add grpc shims to helper/plugin

Add shim functions to automatically wrap plugins in the new GRPC
implementations.

helper/plugin/grpc_provider.go

@@ -16,6 +16,20 @@ import (
 	"github.com/zclconf/go-cty/cty/msgpack"
 )
 
+// NewGRPCProviderServerShim wraps a terraform.ResourceProvider in a
+// proto.ProviderServer implementation. If the provided provider is not a
+// *schema.Provider, this will return nil,
+func NewGRPCProviderServerShim(p terraform.ResourceProvider) *GRPCProviderServer {
+	sp, ok := p.(*schema.Provider)
+	if !ok {
+		return nil
+	}
+
+	return &GRPCProviderServer{
+		provider: sp,
+	}
+}
+
 // GRPCProviderServer handles the server, or plugin side of the rpc connection.
 type GRPCProviderServer struct {
 	provider *schema.Provider

helper/plugin/grpc_provisioner.go

@@ -12,6 +12,19 @@ import (
 	context "golang.org/x/net/context"
 )
 
+// NewGRPCProvisionerServerShim wraps a terraform.ResourceProvisioner in a
+// proto.ProvisionerServer implementation. If the provided provisioner is not a
+// *schema.Provisioner, this will return nil,
+func NewGRPCProvisionerServerShim(p terraform.ResourceProvisioner) *GRPCProvisionerServer {
+	sp, ok := p.(*schema.Provisioner)
+	if !ok {
+		return nil
+	}
+	return &GRPCProvisionerServer{
+		provisioner: sp,
+	}
+}
+
 type GRPCProvisionerServer struct {
 	provisioner *schema.Provisioner
 }