From 1930bab51e48a8dcee8b76b9a2b4cc3868ac8d29 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Daniel=20Mart=C3=AD?= <mvdan@mvdan.cc>
Date: Mon, 17 Jan 2022 21:46:56 +0000
Subject: [PATCH] node/bindnode: subtract all absents in Length at the repr
 level

For structs at the repr level, we were only subtracting the number of
trailing absents, which isn't right.

Rod's added checks in the codecs caught this as a failure,
thanks to the node/tests suite already having one such case.
---
 node/bindnode/repr.go | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/node/bindnode/repr.go b/node/bindnode/repr.go
index 39473ea5..dcc3da54 100644
--- a/node/bindnode/repr.go
+++ b/node/bindnode/repr.go
@@ -234,6 +234,7 @@ func (w *_nodeRepr) MapIterator() datamodel.MapIterator {
 	switch stg := reprStrategy(w.schemaType).(type) {
 	case schema.StructRepresentation_Map:
 		itr := (*_node)(w).MapIterator().(*_structIterator)
+		// When we reach the last non-absent field, we should stop.
 		itr.reprEnd = int(w.lengthMinusTrailingAbsents())
 		return (*_structIteratorRepr)(itr)
 	case schema.UnionRepresentation_Keyed:
@@ -291,6 +292,17 @@ func (w *_listIteratorRepr) Done() bool {
 	return w.nextIndex >= w.val.Len()
 }
 
+func (w *_nodeRepr) lengthMinusAbsents() int64 {
+	fields := w.schemaType.(*schema.TypeStruct).Fields()
+	n := int64(len(fields))
+	for i, field := range fields {
+		if field.IsOptional() && w.val.Field(i).IsNil() {
+			n--
+		}
+	}
+	return n
+}
+
 func (w *_nodeRepr) lengthMinusTrailingAbsents() int64 {
 	fields := w.schemaType.(*schema.TypeStruct).Fields()
 	for i := len(fields) - 1; i >= 0; i-- {
@@ -307,9 +319,9 @@ func (w *_nodeRepr) Length() int64 {
 	case schema.StructRepresentation_Stringjoin:
 		return -1
 	case schema.StructRepresentation_Map:
-		return w.lengthMinusTrailingAbsents()
+		return w.lengthMinusAbsents()
 	case schema.StructRepresentation_Tuple:
-		return w.lengthMinusTrailingAbsents()
+		return w.lengthMinusAbsents()
 	case schema.UnionRepresentation_Keyed:
 		return (*_node)(w).Length()
 	case schema.UnionRepresentation_Kinded: