all: consistently treat nil message interface as an empty read-only message

To assist users in migrating from github.com/golang/protobuf
to google.golang.org/protobuf, make it such that functiionality like
proto.Marshal doesn't panic on nil interfaces.

Similar to how the new implementation treats a typed nil message
as an empty message, we treat a nil interface as being equivalent
to an "untyped" empty message.

Change-Id: Ic037f386f855b122f732b34d370e524b7c0d76f1
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/228837
Reviewed-by: Damien Neil <dneil@google.com>

encoding/protojson/encode.go

@@ -116,6 +116,12 @@ func (o MarshalOptions) Marshal(m proto.Message) ([]byte, error) {
 		return nil, err
 	}
 
+	// Treat nil message interface as an empty message,
+	// in which case the output in an empty JSON object.
+	if m == nil {
+		return []byte("{}"), nil
+	}
+
 	enc := encoder{internalEnc, o}
 	if err := enc.marshalMessage(m.ProtoReflect()); err != nil {
 		return nil, err

encoding/prototext/encode.go

@@ -106,6 +106,12 @@ func (o MarshalOptions) Marshal(m proto.Message) ([]byte, error) {
 		return nil, err
 	}
 
+	// Treat nil message interface as an empty message,
+	// in which case there is nothing to output.
+	if m == nil {
+		return []byte{}, nil
+	}
+
 	enc := encoder{internalEnc, o}
 	err = enc.marshalMessage(m.ProtoReflect(), false)
 	if err != nil {

proto/checkinit.go

@@ -12,6 +12,12 @@ import (
 
 // CheckInitialized returns an error if any required fields in m are not set.
 func CheckInitialized(m Message) error {
+	// Treat a nil message interface as an "untyped" empty message,
+	// which we assume to have no required fields.
+	if m == nil {
+		return nil
+	}
+
 	return checkInitialized(m.ProtoReflect())
 }
 

proto/encode.go

@@ -74,12 +74,22 @@ type MarshalOptions struct {
 
 // Marshal returns the wire-format encoding of m.
 func Marshal(m Message) ([]byte, error) {
+	// Treat nil message interface as an empty message; nothing to output.
+	if m == nil {
+		return nil, nil
+	}
+
 	out, err := MarshalOptions{}.marshal(nil, m.ProtoReflect())
 	return out.Buf, err
 }
 
 // Marshal returns the wire-format encoding of m.
 func (o MarshalOptions) Marshal(m Message) ([]byte, error) {
+	// Treat nil message interface as an empty message; nothing to output.
+	if m == nil {
+		return nil, nil
+	}
+
 	out, err := o.marshal(nil, m.ProtoReflect())
 	return out.Buf, err
 }
@@ -87,6 +97,11 @@ func (o MarshalOptions) Marshal(m Message) ([]byte, error) {
 // MarshalAppend appends the wire-format encoding of m to b,
 // returning the result.
 func (o MarshalOptions) MarshalAppend(b []byte, m Message) ([]byte, error) {
+	// Treat nil message interface as an empty message; nothing to append.
+	if m == nil {
+		return b, nil
+	}
+
 	out, err := o.marshal(b, m.ProtoReflect())
 	return out.Buf, err
 }

proto/merge.go

@@ -21,6 +21,9 @@ import (
 // It is semantically equivalent to unmarshaling the encoded form of src
 // into dst with the UnmarshalOptions.Merge option specified.
 func Merge(dst, src Message) {
+	// TODO: Should nil src be treated as semantically equivalent to a
+	// untyped, read-only, empty message? What about a nil dst?
+
 	dstMsg, srcMsg := dst.ProtoReflect(), src.ProtoReflect()
 	if dstMsg.Descriptor() != srcMsg.Descriptor() {
 		panic("descriptor mismatch")

proto/nil_test.go

@@ -27,14 +27,12 @@ func TestNil(t *testing.T) {
 	}{{
 		label: "Size",
 		test:  func() { proto.Size(nil) },
-		panic: true,
 	}, {
 		label: "Size",
 		test:  func() { proto.Size(nilMsg) },
 	}, {
 		label: "Marshal",
 		test:  func() { proto.Marshal(nil) },
-		panic: true,
 	}, {
 		label: "Marshal",
 		test:  func() { proto.Marshal(nilMsg) },

proto/size.go

@@ -18,6 +18,11 @@ func Size(m Message) int {
 
 // Size returns the size in bytes of the wire-format encoding of m.
 func (o MarshalOptions) Size(m Message) int {
+	// Treat a nil message interface as an empty message; nothing to output.
+	if m == nil {
+		return 0
+	}
+
 	return sizeMessage(m.ProtoReflect())
 }