feat: add --config flag support for oras attach command

Add support for customizing the config descriptor when using oras attach,
matching the existing --config flag behavior in oras push. This allows users
to specify a config file with an optional media type using the format
--config <file>[:<type>].

- Add manifestConfigRef field and --config flag to attach command
- Parse config file reference and set ConfigDescriptor in pack options
- Add config and platform mutual exclusivity validation
- Add config annotations support for attach
- Add usage example in command help text

Fixes #1146

Author: ogulcanaydogan

cmd/oras/root/attach.go

@@ -30,6 +30,7 @@ import (
 	"oras.land/oras/cmd/oras/internal/command"
 	"oras.land/oras/cmd/oras/internal/display"
 	oerrors "oras.land/oras/cmd/oras/internal/errors"
+	"oras.land/oras/cmd/oras/internal/fileref"
 	"oras.land/oras/cmd/oras/internal/option"
 	"oras.land/oras/internal/graph"
 	"oras.land/oras/internal/registryutil"
@@ -43,8 +44,9 @@ type attachOptions struct {
 	option.Platform
 	option.Terminal
 
-	artifactType string
-	concurrency  int
+	artifactType      string
+	manifestConfigRef string
+	concurrency       int
 	// Deprecated: verbose is deprecated and will be removed in the future.
 	verbose bool
 }
@@ -92,6 +94,9 @@ Example - Attach file to the manifest tagged 'v1' in an OCI image layout folder
 
 Example - Attach file to the manifest tagged 'example.com:v1' in an OCI image layout folder 'layout-dir':
   oras attach --artifact-type doc/example --oci-layout-path layout-dir example.com:v1 hi.txt
+
+Example - Attach file 'hi.txt' with the custom manifest config "config.json" of the custom media type "application/vnd.me.config":
+  oras attach --artifact-type doc/example --config config.json:application/vnd.me.config localhost:5000/hello:v1 hi.txt
 `,
 		Args: oerrors.CheckArgs(argument.AtLeast(1), "the destination artifact for attaching."),
 		PreRunE: func(cmd *cobra.Command, args []string) error {
@@ -100,6 +105,9 @@ Example - Attach file to the manifest tagged 'example.com:v1' in an OCI image la
 			err := option.Parse(cmd, &opts)
 			if err == nil {
 				opts.DisableTTY(opts.Debug, false)
+				if err = oerrors.CheckMutuallyExclusiveFlags(cmd.Flags(), "config", "platform"); err != nil {
+					return err
+				}
 				if err = opts.EnsureReferenceNotEmpty(cmd, true); err == nil {
 					return nil
 				}
@@ -121,6 +129,7 @@ Example - Attach file to the manifest tagged 'example.com:v1' in an OCI image la
 	}
 
 	cmd.Flags().StringVarP(&opts.artifactType, "artifact-type", "", "", "artifact type")
+	cmd.Flags().StringVarP(&opts.manifestConfigRef, "config", "", "", "`path` of image config file")
 	cmd.Flags().IntVarP(&opts.concurrency, "concurrency", "", 5, "concurrency level")
 	cmd.Flags().BoolVarP(&opts.verbose, "verbose", "v", true, "print status output for unnamed blobs")
 	opts.FlagDescription = "attach to an arch-specific subject"
@@ -184,9 +193,22 @@ func runAttach(cmd *cobra.Command, opts *attachOptions) error {
 
 	packOpts := oras.PackManifestOptions{
 		Subject:             &subject,
+		ConfigAnnotations:   opts.Annotations[option.AnnotationConfig],
 		ManifestAnnotations: opts.Annotations[option.AnnotationManifest],
 		Layers:              descs,
 	}
+	if opts.manifestConfigRef != "" {
+		path, cfgMediaType, err := fileref.Parse(opts.manifestConfigRef, oras.MediaTypeUnknownConfig)
+		if err != nil {
+			return err
+		}
+		desc, err := addFile(ctx, store, option.AnnotationConfig, cfgMediaType, path)
+		if err != nil {
+			return err
+		}
+		desc.Annotations = packOpts.ConfigAnnotations
+		packOpts.ConfigDescriptor = &desc
+	}
 	pack := func() (ocispec.Descriptor, error) {
 		return oras.PackManifest(ctx, store, oras.PackManifestVersion1_1, opts.artifactType, packOpts)
 	}