[28.x backport] cli-plugins/manager: various fixes and deprecations

[thaJeztah]

• backport cli-plugins/manager: various fixes and deprecations #6237
• depends on / stacked on [28.x] add internal fork of docker/docker/registry #6221
• depends on / stacked on [28.x backport] remove aliases for containerd/errdefs, disallow docker/errdefs #6272

---

cli-plugins/manager: un-export "Candidate" interface

It is for internal use for mocking purposes, and is not part
of any public interface / signature.

cli-plugins/manager: fix Plugin marshaling with regular errors

Go does not by default marshal error type fields to JSON. The manager
package therefore implemented a pluginError type that implements
encoding.TextMarshaler. However, the field was marked as a regular
error, which made it brittle; assining any other type of error would
result in the error being discarded in the marshaled JSON (as used in
docker info output), resulting in the error being marshaled as {}.

This patch adds a custom MarshalJSON() on the Plugin type itself
so that any error is rendered. It checks if the error used already
implements encoding.TextMarshaler, otherwise wraps the error in
a pluginError.

cli-plugins/manager: un-export "NewPluginError"

It is for internal use, and no longer needed for testing, now that
the Plugin type handles marshalling errors.

cli-plugins/manager: deprecate "IsNotFound"

These errors satisfy errdefs.IsNotFound, so make it a wrapper, and
deprecate it.

cli-plugins/manager: pluginError: remove Causer interface

We no longer depend on this interface and it implements Unwrap for
native handling by go stdlib.

cli-plugins/manager: wrapAsPluginError: don't special-case nil

This was a pattern inheritted from pkg/errors.Wrapf, which ignored
nil errors for convenience. However, it is error-prone, as it is
not obvious when returning a nil-error.

All call-sites using wrapAsPluginError already do a check for
nil errors, so remove this code to prevent hard to find bugs.

cli-plugins/manager: deprecate metadata aliases

These aliases were added in 4321293
(part of v28.0), but did not deprecate them. They are no longer used
in the CLI itself, but may be used by cli-plugin implementations.

This deprecates the aliases in cli-plugins/manager in favor of
their equivalent in cli-plugins/manager/metadata:

• NamePrefix
• MetadataSubcommandName
• HookSubcommandName
• Metadata
• ReexecEnvvar

cli-plugins/manager: remove deprecated ResourceAttributesEnvvar

This const was deprecated in 9dc175d,
which is part of v28.0, so let's remove it.

- Human readable description for the release notes

Go SDK: `cli-plugins/manager`: remove `Candidate` interface, which was only for internal use.
Go SDK: `cli-plugins/manager`: remove `NewPluginError` function, which was only for internal use.
Go SDK: `cli-plugins/manager`: remove deprecated `ResourceAttributesEnvvar` const.
Go SDK: `cli-plugins/manager`: deprecate metadata aliases in favor of their equivalent in `cli-plugins/manager/metadata`:

- `NamePrefix`
- `MetadataSubcommandName`
- `HookSubcommandName`
- `Metadata`
- `ReexecEnvvar`

- A picture of a cute animal (not mandatory but encouraged)

[thaJeztah]

Ah; we're probably still using cerrdefs as alias in some places;

30.99 cmd/docker/docker.go:204:8: undefined: errdefs
30.99 cmd/docker/docker.go:243:8: undefined: errdefs
30.99 cmd/docker/docker.go:476:8: undefined: errdefs

Let me have a look

[thaJeztah]

OK, that would need #6217, but that one needs #6221

Let me open a draft PR for that, and move this into draft as well

[codecov-commenter]

Codecov Report

❌ Patch coverage is 79.16667% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cmd/docker/docker.go	0.00%	2 Missing and 1 partial ⚠️
cli-plugins/manager/manager.go	66.66%	1 Missing ⚠️
cmd/docker/builder.go	0.00%	0 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!