rlp: improve nil pointer handling (#20064)

* rlp: improve nil pointer handling

In both encoder and decoder, the rules for encoding nil pointers were a
bit hard to understand, and didn't leave much choice. Since RLP allows
two empty values (empty list, empty string), any protocol built on RLP
must choose either of these values to represent the null value in a
certain context.

This change adds choice in the form of two new struct tags, "nilString"
and "nilList". These can be used to specify how a nil pointer value is
encoded. The "nil" tag still exists, but its implementation is now
explicit and defines exactly how nil pointers are handled in a single
place.

Another important change in this commit is how nil pointers and the
Encoder interface interact. The EncodeRLP method was previously called
even on nil values, which was supposed to give users a choice of how
their value would be handled when nil. It turns out this is a stupid
idea. If you create a network protocol containing an object defined in
another package, it's better to be able to say that the object should be
a list or string when nil in the definition of the protocol message
rather than defining the encoding of nil on the object itself.

As of this commit, the encoding rules for pointers now take precedence
over the Encoder interface rule. I think the "nil" tag will work fine
for most cases. For special kinds of objects which are a struct in Go
but strings in RLP, code using the object can specify the desired
encoding of nil using the "nilString" and "nilList" tags.

* rlp: propagate struct field type errors

If a struct contained fields of undecodable type, the encoder and
decoder would panic instead of returning an error. Fix this by
propagating type errors in makeStruct{Writer,Decoder} and add a test.

rlp/typecache.go

@@ -35,22 +35,28 @@ type typeinfo struct {
 	writerErr  error // error from makeWriter
 }
 
-// represents struct tags
+// tags represents struct tags.
 type tags struct {
 	// rlp:"nil" controls whether empty input results in a nil pointer.
 	nilOK bool
+
+	// This controls whether nil pointers are encoded/decoded as empty strings
+	// or empty lists.
+	nilKind Kind
+
 	// rlp:"tail" controls whether this field swallows additional list
 	// elements. It can only be set for the last field, which must be
 	// of slice type.
 	tail bool
+
 	// rlp:"-" ignores fields.
 	ignored bool
 }
 
+// typekey is the key of a type in typeCache. It includes the struct tags because
+// they might generate a different decoder.
 type typekey struct {
 	reflect.Type
-	// the key must include the struct tags because they
-	// might generate a different decoder.
 	tags
 }
 
@@ -120,6 +126,25 @@ func structFields(typ reflect.Type) (fields []field, err error) {
 	return fields, nil
 }
 
+type structFieldError struct {
+	typ   reflect.Type
+	field int
+	err   error
+}
+
+func (e structFieldError) Error() string {
+	return fmt.Sprintf("%v (struct field %v.%s)", e.err, e.typ, e.typ.Field(e.field).Name)
+}
+
+type structTagError struct {
+	typ             reflect.Type
+	field, tag, err string
+}
+
+func (e structTagError) Error() string {
+	return fmt.Sprintf("rlp: invalid struct tag %q for %v.%s (%s)", e.tag, e.typ, e.field, e.err)
+}
+
 func parseStructTag(typ reflect.Type, fi, lastPublic int) (tags, error) {
 	f := typ.Field(fi)
 	var ts tags
@@ -128,15 +153,26 @@ func parseStructTag(typ reflect.Type, fi, lastPublic int) (tags, error) {
 		case "":
 		case "-":
 			ts.ignored = true
-		case "nil":
+		case "nil", "nilString", "nilList":
 			ts.nilOK = true
+			if f.Type.Kind() != reflect.Ptr {
+				return ts, structTagError{typ, f.Name, t, "field is not a pointer"}
+			}
+			switch t {
+			case "nil":
+				ts.nilKind = defaultNilKind(f.Type.Elem())
+			case "nilString":
+				ts.nilKind = String
+			case "nilList":
+				ts.nilKind = List
+			}
 		case "tail":
 			ts.tail = true
 			if fi != lastPublic {
-				return ts, fmt.Errorf(`rlp: invalid struct tag "tail" for %v.%s (must be on last field)`, typ, f.Name)
+				return ts, structTagError{typ, f.Name, t, "must be on last field"}
 			}
 			if f.Type.Kind() != reflect.Slice {
-				return ts, fmt.Errorf(`rlp: invalid struct tag "tail" for %v.%s (field type is not slice)`, typ, f.Name)
+				return ts, structTagError{typ, f.Name, t, "field type is not slice"}
 			}
 		default:
 			return ts, fmt.Errorf("rlp: unknown struct tag %q on %v.%s", t, typ, f.Name)
@@ -160,6 +196,20 @@ func (i *typeinfo) generate(typ reflect.Type, tags tags) {
 	i.writer, i.writerErr = makeWriter(typ, tags)
 }
 
+// defaultNilKind determines whether a nil pointer to typ encodes/decodes
+// as an empty string or empty list.
+func defaultNilKind(typ reflect.Type) Kind {
+	k := typ.Kind()
+	if isUint(k) || k == reflect.String || k == reflect.Bool || isByteArray(typ) {
+		return String
+	}
+	return List
+}
+
 func isUint(k reflect.Kind) bool {
 	return k >= reflect.Uint && k <= reflect.Uintptr
 }
+
+func isByteArray(typ reflect.Type) bool {
+	return (typ.Kind() == reflect.Slice || typ.Kind() == reflect.Array) && isByte(typ.Elem())
+}