meta: make a read-only version to enforce token immutability

token/delegation/delegation.go

@@ -142,8 +142,8 @@ func (t *Token) Nonce() []byte {
 }
 
 // Meta returns the Token's metadata.
-func (t *Token) Meta() *meta.Meta {
-	return t.meta
+func (t *Token) Meta() meta.ReadOnly {
+	return t.meta.ReadOnly()
 }
 
 // NotBefore returns the time at which the Token becomes "active".

token/interface.go

@@ -17,7 +17,7 @@ type Token interface {
 	// Issuer returns the did.DID representing the Token's issuer.
 	Issuer() did.DID
 	// Meta returns the Token's metadata.
-	Meta() *meta.Meta
+	Meta() meta.ReadOnly
 }
 
 type Marshaller interface {

token/invocation/invocation.go

@@ -71,8 +71,8 @@ func (t *Token) Nonce() []byte {
 }
 
 // Meta returns the Token's metadata.
-func (t *Token) Meta() *meta.Meta {
-	return t.meta
+func (t *Token) Meta() meta.ReadOnly {
+	return t.meta.ReadOnly()
 }
 
 // Expiration returns the time at which the Token expires.