From ba0cfb20bcebc10ea431e2956047af183dfe95a9 Mon Sep 17 00:00:00 2001
From: Yawning Angel <yawning@schwanenlied.me>
Date: Thu, 25 Jun 2020 11:30:20 +0000
Subject: [PATCH] go/common/cbor: Add support for generic versioned blobs

---
 .changelog/3052.internal.md      |  1 +
 go/common/cbor/versioned.go      | 51 ++++++++++++++++++++++++++++++++
 go/common/cbor/versioned_test.go | 39 ++++++++++++++++++++++++
 3 files changed, 91 insertions(+)
 create mode 100644 .changelog/3052.internal.md
 create mode 100644 go/common/cbor/versioned.go
 create mode 100644 go/common/cbor/versioned_test.go

diff --git a/.changelog/3052.internal.md b/.changelog/3052.internal.md
new file mode 100644
index 00000000000..45db96f11ef
--- /dev/null
+++ b/.changelog/3052.internal.md
@@ -0,0 +1 @@
+go/common/cbor: Add support for generic versioned blobs
diff --git a/go/common/cbor/versioned.go b/go/common/cbor/versioned.go
new file mode 100644
index 00000000000..0461af73a73
--- /dev/null
+++ b/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)
+	}
+}
diff --git a/go/common/cbor/versioned_test.go b/go/common/cbor/versioned_test.go
new file mode 100644
index 00000000000..2d0884b26f2
--- /dev/null
+++ b/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 uint16 = 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")
+}