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reflect/protoreflect: clarify Get semantics on unpopulated fields

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Clearly specify that Get on an unpopulated field:
* returns the default value for scalars
* returns a mutable (but empty) List for repeated fields
* returns a mutable (but empty) Map for map fields
* returns an invalid value for message fields

The difference in semantics between List+Maps and Messages is because
protobuf semantics provide no distinction between an unpopulated and empty list
or map. On the other hand, there is a semantic difference between an unpopulated
message and an empty message.

Default values for scalars is trivial to implement with FieldDescriptor.Default.

A mutable, but empty List and Map is easy to implement for known fields since
known fields are generated as a slice or map field in a struct.
Since struct fields are addressable, the implementation can just return a
reference to the slice or map.

Repeated, extension fields are a little more tricky since extension fields
are implemented under the hood as a map[FieldNumber]Extension.
Rather than allocating an empty list in KnownFields.Get upon first retrieval
(which presents a race), delegate the work to ExtensionFieldTypes.Register,
which must occur before any Get operation. Register is not a concurrent-safe
operation, so that is an excellent time to initilize empty lists.
The implementation of extensions will need to be careful that Clear on a repeated
field simply truncates it zero instead of deleting the object.

For unpopulated messages, we return an invalid value, instead of the prior
behavior of returning a typed nil-pointer to the Go type for the message.
The approach is problematic because it assumes that
1) all messages are always implemented on a pointer reciever
2) a typed nil-pointer is an appropriate "read-only, but empty" message
These assumptions are not true of all message types (e.g., dynamic messages).

Change-Id: Ie96e6744c890308d9de738b6cf01d3b19e7e7c6a
Reviewed-on: https://go-review.googlesource.com/c/150319
Reviewed-by: Damien Neil <dneil@google.com>
This commit is contained in:
Joe Tsai 2018-11-19 14:26:06 -08:00 committed by Joe Tsai
parent 492a476312
commit f6d4a4215f
7 changed files with 47 additions and 41 deletions
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28
internal/impl/legacy_extension.go
View File

@ -59,16 +59,9 @@ func (p legacyExtensionFields) Get(n pref.FieldNumber) pref.Value {
}
t := legacyExtensionTypeOf(x.desc)
if x.val == nil {
if t.Cardinality() == pref.Repeated {
// TODO: What is the zero value for Lists?
// TODO: This logic is racy.
v := t.ValueOf(t.New())
x.val = t.InterfaceOf(v)
p.x.Set(n, x)
return v
}
// NOTE: x.val is never nil for Lists since they are always populated
// during ExtensionFieldTypes.Register.
if t.Kind() == pref.MessageKind || t.Kind() == pref.GroupKind {
// TODO: What is the zero value for Messages?
return pref.Value{}
}
return t.Default()
@ -91,6 +84,11 @@ func (p legacyExtensionFields) Clear(n pref.FieldNumber) {
if x.desc == nil {
return
}
t := legacyExtensionTypeOf(x.desc)
if t.Cardinality() == pref.Repeated {
t.ValueOf(x.val).List().Truncate(0)
return
}
x.val = nil
p.x.Set(n, x)
}
@ -146,6 +144,11 @@ func (p legacyExtensionTypes) Register(t pref.ExtensionType) {
panic("extension descriptor already registered")
}
x.desc = legacyExtensionDescOf(t, p.mi.goType)
if t.Cardinality() == pref.Repeated {
// If the field is repeated, initialize the entry with an empty list
// so that future Get operations can return a mutable and concrete list.
x.val = t.InterfaceOf(t.ValueOf(t.New()))
}
p.x.Set(t.Number(), x)
}
@ -154,6 +157,13 @@ func (p legacyExtensionTypes) Remove(t pref.ExtensionType) {
return
}
x := p.x.Get(t.Number())
if t.Cardinality() == pref.Repeated {
// Treat an empty repeated field as unpopulated.
v := reflect.ValueOf(x.val)
if x.val == nil || v.IsNil() || v.Elem().Len() == 0 {
x.val = nil
}
}
if x.val != nil {
panic("value for extension descriptor still populated")
}

16
internal/impl/legacy_test.go
View File

@ -831,11 +831,8 @@ func TestLegactExtensions(t *testing.T) {
}
for i, xt := range extensions {
var got interface{}
v := fs.Get(xt.Number())
if xt.Cardinality() != pref.Repeated && xt.Kind() == pref.MessageKind {
got = v.Interface()
} else {
got = xt.InterfaceOf(v) // TODO: Simplify this if InterfaceOf allows nil
if v := fs.Get(xt.Number()); v.IsValid() {
got = xt.InterfaceOf(v)
}
want := defaultValues[i]
if diff := cmp.Diff(want, got, opts); diff != "" {
@ -924,9 +921,16 @@ func TestLegactExtensions(t *testing.T) {
}
// Clear the field for all extension types.
for _, xt := range extensions {
for _, xt := range extensions[:len(extensions)/2] {
fs.Clear(xt.Number())
}
for i, xt := range extensions[len(extensions)/2:] {
if i%2 == 0 {
fs.Clear(xt.Number())
} else {
fs.Get(xt.Number()).List().Truncate(0)
}
}
if n := fs.Len(); n != 0 {
t.Errorf("KnownFields.Len() = %v, want 0", n)
}

19
internal/impl/message_field.go
View File

@ -61,9 +61,7 @@ func fieldInfoForOneof(fd pref.FieldDescriptor, fs reflect.StructField, ot refle
rv := p.apply(fieldOffset).asType(fs.Type).Elem()
if rv.IsNil() || rv.Elem().Type().Elem() != ot {
if fd.Kind() == pref.MessageKind || fd.Kind() == pref.GroupKind {
// TODO: Should this return an invalid protoreflect.Value?
rv = reflect.Zero(ot.Field(0).Type)
return conv.PBValueOf(rv)
return pref.Value{}
}
return fd.Default()
}
@ -96,7 +94,7 @@ func fieldInfoForOneof(fd pref.FieldDescriptor, fs reflect.StructField, ot refle
pv := pref.ValueOf(conv.MessageType.New().ProtoReflect())
rv.Set(conv.GoValueOf(pv))
}
return rv.Interface().(pref.Message)
return conv.PBValueOf(rv).Message()
},
}
}
@ -253,19 +251,18 @@ func fieldInfoForMessage(fd pref.FieldDescriptor, fs reflect.StructField) fieldI
return !rv.IsNil()
},
get: func(p pointer) pref.Value {
// TODO: If rv.IsNil(), should this return a typed-nil pointer or
// an invalid protoreflect.Value?
//
// Returning a typed nil pointer assumes that such values
// are valid for all possible custom message types,
// which may not be case for dynamic messages.
rv := p.apply(fieldOffset).asType(fs.Type).Elem()
if rv.IsNil() {
return pref.Value{}
}
return conv.PBValueOf(rv)
},
set: func(p pointer, v pref.Value) {
// TODO: Similarly, is it valid to set this to a typed nil pointer?
rv := p.apply(fieldOffset).asType(fs.Type).Elem()
rv.Set(conv.GoValueOf(v))
if rv.IsNil() {
panic("invalid nil pointer")
}
},
clear: func(p pointer) {
rv := p.apply(fieldOffset).asType(fs.Type).Elem()

3
internal/impl/message_test.go
View File

@ -1017,7 +1017,6 @@ func (*EnumMessages_OneofM2) isEnumMessages_Union() {}
func (*EnumMessages_OneofM3) isEnumMessages_Union() {}
func TestEnumMessages(t *testing.T) {
// TODO: Test behavior of Get on unpopulated message.
wantL := MessageOf(&proto2_20180125.Message{OptionalFloat: protoV1.Float32(math.E)})
wantM := &EnumMessages{EnumP2: EnumProto2(1234).Enum()}
wantM2a := &ScalarProto2{Float32: protoV1.Float32(math.Pi)}
@ -1033,7 +1032,7 @@ func TestEnumMessages(t *testing.T) {
testMessage(t, nil, &EnumMessages{}, messageOps{
hasFields{1: false, 2: false, 3: false, 4: false, 5: false, 6: false, 7: false, 8: false, 9: false, 10: false, 11: false, 12: false},
getFields{1: VE(0xbeef), 2: VE(1), 9: VE(0xbeef), 10: VE(1)},
getFields{1: VE(0xbeef), 2: VE(1), 3: V(nil), 4: V(nil), 9: VE(0xbeef), 10: VE(1)},
// Test singular enums.
setFields{1: VE(0xdead), 2: VE(0)},

10
internal/value/list.go
View File

@ -35,19 +35,13 @@ func (ls listReflect) Append(v pref.Value) {
}
func (ls listReflect) Mutable(i int) pref.Mutable {
// Mutable is only valid for messages and panics for other kinds.
rv := ls.v.Index(i)
if rv.IsNil() {
// TODO: Is checking for nil proper behavior for custom messages?
pv := pref.ValueOf(ls.conv.MessageType.New().ProtoReflect())
rv.Set(ls.conv.GoValueOf(pv))
}
return rv.Interface().(pref.Message)
return ls.conv.PBValueOf(ls.v.Index(i)).Message()
}
func (ls listReflect) MutableAppend() pref.Mutable {
// MutableAppend is only valid for messages and panics for other kinds.
pv := pref.ValueOf(ls.conv.MessageType.New().ProtoReflect())
ls.v.Set(reflect.Append(ls.v, ls.conv.GoValueOf(pv)))
return ls.v.Index(ls.Len() - 1).Interface().(pref.Message)
return pv.Message()
}
func (ls listReflect) Truncate(i int) {
ls.v.Set(ls.v.Slice(0, i))

5
internal/value/map.go
View File

@ -57,13 +57,12 @@ func (ms mapReflect) Mutable(k pref.MapKey) pref.Mutable {
}
rk := ms.keyConv.GoValueOf(k.Value())
rv := ms.v.MapIndex(rk)
if !rv.IsValid() || rv.IsNil() {
// TODO: Is checking for nil proper behavior for custom messages?
if !rv.IsValid() {
pv := pref.ValueOf(ms.valConv.MessageType.New().ProtoReflect())
rv = ms.valConv.GoValueOf(pv)
ms.v.SetMapIndex(rk, rv)
}
return rv.Interface().(pref.Message)
return ms.valConv.PBValueOf(rv).Message()
}
func (ms mapReflect) Range(f func(pref.MapKey, pref.Value) bool) {
for _, k := range ms.v.MapKeys() {

7
reflect/protoreflect/value.go
View File

@ -69,8 +69,11 @@ type KnownFields interface {
Has(FieldNumber) bool
// Get retrieves the value for a field with the given field number.
// It returns the default value for unpopulated fields.
// It returns an invalid value for unknown fields.
// If the field is unpopulated, it returns the default value for scalars,
// a mutable empty List for empty repeated fields, a mutable empty Map for
// empty map fields, and an invalid value for message fields.
// If the field is unknown (does not appear in MessageDescriptor.Fields
// or ExtensionFieldTypes), it returns an invalid value.
Get(FieldNumber) Value
// Set stores the value for a field with the given field number.