Use stdlib context instead of golang.org/x/net/context

[peterbourgon]

Addresses #420 (heyoh)

[peterbourgon]

Hmm, gRPC still uses x/net/context, which means it can't produce or use endpoints that use plain context. Unless someone has a coercion trick I think we're stuck until gRPC makes the switch.

Relevant issue grpc/grpc-go#711

[harlow]

can remove empty line-break

[Ayiga]

Since grpc uses golang.org/x/net/context.Context, and it's an interface, the only real issue that we're having is grpc Server method signature's aren't directly equivalent, right?

Would it be a decent solution, for the time being, to have a partial mix?

For example, I can get the addsvc working by changing one file in 3 places transport_grpc.go:

// ...
import (
	"context"

	gcontext "golang.org/x/net/context"
// ...
)
// ...
func (s *grpcServer) Sum(ctx gcontext.Context, req *pb.SumRequest) (*pb.SumReply, error) {
// ...
}
// ...
func (s *grpcServer) Concat(ctx gcontext.Context, req *pb.ConcatRequest) (*pb.ConcatReply, error) {
// ...
}
// ...

Is this a simple enough solution? I understand the need to try and make this as seamless and easy to use as possible for people. But the only other possible solution, I can see as being reasonable, is to create a Middleware / Shim / Adapter for submitting to grpc itself.

func MakeGRPCShimServer(server *grpcServer) pb.AddServer {
    return grpcShimServer {
        grpcServer: server,
   }
}

type grpcShimServer struct {
    grpcServer *grpcServer
}

// A shim Sum method for function matching.
func (s *grpcShimServer) Sum(ctx gcontext.Context, req *pb.SumRequest) (*pb.SumReply, error) {
   return  s.grpcServer.Sum(ctx, req)
}

func (s *grpcShimServer) Concat(ctx gcontext.Context, req *pb.ConcatRequest) (*pb.ConcatReply, error) {
    return s.grpcServer.Concat(ctx, req)
}

Then, of course, you'd have to the return type of the MakeGRPCServer function, and probably introduce a new interface so it can be used still:

// Server API for Add service
type AddServer interface {
	// Sums two integers.
	Sum(context.Context, *SumRequest) (*SumReply, error)
	// Concatenates two strings
	Concat(context.Context, *ConcatRequest) (*ConcatReply, error)
}

// MakeGRPCServer makes a set of endpoints available as a gRPC AddServer.
func MakeGRPCServer(ctx context.Context, endpoints Endpoints, tracer stdopentracing.Tracer, logger log.Logger) AddServer {
// ...
}