`protoc-gen-fastmarshal` generated code fails to unmarshal empty protov2 message

[wtrep]

Title

protoc-gen-fastmarshal generated code fails to unmarshal empty protov2 message

Version

0.14.0

Description

Reflection-free generated code (*.pb.fm.go) doesn't seem to support "unmarshalling" empty messages.

The Unmarshal method first validates that the buffer isn't empty:

// Unmarshal decodes a binary encoded Protobuf message from p and populates m with the result.
func (m *BogusRequest) Unmarshal(p []byte) error {
    if len(p) == 0 {
        return fmt.Errorf("cannot unmarshal from an empty buffer")
    }
        // ...
}

This seems to not support empty messages.

Repro steps

1. Create a simple proto:

   
   syntax = "proto3";

package bogus.v1alpha1;

option go_package = "github.com/bogus/api/proto/v1alpha1";

message BogusRequest {}

message BogusResponse { string response = 1; }

service BogusService { rpc BogusEmptyBody(BogusRequest) returns (BogusResponse) {} }

2. Generate the `*.pb.fm.go` files with `--fastmarshal_opt=apiversion=v2,paths=source_relative`
3. Register the codec with `encoding.RegisterCodec(csproto.GrpcCodec{})`
4. Implement the server:
```go
package bogus

import (
    "context"

    "github.com/bogus/api/proto/v1alpha1"
)

type Server struct {
    v1alpha1.UnimplementedBogusServiceServer
}

func (s *Server) BogusEmptyBody(_ context.Context, _ *v1alpha1.BogusRequest) (*v1alpha1.BogusResponse, error) {
    return &v1alpha1.BogusResponse{Response: "bogus"}, nil
}

5. Make a simple request:

   ❯ grpcurl -plaintext localhost:8080 bogus.v1alpha1.BogusService/BogusEmptyBody
   ERROR:
   Code: Internal
   Message: grpc: error unmarshalling request: cannot unmarshal from an empty buffer

6. Comment out encoding.RegisterCodec(csproto.GrpcCodec{})

7. Validate that it works now:

   ❯ grpcurl -plaintext localhost:8080  bogus.v1alpha1.BogusService/BogusEmptyBody
   {
   "response": "bogus"
   }

[dylan-bourque]

@wtrep this was a miss on my part. I wrongly assumed that trying to unmarshal no data should be an error.

That snippet in the generated code should just return nil instead of an error and leave the message unmodified. Happy to review a PR if you want to submit one. If not, I'll fix this as soon as I finish up some other in-flight work I have.

The templates are here and you'd need to update both singlefile.go.tmpl and permessage.go.tmpl. Go text templates aren't great at sharing code. 😢

[wtrep]

No problem ! I'll submit a PR

[dylan-bourque]

@wtrep I had some spare bandwidth so I went ahead and knocked this out. This should resolve your issue.

[wtrep]

Thanks @dylan-bourque !

[dylan-bourque]

@wtrep I just tagged v0.18.0. Let me know if that doesn't work for you.