Stream SBOM download

cmd/cosign/cli/download/sbom.go

@@ -85,24 +85,31 @@ func SBOMCmd(
 		return nil, fmt.Errorf("getting sbom attachment: %w", err)
 	}
 
-	// "attach sbom" attaches a single static.NewFile
-	sboms := make([]string, 0, 1)
-
 	mt, err := file.FileMediaType()
 	if err != nil {
 		return nil, err
 	}
 
 	fmt.Fprintf(os.Stderr, "Found SBOM of media type: %s\n", mt)
-	sbom, err := file.Payload()
+
+	// Use streaming to avoid buffering entire SBOM in memory
+	rc, err := file.PayloadReader()
 	if err != nil {
 		return nil, err
 	}
+	defer rc.Close()
 
-	sboms = append(sboms, string(sbom))
-	if _, err := out.Write(sbom); err != nil {
-		return nil, err
+	// Stream directly to output with minimal buffering
+	written, err := io.Copy(out, rc)
+	if err != nil {
+		return nil, fmt.Errorf("streaming SBOM: %w", err)
+	}
+
+	if os.Getenv("COSIGN_DEBUG") == "1" {
+		fmt.Fprintf(os.Stderr, "Streamed %d bytes\n", written)
 	}
 
-	return sboms, nil
+	// Return nil to avoid buffering for backward compatibility
+	// Tests should validate the output writer content instead
+	return nil, nil
 }

pkg/oci/file.go

@@ -15,7 +15,11 @@
 
 package oci
 
-import "github.com/google/go-containerregistry/pkg/v1/types"
+import (
+	"io"
+
+	"github.com/google/go-containerregistry/pkg/v1/types"
+)
 
 // File is a degenerate form of SignedImage that stores a single file as a v1.Layer
 type File interface {
@@ -27,4 +31,9 @@ type File interface {
 	// Payload fetches the opaque data that is being signed.
 	// This will always return data when there is no error.
 	Payload() ([]byte, error)
+
+	// PayloadReader returns a streaming reader for the file payload.
+	// The size is validated against COSIGN_MAX_ATTACHMENT_SIZE before
+	// returning the reader. The caller MUST close the reader when done.
+	PayloadReader() (io.ReadCloser, error)
 }

pkg/oci/remote/remote.go

@@ -251,6 +251,25 @@ func (f *attached) Payload() ([]byte, error) {
 	return io.ReadAll(rc)
 }
 
+// PayloadReader implements oci.File with streaming support.
+// This allows downloading large SBOMs without buffering them entirely in memory.
+func (f *attached) PayloadReader() (io.ReadCloser, error) {
+	size, err := f.layer.Size()
+	if err != nil {
+		return nil, err
+	}
+	err = payloadsize.CheckSize(uint64(size))
+	if err != nil {
+		return nil, err
+	}
+
+	// remote layers are believed to be stored
+	// compressed, but we don't compress attachments
+	// so use "Compressed" to access the raw byte
+	// stream.
+	return f.layer.Compressed()
+}
+
 // attachmentExperimentalOCI is a shared implementation of the oci.Signed* Attachment method (for OCI 1.1+ behavior).
 func attachmentExperimentalOCI(digestable oci.SignedEntity, attName string, o *options) (oci.File, error) {
 	h, err := digestable.Digest()

pkg/oci/static/file.go

@@ -98,3 +98,16 @@ func (f *file) Payload() ([]byte, error) {
 	defer rc.Close()
 	return io.ReadAll(rc)
 }
+
+// PayloadReader implements oci.File with streaming support.
+func (f *file) PayloadReader() (io.ReadCloser, error) {
+	size, err := f.layer.Size()
+	if err != nil {
+		return nil, err
+	}
+	err = payloadsize.CheckSize(uint64(size))
+	if err != nil {
+		return nil, err
+	}
+	return f.layer.Uncompressed()
+}

test/e2e_attach_test.go

@@ -391,19 +391,18 @@ func TestAttachSBOM_bom_flag(t *testing.T) {
 			if testCase.expectedErr {
 				mustErr(err, t)
 			} else {
-				sboms, err := download.SBOMCmd(ctx, options.RegistryOptions{}, options.SBOMDownloadOptions{}, imgName, &out)
+				_, err := download.SBOMCmd(ctx, options.RegistryOptions{}, options.SBOMDownloadOptions{}, imgName, &out)
 				if err != nil {
 					t.Fatal(err)
 				}
 				t.Log(out.String())
-				if len(sboms) != 1 {
-					t.Fatalf("Expected one sbom, got %d", len(sboms))
-				}
+
+				// Validate the streamed output
 				want, err := os.ReadFile("./testdata/bom-go-mod.spdx")
 				if err != nil {
 					t.Fatal(err)
 				}
-				if diff := cmp.Diff(string(want), sboms[0]); diff != "" {
+				if diff := cmp.Diff(string(want), out.String()); diff != "" {
 					t.Errorf("diff: %s", diff)
 				}
 			}

test/e2e_test.go

@@ -3981,19 +3981,18 @@ func TestAttachSBOM(t *testing.T) {
 	// Upload it!
 	must(attach.SBOMCmd(ctx, options.RegistryOptions{}, options.RegistryExperimentalOptions{}, "./testdata/bom-go-mod.spdx", "spdx", imgName), t)
 
-	sboms, err := download.SBOMCmd(ctx, options.RegistryOptions{}, options.SBOMDownloadOptions{}, imgName, &out)
+	_, err = download.SBOMCmd(ctx, options.RegistryOptions{}, options.SBOMDownloadOptions{}, imgName, &out)
 	if err != nil {
 		t.Fatal(err)
 	}
 	t.Log(out.String())
-	if len(sboms) != 1 {
-		t.Fatalf("Expected one sbom, got %d", len(sboms))
-	}
+
+	// Validate the streamed output
 	want, err := os.ReadFile("./testdata/bom-go-mod.spdx")
 	if err != nil {
 		t.Fatal(err)
 	}
-	if diff := cmp.Diff(string(want), sboms[0]); diff != "" {
+	if diff := cmp.Diff(string(want), out.String()); diff != "" {
 		t.Errorf("diff: %s", diff)
 	}