go/common/cbor: Add support for generic versioned blobs

.changelog/3052.internal.md

@@ -0,0 +1,4 @@
+go/common/cbor: Add support for generic versioned blobs
+
+Having to push, get a PR number, and then write these things is stupid
+and annoying.

go/common/cbor/versioned.go

@@ -0,0 +1,51 @@
+package cbor
+
+import (
+	"errors"
+	"math"
+
+	"github.com/fxamacker/cbor/v2"
+)
+
+const invalidVersion = math.MaxUint16
+
+var (
+	// ErrInvalidVersion is the error returned when a versioned
+	// serialized blob is either missing, or has an invalid version.
+	ErrInvalidVersion = errors.New("cbor: missing or invalid version")
+
+	decOptionsVersioned = decOptions
+
+	decModeVersioned cbor.DecMode
+)
+
+// Versioned is a generic versioned serializable data structure.
+type Versioned struct {
+	V uint16 `json:"v"`
+}
+
+// GetVersion returns the version of a versioned serializable data
+// structure, if any.
+func GetVersion(data []byte) (uint16, error) {
+	vblob := Versioned{
+		V: invalidVersion,
+	}
+	if err := decModeVersioned.Unmarshal(data, &vblob); err != nil {
+		return 0, err
+	}
+	if vblob.V == invalidVersion {
+		return 0, ErrInvalidVersion
+	}
+	return vblob.V, nil
+}
+
+func init() {
+	// Use the untrusted decode options, but ignore unknown fields.
+	// FIXME: https://github.com/fxamacker/cbor/issues/240
+	decOptionsVersioned.ExtraReturnErrors = int(cbor.ExtraDecErrorNone)
+
+	var err error
+	if decModeVersioned, err = decOptionsVersioned.DecMode(); err != nil {
+		panic(err)
+	}
+}

go/common/cbor/versioned_test.go

@@ -0,0 +1,39 @@
+package cbor
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestVersioned(t *testing.T) {
+	require := require.New(t)
+
+	type a struct {
+		A string
+	}
+
+	raw := Marshal(&a{
+		A: "Open still remaineth the earth for great souls.",
+	})
+	_, err := GetVersion(raw)
+	require.Equal(ErrInvalidVersion, err, "missing version should error")
+
+	type b struct {
+		Versioned
+		a
+	}
+
+	const testVersion = 451
+	raw = Marshal(&b{
+		Versioned: Versioned{
+			V: testVersion,
+		},
+		a: a{
+			A: "Empty are still many sites for lone ones and twain ones",
+		},
+	})
+	version, err := GetVersion(raw)
+	require.NoError(err, "versioned blobs should deserialize")
+	require.Equal(testVersion, version, "the version should be correct")
+}