Add mediatype string builder to bundle package

pkg/bundle/bundle.go

@@ -42,6 +42,8 @@ var ErrMissingEnvelope = fmt.Errorf("%w: missing envelope", ErrInvalidAttestatio
 var ErrDecodingJSON = fmt.Errorf("%w: decoding json", ErrInvalidAttestation)
 var ErrDecodingB64 = fmt.Errorf("%w: decoding base64", ErrInvalidAttestation)
 
+const mediaTypeBase = "application/vnd.dev.sigstore.bundle"
+
 func ErrValidationError(err error) error {
 	return fmt.Errorf("%w: %w", ErrValidation, err)
 }
@@ -113,17 +115,40 @@ func (b *ProtobufBundle) validate() error {
 	return nil
 }
 
+// MediaTypeString returns a mediatype string for the specified bundle version.
+// The function returns an error if the resulting string does validate.
+func MediaTypeString(version string) (string, error) {
+	if version == "" {
+		return "", fmt.Errorf("unable to build media type string, no version defined")
+	}
+
+	var mtString string
+
+	version = strings.TrimPrefix(version, "v")
+	mtString = fmt.Sprintf("%s.v%s+json", mediaTypeBase, strings.TrimPrefix(version, "v"))
+
+	if version == "0.1" || version == "0.2" {
+		mtString = fmt.Sprintf("%s+json;version=%s", mediaTypeBase, strings.TrimPrefix(version, "v"))
+	}
+
+	if _, err := getBundleVersion(mtString); err != nil {
+		return "", fmt.Errorf("unable to build mediatype: %w", err)
+	}
+
+	return mtString, nil
+}
+
 func getBundleVersion(mediaType string) (string, error) {
 	switch mediaType {
-	case "application/vnd.dev.sigstore.bundle+json;version=0.1":
+	case mediaTypeBase + "+json;version=0.1":
 		return "v0.1", nil
-	case "application/vnd.dev.sigstore.bundle+json;version=0.2":
+	case mediaTypeBase + "+json;version=0.2":
 		return "v0.2", nil
-	case "application/vnd.dev.sigstore.bundle+json;version=0.3":
+	case mediaTypeBase + "+json;version=0.3":
 		return "v0.3", nil
 	}
-	if strings.HasPrefix(mediaType, "application/vnd.dev.sigstore.bundle.v") && strings.HasSuffix(mediaType, "+json") {
-		version := strings.TrimPrefix(mediaType, "application/vnd.dev.sigstore.bundle.")
+	if strings.HasPrefix(mediaType, mediaTypeBase+".v") && strings.HasSuffix(mediaType, "+json") {
+		version := strings.TrimPrefix(mediaType, mediaTypeBase+".")
 		version = strings.TrimSuffix(version, "+json")
 		if semver.IsValid(version) {
 			return version, nil

pkg/bundle/bundle_test.go

@@ -17,6 +17,8 @@ package bundle
 import (
 	"fmt"
 	"testing"
+
+	"github.com/stretchr/testify/require"
 )
 
 func Test_getBundleVersion(t *testing.T) {
@@ -94,3 +96,31 @@ func Test_getBundleVersion(t *testing.T) {
 		})
 	}
 }
+
+func TestMediaTypeString(t *testing.T) {
+	t.Parallel()
+	for _, tc := range []struct {
+		name     string
+		ver      string
+		expected string
+		mustErr  bool
+	}{
+		{"normal-semver", "v0.3", "application/vnd.dev.sigstore.bundle.v0.3+json", false},
+		{"old-semver1", "v0.1", "application/vnd.dev.sigstore.bundle+json;version=0.1", false},
+		{"old-semver2", "v0.2", "application/vnd.dev.sigstore.bundle+json;version=0.2", false},
+		{"blank", "", "", true},
+		{"invalid", "garbage", "", true},
+	} {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			res, err := MediaTypeString(tc.ver)
+			if tc.mustErr {
+				require.Error(t, err)
+				return
+			}
+			require.NoError(t, err)
+			require.Equal(t, tc.expected, res)
+		})
+	}
+}