Decoder: disallow modification of existing table (#704)

Fixes #703

internal/ast/ast.go

@@ -20,8 +20,8 @@ type Iterator struct {
 	node    *Node
 }
 
-// Next moves the iterator forward and returns true if points to a node, false
-// otherwise.
+// Next moves the iterator forward and returns true if points to a
+// node, false otherwise.
 func (c *Iterator) Next() bool {
 	if !c.started {
 		c.started = true
@@ -31,8 +31,8 @@ func (c *Iterator) Next() bool {
 	return c.node.Valid()
 }
 
-// IsLast returns true if the current node of the iterator is the last one.
-// Subsequent call to Next() will return false.
+// IsLast returns true if the current node of the iterator is the last
+// one.  Subsequent call to Next() will return false.
 func (c *Iterator) IsLast() bool {
 	return c.node.next == 0
 }
@@ -62,20 +62,20 @@ func (r *Root) at(idx Reference) *Node {
 	return &r.nodes[idx]
 }
 
-// Arrays have one child per element in the array.
-// InlineTables have one child per key-value pair in the table.
-// KeyValues have at least two children. The first one is the value. The
-// rest make a potentially dotted key.
-// Table and Array table have one child per element of the key they
-// represent (same as KeyValue, but without the last node being the value).
-// children []Node
+// Arrays have one child per element in the array.  InlineTables have
+// one child per key-value pair in the table.  KeyValues have at least
+// two children. The first one is the value. The rest make a
+// potentially dotted key.  Table and Array table have one child per
+// element of the key they represent (same as KeyValue, but without
+// the last node being the value).
 type Node struct {
 	Kind Kind
 	Raw  Range  // Raw bytes from the input.
-	Data []byte // Node value (could be either allocated or referencing the input).
+	Data []byte // Node value (either allocated or referencing the input).
 
-	// References to other nodes, as offsets in the backing array from this
-	// node. References can go backward, so those can be negative.
+	// References to other nodes, as offsets in the backing array
+	// from this node. References can go backward, so those can be
+	// negative.
 	next  int // 0 if last element
 	child int // 0 if no child
 }
@@ -85,8 +85,8 @@ type Range struct {
 	Length uint32
 }
 
-// Next returns a copy of the next node, or an invalid Node if there is no
-// next node.
+// Next returns a copy of the next node, or an invalid Node if there
+// is no next node.
 func (n *Node) Next() *Node {
 	if n.next == 0 {
 		return nil
@@ -96,9 +96,9 @@ func (n *Node) Next() *Node {
 	return (*Node)(danger.Stride(ptr, size, n.next))
 }
 
-// Child returns a copy of the first child node of this node. Other children
-// can be accessed calling Next on the first child.
-// Returns an invalid Node if there is none.
+// Child returns a copy of the first child node of this node. Other
+// children can be accessed calling Next on the first child.  Returns
+// an invalid Node if there is none.
 func (n *Node) Child() *Node {
 	if n.child == 0 {
 		return nil
@@ -113,10 +113,9 @@ func (n *Node) Valid() bool {
 	return n != nil
 }
 
-// Key returns the child nodes making the Key on a supported node. Panics
-// otherwise.
-// They are guaranteed to be all be of the Kind Key. A simple key would return
-// just one element.
+// Key returns the child nodes making the Key on a supported
+// node. Panics otherwise.  They are guaranteed to be all be of the
+// Kind Key. A simple key would return just one element.
 func (n *Node) Key() Iterator {
 	switch n.Kind {
 	case KeyValue:
@@ -133,8 +132,8 @@ func (n *Node) Key() Iterator {
 }
 
 // Value returns a pointer to the value node of a KeyValue.
-// Guaranteed to be non-nil.
-// Panics if not called on a KeyValue node, or if the Children are malformed.
+// Guaranteed to be non-nil.  Panics if not called on a KeyValue node,
+// or if the Children are malformed.
 func (n *Node) Value() *Node {
 	return n.Child()
 }