feat: support --platform for oras attach

cmd/oras/internal/option/platform.go

@@ -27,13 +27,17 @@ import (
 
 // Platform option struct.
 type Platform struct {
-	platform string
-	Platform *ocispec.Platform
+	platform        string
+	Platform        *ocispec.Platform
+	FlagDescription string
 }
 
 // ApplyFlags applies flags to a command flag set.
 func (opts *Platform) ApplyFlags(fs *pflag.FlagSet) {
-	fs.StringVarP(&opts.platform, "platform", "", "", "request platform in the form of `os[/arch][/variant][:os_version]`")
+	if opts.FlagDescription == "" {
+		opts.FlagDescription = "request platform"
+	}
+	fs.StringVarP(&opts.platform, "platform", "", "", opts.FlagDescription+" in the form of `os[/arch][/variant][:os_version]`")
 }
 
 // parse parses the input platform flag to an oci platform type.

cmd/oras/internal/option/platform_test.go

@@ -35,11 +35,11 @@ func TestPlatform_Parse_err(t *testing.T) {
 		name string
 		opts *Platform
 	}{
-		{name: "empty arch 1", opts: &Platform{"os/", nil}},
-		{name: "empty arch 2", opts: &Platform{"os//variant", nil}},
-		{name: "empty os", opts: &Platform{"/arch", nil}},
-		{name: "empty os with variant", opts: &Platform{"/arch/variant", nil}},
-		{name: "trailing slash", opts: &Platform{"os/arch/variant/llama", nil}},
+		{name: "empty arch 1", opts: &Platform{"os/", nil, ""}},
+		{name: "empty arch 2", opts: &Platform{"os//variant", nil, ""}},
+		{name: "empty os", opts: &Platform{"/arch", nil, ""}},
+		{name: "empty os with variant", opts: &Platform{"/arch/variant", nil, ""}},
+		{name: "trailing slash", opts: &Platform{"os/arch/variant/llama", nil, ""}},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {

cmd/oras/root/attach.go

@@ -39,6 +39,7 @@ type attachOptions struct {
 	option.Packer
 	option.Target
 	option.Format
+	option.Platform
 
 	artifactType string
 	concurrency  int
@@ -56,6 +57,9 @@ func attachCmd() *cobra.Command {
 Example - Attach file 'hi.txt' with aritifact type 'doc/example' to manifest 'hello:v1' in registry 'localhost:5000':
   oras attach --artifact-type doc/example localhost:5000/hello:v1 hi.txt
 
+Example - Attach file 'hi.txt' to a specific artifact with platform 'linux/amd64' in multi-arch index 'hello:v1'
+  oras attach --artifact-type doc/example --platform linux/amd64 localhost:5000/hello:v1 hi.txt
+
 Example - Push file "hi.txt" with the custom layer media type 'application/vnd.me.hi':
   oras attach --artifact-type doc/example localhost:5000/hello:v1 hi.txt:application/vnd.me.hi
 
@@ -94,6 +98,7 @@ Example - Attach file to the manifest tagged 'v1' in an OCI image layout folder
 
 	cmd.Flags().StringVarP(&opts.artifactType, "artifact-type", "", "", "artifact type")
 	cmd.Flags().IntVarP(&opts.concurrency, "concurrency", "", 5, "concurrency level")
+	opts.FlagDescription = "attach to an arch-specific subject"
 	_ = cmd.MarkFlagRequired("artifact-type")
 	opts.EnableDistributionSpecFlag()
 	option.ApplyFlags(&opts, cmd.Flags())
@@ -132,9 +137,11 @@ func runAttach(cmd *cobra.Command, opts *attachOptions) error {
 	// add both pull and push scope hints for dst repository
 	// to save potential push-scope token requests during copy
 	ctx = registryutil.WithScopeHint(ctx, dst, auth.ActionPull, auth.ActionPush)
-	subject, err := dst.Resolve(ctx, opts.Reference)
+	fetchOpts := oras.DefaultResolveOptions
+	fetchOpts.TargetPlatform = opts.Platform.Platform
+	subject, err := oras.Resolve(ctx, dst, opts.Reference, fetchOpts)
 	if err != nil {
-		return err
+		return fmt.Errorf("failed to resolve %s: %w", opts.Reference, err)
 	}
 	descs, err := loadFiles(ctx, store, annotations, opts.FileRefs, displayStatus)
 	if err != nil {

test/e2e/suite/command/attach.go

@@ -29,6 +29,7 @@ import (
 	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
 	"oras.land/oras/test/e2e/internal/testdata/feature"
 	"oras.land/oras/test/e2e/internal/testdata/foobar"
+	"oras.land/oras/test/e2e/internal/testdata/multi_arch"
 	. "oras.land/oras/test/e2e/internal/utils"
 	"oras.land/oras/test/e2e/internal/utils/match"
 )
@@ -99,6 +100,21 @@ var _ = Describe("1.1 registry users:", func() {
 				MatchStatus([]match.StateKey{foobar.AttachFileStateKey}, false, 1).Exec()
 		})
 
+		It("should attach a file to an arch-specific subject", func() {
+			testRepo := attachTestRepo("arch-specific")
+			// Below line will cause unexpected 500
+			// pending for https://github.com/project-zot/zot/pull/2351 to be released
+			// CopyZOTRepo(ImageRepo, testRepo)
+			subjectRef := RegistryRef(ZOTHost, testRepo, multi_arch.Tag)
+			ORAS("cp", RegistryRef(ZOTHost, ImageRepo, multi_arch.Tag), subjectRef).Exec()
+			artifactType := "test/attach"
+			// test
+			out := ORAS("attach", "--artifact-type", artifactType, subjectRef, fmt.Sprintf("%s:%s", foobar.AttachFileName, foobar.AttachFileMedia), "--format", "{{.Digest}}", "--platform", "linux/amd64").
+				WithWorkDir(PrepareTempFiles()).Exec().Out.Contents()
+			// validate
+			ORAS("discover", "--artifact-type", artifactType, RegistryRef(ZOTHost, testRepo, multi_arch.LinuxAMD64.Digest.String())).MatchKeyWords(string(out)).Exec()
+		})
+
 		It("should attach a file to a subject and export the built manifest", func() {
 			// prepare
 			testRepo := attachTestRepo("export-manifest")
@@ -282,6 +298,18 @@ var _ = Describe("OCI image layout users:", func() {
 			fetched := ORAS("manifest", "fetch", Flags.Layout, LayoutRef(root, index.Manifests[0].Digest.String())).Exec().Out.Contents()
 			MatchFile(filepath.Join(root, exportName), string(fetched), DefaultTimeout)
 		})
+
+		It("should attach a file to an arch-specific subject", func() {
+			root := PrepareTempOCI(ImageRepo)
+			subjectRef := LayoutRef(root, multi_arch.Tag)
+			artifactType := "test/attach"
+			// test
+			out := ORAS("attach", Flags.Layout, "--artifact-type", artifactType, subjectRef, fmt.Sprintf("%s:%s", foobar.AttachFileName, foobar.AttachFileMedia), "--format", "{{.Digest}}", "--platform", "linux/amd64").
+				WithWorkDir(PrepareTempFiles()).Exec().Out.Contents()
+			// validate
+			ORAS("discover", Flags.Layout, "--artifact-type", artifactType, LayoutRef(root, multi_arch.LinuxAMD64.Digest.String())).MatchKeyWords(string(out)).Exec()
+		})
+
 		It("should attach a file via a OCI Image", func() {
 			root := PrepareTempOCI(ImageRepo)
 			subjectRef := LayoutRef(root, foobar.Tag)