reflect/protoreflect: add Has and Clear to KnownFields and Map

Change the API to use explicit Has/Clear methods instead of relying on the
"invalid" form of Value to represent nullability.

There are several reasons for this change:

* Much of the ecosystem around protobufs do not care about nullability alone.
For example, for the encoder to determine whether to emit a field:
it needs to first check if a field is nulled, and if not, it still needs to go
through a series of type-assertion to check whether the value is the zero value.
It is much easier to be able to just call Has.

* It enables representing the default value as part of the value API, rather
than only as part of the descriptor API.

* The C++ API also uses explicit has and clear methods. However, we differ from
them by defining Has for proto3 scalars (while C++ panics instead).

For internal consistency, we also use a Has/Clear API for Maps.

Change-Id: I30eda482c959d3e1454d72d9fc761c761ace27a6
Reviewed-on: https://go-review.googlesource.com/134998
Reviewed-by: Damien Neil <dneil@google.com>

reflect/protoreflect/type.go

@@ -141,11 +141,11 @@ type DescriptorOptions interface {
 
 	// Get returns the ith field. It panics if out of bounds.
 	// The FieldDescriptor is guaranteed to be non-nil, while the Value
-	// may be Null if the proto file did not specify this option.
+	// may be invalid if the proto file did not specify this option.
 	Get(i int) (FieldDescriptor, Value)
 
 	// ByName looks a field up by full name and
-	// returns (nil, Null) if not found.
+	// returns (nil, Value{}) if not found.
 	//
 	// As a special-case, non-extension fields in the options type can be
 	// directly accessed by the field name alone (e.g., "map_entry" may be used
@@ -153,7 +153,7 @@ type DescriptorOptions interface {
 	ByName(s FullName) (FieldDescriptor, Value)
 
 	// ByNumber looks a field up by the field number and
-	// returns (nil, Null) if not found.
+	// returns (nil, Value{}) if not found.
 	ByNumber(n FieldNumber) (FieldDescriptor, Value)
 
 	doNotImplement

reflect/protoreflect/value.go

@@ -20,7 +20,7 @@ type Enum interface {
 	Number() EnumNumber
 }
 
-// Message is a reflection interface to a concrete message value,
+// Message is a reflective interface for a concrete message value,
 // which provides type information and getters/setters for individual fields.
 //
 // Concrete types may implement interfaces defined in proto/protoiface,
@@ -51,22 +51,12 @@ type Message interface {
 // underlying repeated element type is determined by the FieldDescriptor.Kind.
 // See Value for a list of Go types associated with each Kind.
 //
-// Some fields have the property of nullability where it is possible to
-// distinguish between the zero value of a field and whether the field was
-// explicitly populated with the zero value. Only scalars in proto2,
-// members of a oneof field, and singular messages are nullable.
-// In the presence of unset fields, KnownFields.Get does not return defaults;
-// use the corresponding FieldDescriptor.DefaultValue for that information.
-//
 // Field extensions are handled as known fields once the extension type has been
 // registered with KnownFields.ExtensionTypes.
 //
-// List, Len, Get, Range, and ExtensionTypes are safe for concurrent access.
+// List, Len, Has, Get, Range, and ExtensionTypes are safe for concurrent use.
 type KnownFields interface {
-	// List returns a new, unordered list of all fields that are populated.
+	// List returns a new, unordered list of all populated fields.
-	// A nullable field is populated only if explicitly set.
-	// A scalar field in proto3 is populated if it contains a non-zero value.
-	// A repeated field is populated only if it is non-empty.
 	List() []FieldNumber
 
 	// Len reports the number of fields that are populated.
@@ -74,22 +64,26 @@ type KnownFields interface {
 	// Invariant: f.Len() == len(f.List())
 	Len() int
 
-	// TODO: Should Get return FieldDescriptor.Default if unpopulated instead of
+	// Has reports whether a field is populated.
-	// returning the Null variable? If so, we loose the ability to represent
+	//
-	// nullability in Get and Set calls and also need to add Has and Clear.
+	// Some fields have the property of nullability where it is possible to
+	// distinguish between the default value of a field and whether the field
+	// was explicitly populated with the default value. Only scalars in proto2,
+	// member fields of a oneof, and singular messages are nullable.
+	//
+	// A nullable field is populated only if explicitly set.
+	// A scalar field in proto3 is populated if it contains a non-zero value.
+	// A repeated field is populated only if it is non-empty.
+	Has(FieldNumber) bool
 
-	// Get retrieves the value for field with the given field number.
+	// Get retrieves the value for a field with the given field number.
-	// It returns Null for non-existent or nulled fields.
+	// It returns the default value for unpopulated fields.
+	// It returns an invalid value for unknown fields.
 	Get(FieldNumber) Value
 
-	// TODO: Document memory aliasing behavior when a field is cleared?
-	// For example, if Mutable is called later, can it reuse memory?
-
 	// Set stores the value for a field with the given field number.
 	// Setting a field belonging to a oneof implicitly clears any other field
 	// that may be currently set by the same oneof.
-	// Null may be used to explicitly clear a field containing a proto2 scalar,
-	// a member of oneof, or a singular message.
 	//
 	// When setting a composite type, it is unspecified whether the set
 	// value aliases the source's memory in any way.
@@ -98,6 +92,15 @@ type KnownFields interface {
 	// in MessageDescriptor.Fields or an extension field in ExtensionTypes.
 	Set(FieldNumber, Value)
 
+	// TODO: Document memory aliasing behavior when a field is cleared?
+	// For example, if Mutable is called later, can it reuse memory?
+
+	// Clear clears the field such that a subsequent call to Has reports false.
+	//
+	// It panics if the field number does not correspond with a known field
+	// in MessageDescriptor.Fields or an extension field in ExtensionTypes.
+	Clear(FieldNumber)
+
 	// Mutable returns a reference value for a field with a given field number.
 	// If the field is unset, Mutable implicitly initializes the field with
 	// a zero value instance of the Go type for that field.
@@ -105,7 +108,7 @@ type KnownFields interface {
 	// The returned Mutable reference is never nil, and is only valid until the
 	// next Set or Mutable call.
 	//
-	// It panics if FieldNumber does not correspond with a known field
+	// It panics if the field number does not correspond with a known field
 	// in MessageDescriptor.Fields or an extension field in ExtensionTypes.
 	Mutable(FieldNumber) Mutable
 
@@ -124,7 +127,7 @@ type KnownFields interface {
 // However, the relative ordering of fields with different field numbers
 // is undefined.
 //
-// List, Len, Get, and Range are safe for concurrent access.
+// List, Len, Get, and Range are safe for concurrent use.
 type UnknownFields interface {
 	// List returns a new, unordered list of all fields that are set.
 	List() []FieldNumber
@@ -184,7 +187,7 @@ func (b RawFields) IsValid() bool {
 // ExtensionFieldTypes are the extension field types that this message instance
 // has been extended with.
 //
-// List, Len, Get, and Range are safe for concurrent access.
+// List, Len, Get, and Range are safe for concurrent use.
 type ExtensionFieldTypes interface {
 	// List returns a new, unordered list of known extension field types.
 	List() []ExtensionType
@@ -219,12 +222,12 @@ type ExtensionFieldTypes interface {
 	Range(f func(ExtensionType) bool)
 }
 
-// Vector is an ordered list. Every element is always considered populated
+// Vector is an ordered list. Every element is considered populated
-// (i.e., Get never provides and Set never accepts Null).
+// (i.e., Get never provides and Set never accepts invalid Values).
 // The element Value type is determined by the associated FieldDescriptor.Kind
 // and cannot be a Map or Vector.
 //
-// Len and Get are safe for concurrent access.
+// Len and Get are safe for concurrent use.
 type Vector interface {
 	// Len reports the number of entries in the Vector.
 	// Get, Set, Mutable, and Truncate panic with out of bound indexes.
@@ -237,16 +240,12 @@ type Vector interface {
 	//
 	// When setting a composite type, it is unspecified whether the set
 	// value aliases the source's memory in any way.
-	//
-	// It panics if the value is Null.
 	Set(int, Value)
 
 	// Append appends the provided value to the end of the vector.
 	//
 	// When appending a composite type, it is unspecified whether the appended
 	// value aliases the source's memory in any way.
-	//
-	// It panics if the value is Null.
 	Append(Value)
 
 	// Mutable returns a Mutable reference for the element with a given index.
@@ -274,7 +273,7 @@ type Vector interface {
 // is considered populated. The entry Value type is determined by the associated
 // FieldDescripto.Kind and cannot be a Map or Vector.
 //
-// List, Len, Get, and Range are safe for concurrent access.
+// List, Len, Has, Get, and Range are safe for concurrent use.
 type Map interface {
 	// List returns an unordered list of keys for all entries in the map.
 	List() []MapKey
@@ -284,7 +283,11 @@ type Map interface {
 	// Invariant: f.Len() == len(f.List())
 	Len() int
 
+	// Has reports whether an entry with the given key is in the map.
+	Has(MapKey) bool
+
 	// Get retrieves the value for an entry with the given key.
+	// It returns an invalid value for non-existent entries.
 	Get(MapKey) Value
 
 	// Set stores the value for an entry with the given key.
@@ -292,9 +295,12 @@ type Map interface {
 	// When setting a composite type, it is unspecified whether the set
 	// value aliases the source's memory in any way.
 	//
-	// It panics if either the key or value are Null.
+	// It panics if either the key or value are invalid.
 	Set(MapKey, Value)
 
+	// Clear clears the entry associated with they given key.
+	Clear(MapKey)
+
 	// Mutable returns a Mutable reference for the element with a given key,
 	// allocating a new entry if necessary.
 	//
@@ -303,6 +309,7 @@ type Map interface {
 	Mutable(MapKey) Mutable
 
 	// Range calls f sequentially for each key and value present in the map.
+	// The Value provided is guaranteed to be valid.
 	// If f returns false, range stops the iteration.
 	Range(f func(MapKey, Value) bool)
 

reflect/protoreflect/value_test.go

@@ -20,7 +20,6 @@ func TestValue(t *testing.T) {
 		in   Value
 		want interface{}
 	}{
-		{in: Null},
 		{in: Value{}},
 		{in: ValueOf(nil)},
 		{in: ValueOf(true), want: true},
@@ -43,8 +42,8 @@ func TestValue(t *testing.T) {
 			t.Errorf("Value(%v).Interface() = %v, want %v", tt.in, got, tt.want)
 		}
 
-		if got := tt.in.IsNull(); got != (tt.want == nil) {
+		if got := tt.in.IsValid(); got != (tt.want != nil) {
-			t.Errorf("Value(%v).IsNull() = %v, want %v", tt.in, got, tt.want == nil)
+			t.Errorf("Value(%v).IsValid() = %v, want %v", tt.in, got, tt.want != nil)
 		}
 		switch want := tt.want.(type) {
 		case int32:

reflect/protoreflect/value_union.go

@@ -35,7 +35,7 @@ import (
 //
 // Multiple protobuf Kinds may be represented by a single Go type if the type
 // can losslessly represent the information for the proto kind. For example,
-// Int64Kind, Sint64Kind, and Sfixed64Kind all represent int64,
+// Int64Kind, Sint64Kind, and Sfixed64Kind are all represented by int64,
 // but use different integer encoding methods.
 //
 // The Vector or Map types are used if the FieldDescriptor.Cardinality of the
@@ -47,14 +47,6 @@ import (
 // retrieve an int64 from a string.
 type Value value
 
-// Null is an unpopulated Value.
-//
-// Since Value is incomparable, call Value.IsNull instead to test whether
-// a Value is empty.
-//
-// It is equivalent to Value{} or ValueOf(nil).
-var Null Value
-
 // The protoreflect API uses a custom Value union type instead of interface{}
 // to keep the future open for performance optimizations. Using an interface{}
 // always incurs an allocation for primitives (e.g., int64) since it needs to
@@ -110,9 +102,9 @@ func ValueOf(v interface{}) Value {
 	}
 }
 
-// IsNull reports whether v is empty (has no value).
+// IsValid reports whether v is populated with a value.
-func (v Value) IsNull() bool {
+func (v Value) IsValid() bool {
-	return v.typ == nilType
+	return v.typ != nilType
 }
 
 // Interface returns v as an interface{}.
@@ -282,9 +274,9 @@ func (v Value) MapKey() MapKey {
 // converting a Value to a MapKey with an invalid type panics.
 type MapKey value
 
-// IsNull reports whether v is empty (has no value).
+// IsValid reports whether k is populated with a value.
-func (k MapKey) IsNull() bool {
+func (k MapKey) IsValid() bool {
-	return Value(k).IsNull()
+	return Value(k).IsValid()
 }
 
 // Interface returns k as an interface{}.

reflect/prototype/protofile_desc.go

@@ -386,5 +386,5 @@ func parseDefault(s string, k protoreflect.Kind) (protoreflect.Value, error) {
 			return protoreflect.ValueOf([]byte(s)), nil
 		}
 	}
-	return protoreflect.Null, errors.New("invalid default value for %v: %q", k, s)
+	return protoreflect.Value{}, errors.New("invalid default value for %v: %q", k, s)
 }

reflect/prototype/protofile_type.go

@@ -439,7 +439,7 @@ var (
 func (p *defaultValue) lazyInit(t pref.FieldDescriptor, v pref.Value) pref.Value {
 	p.once.Do(func() {
 		p.val = v
-		if !v.IsNull() {
+		if v.IsValid() {
 			switch t.Kind() {
 			case pref.EnumKind:
 				// Treat a string value as an identifier referencing some enum