't provide any other information.

Could this return false, nil if headBlob returns a 404? (Indicating that it successfully determined that the blob doesn't exist)

[jonjohnsonjr]

You caught me being lazy, look again.