Panic with query parameters

[fische]

Hi!
It appears that the runtime panics when you try to set a subfield of a non-object field in the query parameters.

For example, we will take the following protobuf:

syntax = "proto3";

package pb;

import "google/api/annotations.proto";

service Service {
	rpc Method (Message) returns (Message) {
		option (google.api.http) = {
			get: "/method"
		};
	}
}

message Message {
	string msg = 1;
}

With this protobuf, the runtime will panic, if you try to set a subfield (let's say sth) of msg in the query parameters, like this:

curl http://$HOST:$PORT/method?msg.sth=test

Here is the stacktrace from the panic:

2017/03/13 14:35:47 http: panic serving [::1]:49154: proto: type must have kind struct
goroutine 50 [running]:
net/http.(*conn).serve.func1(0xc4201f0000)
	/usr/lib/go/src/net/http/server.go:1491 +0x12a
panic(0x79cb80, 0xc4201fe320)
	/usr/lib/go/src/runtime/panic.go:458 +0x243
github.com/golang/protobuf/proto.GetProperties(0xa0c760, 0x79cb80, 0x198)
	/home/fische_m/Project/Go/src/github.com/golang/protobuf/proto/properties.go:643 +0x17a
github.com/grpc-ecosystem/grpc-gateway/runtime.fieldByProtoName(0x79cb80, 0xc4201fe2c0, 0x198, 0xc4201ec0d0, 0x3, 0x79cb80, 0xc4201fe2c0, 0x198, 0xc42021a000)
	/home/fische_m/Project/Go/src/github.com/grpc-ecosystem/grpc-gateway/runtime/query.go:93 +0x5e
github.com/grpc-ecosystem/grpc-gateway/runtime.populateFieldValueFromPath(0xa06460, 0xc4201fe2c0, 0xc4201fc2c0, 0x2, 0x2, 0xc4201fe2e0, 0x1, 0x1, 0xc4201ec0cc, 0x7)
	/home/fische_m/Project/Go/src/github.com/grpc-ecosystem/grpc-gateway/runtime/query.go:51 +0x12e
github.com/grpc-ecosystem/grpc-gateway/runtime.PopulateQueryParameters(0xa06460, 0xc4201fe2c0, 0xc4201ec2d0, 0xa29780, 0x79cb80, 0xc4201fe1c0)
	/home/fische_m/Project/Go/src/github.com/grpc-ecosystem/grpc-gateway/runtime/query.go:24 +0x1c3
_/home/fische_m/local/test/gateway/pb.request_Service_Method_0(0x7ff3ca2b9028, 0xc4201ec2a0, 0xa08580, 0xa1e0c0, 0xa00a20, 0xc42003c050, 0xc420208000, 0xc4201ec1e0, 0x0, 0xc4201ec1e0, ...)
	/home/fische_m/local/test/gateway/pb/example.pb.gw.go:38 +0xd8
_/home/fische_m/local/test/gateway/pb.RegisterServiceHandler.func1(0xa07160, 0xc420210000, 0xc420208000, 0xc4201ec1e0)
	/home/fische_m/local/test/gateway/pb/example.pb.gw.go:94 +0x228
github.com/grpc-ecosystem/grpc-gateway/runtime.(*ServeMux).ServeHTTP(0xc420017e60, 0xa07160, 0xc420210000, 0xc420208000)
	/home/fische_m/Project/Go/src/github.com/grpc-ecosystem/grpc-gateway/runtime/mux.go:89 +0x7f9
net/http.serverHandler.ServeHTTP(0xc420026300, 0xa07160, 0xc420210000, 0xc420208000)
	/usr/lib/go/src/net/http/server.go:2202 +0x7d
net/http.(*conn).serve(0xc4201f0000, 0xa079a0, 0xc4201e8140)
	/usr/lib/go/src/net/http/server.go:1579 +0x4b7
created by net/http.(*Server).Serve
	/usr/lib/go/src/net/http/server.go:2293 +0x44d

[tmc]

@fische thanks for the issue report. This scenario should be handled more gracefullly. Perhaps a codes.InvalidArgument/http.StatusBadRequest with a helpful message would be best.