codecs: error upon encountering an 'absent' value during encodes.

CHANGELOG.md

@@ -37,7 +37,9 @@ Unreleased on master
 Changes here are on the master branch, but not in any tagged release yet.
 When a release tag is made, this block of bullet points will just slide down to the [Released Changes](#released-changes) section.
 
-- _nothing yet :)_
+- Changed: encoding functions for codecs in this repo (dag-cbor, dag-json, cbor, json) will now **error** if they encounter an `ipld.Absent` node (or more specifically, if `IsAbsent() == true`). [[]]
+	- Previously, these codecs would quietly emit a serial "null" token in this scenario -- which was effectively implicitly coercing a value into null.  Implicit coercion creates bugs and is bad, so we will not be doing it anymore.
+	- This situation is only reachable when working with schema-typed data (since otherwise `Absent` values aren't used).  In that situation, handing that typed value to an encode function without having called `.Representation()` first is also typically a logical error, so, returning an explicit error now is also probably the right thing to do.
 
 
 

codec/dagcbor/marshal.go

@@ -22,8 +22,11 @@ func Marshal(n ipld.Node, sink shared.TokenSink, allowLinks bool) error {
 func marshal(n ipld.Node, tk *tok.Token, sink shared.TokenSink, allowLinks bool) error {
 	switch n.Kind() {
 	case ipld.Kind_Invalid:
-		return fmt.Errorf("cannot traverse a node that is absent")
+		return fmt.Errorf("invalid node encountered")
 	case ipld.Kind_Null:
+		if n.IsAbsent() {
+			return fmt.Errorf("cannot encode a node that is absent")
+		}
 		tk.Type = tok.TNull
 		_, err := sink.Step(tk)
 		return err

codec/dagjson/marshal.go

@@ -19,7 +19,11 @@ func Marshal(n ipld.Node, sink shared.TokenSink, allowLinks bool) error {
 	var tk tok.Token
 	switch n.Kind() {
 	case ipld.Kind_Invalid:
-		return fmt.Errorf("cannot traverse a node that is absent")
+		return fmt.Errorf("invalid node encountered")
+	case ipld.Kind_Null:
+		if n.IsAbsent() {
+			return fmt.Errorf("cannot encode a node that is absent")
+		}
 	case ipld.Kind_Null:
 		tk.Type = tok.TNull
 		_, err := sink.Step(&tk)

codec/marshal.go

@@ -25,8 +25,11 @@ func Marshal(n ipld.Node, sink shared.TokenSink) error {
 func marshal(n ipld.Node, tk *tok.Token, sink shared.TokenSink) error {
 	switch n.Kind() {
 	case ipld.Kind_Invalid:
-		return fmt.Errorf("cannot traverse a node that is absent")
+		return fmt.Errorf("invalid node encountered")
 	case ipld.Kind_Null:
+		if n.IsAbsent() {
+			return fmt.Errorf("cannot encode a node that is absent")
+		}
 		tk.Type = tok.TNull
 		_, err := sink.Step(tk)
 		return err

unit.go

@@ -67,7 +67,7 @@ var Absent Node = absentNode{}
 type absentNode struct{}
 
 func (absentNode) Kind() Kind {
-	return Kind_Null
+	return Kind_Null // It's questionable if this is a good idea.  Perhaps it should be Kind_Invalid.  See https://github.com/ipld/go-ipld-prime/issues/191 .
 }
 func (absentNode) LookupByString(key string) (Node, error) {
 	return nil, ErrWrongKind{TypeName: "absent", MethodName: "LookupByString", AppropriateKind: KindSet_JustMap, ActualKind: Kind_Null}