go/common/cbor: Add UnmarshalTrusted for trusted inputs

The new method relaxes some decoding restrictions for cases where the inputs are
trusted (e.g., because they are known to be generated by the local node itself).

.changelog/2800.internal.md

@@ -0,0 +1,5 @@
+go/common/cbor: Add UnmarshalTrusted for trusted inputs
+
+The new method relaxes some decoding restrictions for cases where the inputs
+are trusted (e.g., because they are known to be generated by the local node
+itself).

go/common/cbor/cbor.go

@@ -27,14 +27,23 @@ var (
 		TagsMd:        cbor.TagsForbidden,
 	}
 
+	// decOptions are decoding options for UNTRUSTED inputs (used by default).
 	decOptions = cbor.DecOptions{
 		DupMapKey:   cbor.DupMapKeyEnforcedAPF,
 		IndefLength: cbor.IndefLengthForbidden,
 		TagsMd:      cbor.TagsForbidden,
 	}
 
-	encMode cbor.EncMode
-	decMode cbor.DecMode
+	// decOptionsTrusted are decoding options for TRUSTED inputs. They are only used when explicitly
+	// requested by using the UnmarshalTrusted method.
+	decOptionsTrusted = cbor.DecOptions{
+		MaxArrayElements: 134217728, // Maximum allowed.
+		MaxMapPairs:      134217728, // Maximum allowed.
+	}
+
+	encMode        cbor.EncMode
+	decMode        cbor.DecMode
+	decModeTrusted cbor.DecMode
 )
 
 func init() {
@@ -45,6 +54,9 @@ func init() {
 	if decMode, err = decOptions.DecMode(); err != nil {
 		panic(err)
 	}
+	if decModeTrusted, err = decOptionsTrusted.DecMode(); err != nil {
+		panic(err)
+	}
 }
 
 // FixSliceForSerde will convert `nil` to `[]byte` to work around serde
@@ -74,6 +86,17 @@ func Unmarshal(data []byte, dst interface{}) error {
 	return decMode.Unmarshal(data, dst)
 }
 
+// UnmarshalTrusted deserializes a CBOR byte vector into a given type.
+//
+// This method MUST ONLY BE USED FOR TRUSTED INPUTS as it relaxes some decoding restrictions.
+func UnmarshalTrusted(data []byte, dst interface{}) error {
+	if data == nil {
+		return nil
+	}
+
+	return decModeTrusted.Unmarshal(data, dst)
+}
+
 // MustUnmarshal deserializes a CBOR byte vector into a given type.
 // Panics if unmarshal fails.
 func MustUnmarshal(data []byte, dst interface{}) {