Add field type for serialization

Since reference itself may be represented by multiple types which implement the reference inteface, serialization can lead to ambiguous type which cannot be deserialized.
Field wraps the reference object to ensure that the correct type is always deserialized, requiring an extra unwrap of the reference after deserialization.

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)

reference/reference_test.go

@@ -1,6 +1,7 @@
 package reference
 
 import (
+	"encoding/json"
 	"strconv"
 	"strings"
 	"testing"
@@ -265,3 +266,132 @@ func TestSplitHostname(t *testing.T) {
 		}
 	}
 }
+
+type serializationType struct {
+	Description string
+	Field       Field
+}
+
+func TestSerialization(t *testing.T) {
+	testcases := []struct {
+		description string
+		input       string
+		name        string
+		tag         string
+		digest      string
+		err         error
+	}{
+		{
+			description: "empty value",
+			err:         ErrNameEmpty,
+		},
+		{
+			description: "just a name",
+			input:       "example.com:8000/named",
+			name:        "example.com:8000/named",
+		},
+		{
+			description: "name with a tag",
+			input:       "example.com:8000/named:tagged",
+			name:        "example.com:8000/named",
+			tag:         "tagged",
+		},
+		{
+			description: "name with digest",
+			input:       "other.com/named@sha256:1234567890098765432112345667890098765",
+			name:        "other.com/named",
+			digest:      "sha256:1234567890098765432112345667890098765",
+		},
+	}
+	for _, testcase := range testcases {
+		failf := func(format string, v ...interface{}) {
+			t.Logf(strconv.Quote(testcase.input)+": "+format, v...)
+			t.Fail()
+		}
+
+		m := map[string]string{
+			"Description": testcase.description,
+			"Field":       testcase.input,
+		}
+		b, err := json.Marshal(m)
+		if err != nil {
+			failf("error marshalling: %v", err)
+		}
+		t := serializationType{}
+
+		if err := json.Unmarshal(b, &t); err != nil {
+			if testcase.err == nil {
+				failf("error unmarshalling: %v", err)
+			}
+			if err != testcase.err {
+				failf("wrong error, expected %v, got %v", testcase.err, err)
+			}
+
+			continue
+		} else if testcase.err != nil {
+			failf("expected error unmarshalling: %v", testcase.err)
+		}
+
+		if t.Description != testcase.description {
+			failf("wrong description, expected %q, got %q", testcase.description, t.Description)
+		}
+
+		ref := t.Field.Reference()
+
+		if named, ok := ref.(Named); ok {
+			if named.Name() != testcase.name {
+				failf("unexpected repository: got %q, expected %q", named.Name(), testcase.name)
+			}
+		} else if testcase.name != "" {
+			failf("expected named type, got %T", ref)
+		}
+
+		tagged, ok := ref.(Tagged)
+		if testcase.tag != "" {
+			if ok {
+				if tagged.Tag() != testcase.tag {
+					failf("unexpected tag: got %q, expected %q", tagged.Tag(), testcase.tag)
+				}
+			} else {
+				failf("expected tagged type, got %T", ref)
+			}
+		} else if ok {
+			failf("unexpected tagged type")
+		}
+
+		digested, ok := ref.(Digested)
+		if testcase.digest != "" {
+			if ok {
+				if digested.Digest().String() != testcase.digest {
+					failf("unexpected digest: got %q, expected %q", digested.Digest().String(), testcase.digest)
+				}
+			} else {
+				failf("expected digested type, got %T", ref)
+			}
+		} else if ok {
+			failf("unexpected digested type")
+		}
+
+		t = serializationType{
+			Description: testcase.description,
+			Field:       AsField(ref),
+		}
+
+		b2, err := json.Marshal(t)
+		if err != nil {
+			failf("error marshing serialization type: %v", err)
+		}
+
+		if string(b) != string(b2) {
+			failf("unexpected serialized value: expected %q, got %q", string(b), string(b2))
+		}
+
+		// Ensure t.Field is not implementing "Reference" directly, getting
+		// around the Reference type system
+		var fieldInterface interface{} = t.Field
+		if _, ok := fieldInterface.(Reference); ok {
+			failf("field should not implement Reference interface")
+		}
+
+	}
+}