Demonstrate gRPC gateway behavior with nullable values in maps

Using grpc-gateway v2.7.2.

This is a simple service just echoing a map<string, google.protobuf.StringValue>.

Generated Go code is a map[string]*wrapperspb.StringValue, so the value could be nil.

In grpc-gateway version 2.0.0 or greater, passing null value in a JSON fails and does not set *wrapperspb.StringValue to nil :

In grpc-gateway version 1.16.0, passing null value in a JSON set *wrapperspb.StringValue to empty string "".

Reproduce issue

Checkout the right branch associated with the version of grpc-gateway (git checkout) :

• master (v2.7.2)
• v2.0.1
• v1.16.0

Run server and run gateway :

make server
make gateway

Send a HTTP request with body :

{
  "aMap": {
    "someString": "test",
    "emptyString": "",
    "nullableString": null
  }
}

On version 2.7.2 and 2.0.1 :

$ make echo
{"code":3, "message":"proto: (line 1:70): invalid value for string type: null", "details":[]}

On version 1.16.0 :

$ make echo
{"aMap": {"someString": "test", "emptyString": "", "nullableString": ""}}