proto: Prevent Any protos from being deserialized multiple times.

This avoids people specifying protos (inadvertently) as:

some_repeated_any {
  [type.googleapis.com/a/path/proto.Type] {
    blah: 1
  }
  [type.googleapis.com/a/path/proto.Type] {
    blah: 2
  }
}

when they meant:

some_repeated_any {
  [type.googleapis.com/a/path/proto.Type] {
    blah: 1
  }
}
some_repeated_any {
  [type.googleapis.com/a/path/proto.Type] {
    blah: 2
  }
}

proto/any_test.go

@@ -239,7 +239,7 @@ func TestUnmarshalGolden(t *testing.T) {
 	}
 }
 
-func TestMarsahlUnknownAny(t *testing.T) {
+func TestMarshalUnknownAny(t *testing.T) {
 	m := &pb.Message{
 		Anything: &anypb.Any{
 			TypeUrl: "foo",
@@ -260,13 +260,41 @@ func TestMarsahlUnknownAny(t *testing.T) {
 func TestAmbiguousAny(t *testing.T) {
 	pb := &anypb.Any{}
 	err := proto.UnmarshalText(`
-	[type.googleapis.com/proto3_proto.Nested]: <
-	  bunny: "Monty"
-	>
 	type_url: "ttt/proto3_proto.Nested"
+	value: "\n\x05Monty"
 	`, pb)
 	t.Logf("result: %v (error: %v)", expandedMarshaler.Text(pb), err)
 	if err != nil {
 		t.Errorf("failed to parse ambiguous Any message: %v", err)
 	}
 }
+
+func TestUnmarshalOverwriteAny(t *testing.T) {
+	pb := &anypb.Any{}
+	err := proto.UnmarshalText(`
+  [type.googleapis.com/a/path/proto3_proto.Nested]: <
+    bunny: "Monty"
+  >
+  [type.googleapis.com/a/path/proto3_proto.Nested]: <
+    bunny: "Rabbit of Caerbannog"
+  >
+	`, pb)
+	want := `line 7: Any message unpacked multiple times, or "type_url" already set`
+	if err.Error() != want {
+		t.Errorf("incorrect error.\nHave: %v\nWant: %v", err.Error(), want)
+	}
+}
+
+func TestUnmarshalAnyMixAndMatch(t *testing.T) {
+	pb := &anypb.Any{}
+	err := proto.UnmarshalText(`
+	value: "\n\x05Monty"
+  [type.googleapis.com/a/path/proto3_proto.Nested]: <
+    bunny: "Rabbit of Caerbannog"
+  >
+	`, pb)
+	want := `line 5: Any message unpacked multiple times, or "value" already set`
+	if err.Error() != want {
+		t.Errorf("incorrect error.\nHave: %v\nWant: %v", err.Error(), want)
+	}
+}

proto/text_parser.go

@@ -44,6 +44,9 @@ import (
 	"unicode/utf8"
 )
 
+// Error string emitted when deserializing Any and fields are already set
+const anyRepeatedlyUnpacked = "Any message unpacked multiple times, or %q already set"
+
 type ParseError struct {
 	Message string
 	Line    int // 1-based line number
@@ -508,8 +511,16 @@ func (p *textParser) readStruct(sv reflect.Value, terminator string) error {
 				if err != nil {
 					return p.errorf("failed to marshal message of type %q: %v", messageName, err)
 				}
+				if fieldSet["type_url"] {
+					return p.errorf(anyRepeatedlyUnpacked, "type_url")
+				}
+				if fieldSet["value"] {
+					return p.errorf(anyRepeatedlyUnpacked, "value")
+				}
 				sv.FieldByName("TypeUrl").SetString(extName)
 				sv.FieldByName("Value").SetBytes(b)
+				fieldSet["type_url"] = true
+				fieldSet["value"] = true
 				continue
 			}