fix: improve error message when ParseReference fails

[taeold]

When given invalid reference name, name.ParseReference returns a generic error message:

package main

import (
	"fmt"

	"github.com/google/go-containerregistry/pkg/name"
)

var badTagNames = []string{
	"gcr.io/project-id/bad_chars:c@n'tuse",
	"gcr.io/project-id/wrong-length:white space",
	"gcr.io/project-id/too-many-chars:thisisthetagthatneverendsitgoesonandonmyfriendsomepeoplestartedtaggingitnotknowingwhatitwasandtheyllcontinuetaggingitforeverjustbecausethisisthetagthatneverends",
}

func main() {
	for _, n := range badTagNames {
		_, err := name.ParseReference(n)
		fmt.Printf("ParseReference(%q) returned err=%v\n", n, err)
	}
}

# ParseReference("gcr.io/project-id/bad_chars:c@n'tuse") returned err=could not parse reference: gcr.io/project-id/bad_chars:c@n'tuse
# ParseReference("gcr.io/project-id/wrong-length:white space") returned err=could not parse reference: gcr.io/project-id/wrong-length:white space
# ParseReference("gcr.io/project-id/too-many-chars:thisisthetagthatneverendsitgoesonandonmyfriendsomepeoplestartedtaggingitnotknowingwhatitwasandtheyllcontinuetaggingitforeverjustbecausethisisthetagthatneverends") returned err=could not parse reference: gcr.io/project-id/too-many-chars:thisisthetagthatneverendsitgoesonandonmyfriendsomepeoplestartedtaggingitnotknowingwhatitwasandtheyllcontinuetaggingitforeverjustbecausethisisthetagthatneverends

playground

The error message is succinct but unhelpful when the end user is trying to figure out why the given reference name is invalid.

We modify the code to include more descriptive error message on why the parsing failed as a tag or digest:

# could not parse reference 'gcr.io/project-id/bad_chars:c@n'tuse' as tag or digest: tag error: tag can only contain the characters `abcdefghijklmnopqrstuvwxyz0123456789_-.ABCDEFGHIJKLMNOPQRSTUVWXYZ`: c@n'tuse, digest error: unsupported digest algorithm: n'tuse
# could not parse reference 'gcr.io/project-id/wrong-length:white space' as tag or digest: tag error: tag can only contain the characters `abcdefghijklmnopqrstuvwxyz0123456789_-.ABCDEFGHIJKLMNOPQRSTUVWXYZ`: white space, digest error: a digest must contain exactly one '@' separator (e.g. registry/repository@digest) saw: gcr.io/project-id/wrong-length:white space
# could not parse reference 'gcr.io/project-id/too-many-chars:thisisthetagthatneverendsitgoesonandonmyfriendsomepeoplestartedtaggingitnotknowingwhatitwasandtheyllcontinuetaggingitforeverjustbecausethisisthetagthatneverends' as tag or digest: tag error: tag must be between 1 and 128 characters in length: thisisthetagthatneverendsitgoesonandonmyfriendsomepeoplestartedtaggingitnotknowingwhatitwasandtheyllcontinuetaggingitforeverjustbecausethisisthetagthatneverends, digest error: a digest must contain exactly one '@' separator (e.g. registry/repository@digest) saw: gcr.io/project-id/too-many-chars:thisisthetagthatneverendsitgoesonandonmyfriendsomepeoplestartedtaggingitnotknowingwhatitwasandtheyllcontinuetaggingitforeverjustbecausethisisthetagthatneverends

The resulting error message is much more verbose, but clearly identifies the problem with the given resource name.

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