reflect/protoreflect: clarify Get semantics on unpopulated fields
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>
diff --git a/internal/impl/legacy_extension.go b/internal/impl/legacy_extension.go
index d54da34..a7a28cf 100644
--- a/internal/impl/legacy_extension.go
+++ b/internal/impl/legacy_extension.go
@@ -59,16 +59,9 @@
}
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 @@
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 @@
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 @@
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")
}
diff --git a/internal/impl/legacy_test.go b/internal/impl/legacy_test.go
index 4df07a3..2333fc5 100644
--- a/internal/impl/legacy_test.go
+++ b/internal/impl/legacy_test.go
@@ -831,11 +831,8 @@
}
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 @@
}
// 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)
}
diff --git a/internal/impl/message_field.go b/internal/impl/message_field.go
index 2686a97..75cf867 100644
--- a/internal/impl/message_field.go
+++ b/internal/impl/message_field.go
@@ -61,9 +61,7 @@
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 @@
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 @@
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()
diff --git a/internal/impl/message_test.go b/internal/impl/message_test.go
index b3393d4..999bdd6 100644
--- a/internal/impl/message_test.go
+++ b/internal/impl/message_test.go
@@ -1017,7 +1017,6 @@
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 @@
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)},
diff --git a/internal/value/list.go b/internal/value/list.go
index 2eec007..ba82b30 100644
--- a/internal/value/list.go
+++ b/internal/value/list.go
@@ -35,19 +35,13 @@
}
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))
diff --git a/internal/value/map.go b/internal/value/map.go
index d2ef659..db9656e 100644
--- a/internal/value/map.go
+++ b/internal/value/map.go
@@ -57,13 +57,12 @@
}
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() {
diff --git a/reflect/protoreflect/value.go b/reflect/protoreflect/value.go
index 93ff1ff..66cb5da 100644
--- a/reflect/protoreflect/value.go
+++ b/reflect/protoreflect/value.go
@@ -69,8 +69,11 @@
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.
