Read(...) endpoint for the resource service (#16655)

agent/grpc-external/services/resource/read.go

@@ -0,0 +1,54 @@
+package resource
+
+import (
+	"context"
+	"errors"
+	"fmt"
+
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/metadata"
+	"google.golang.org/grpc/status"
+
+	"github.com/hashicorp/consul/internal/resource"
+	"github.com/hashicorp/consul/internal/storage"
+	"github.com/hashicorp/consul/proto-public/pbresource"
+)
+
+func (s *Server) Read(ctx context.Context, req *pbresource.ReadRequest) (*pbresource.ReadResponse, error) {
+	// check type exists
+	_, ok := s.registry.Resolve(req.Id.Type)
+	if !ok {
+		return nil, status.Error(codes.InvalidArgument, fmt.Sprintf("resource type %s not registered", resource.ToGVK(req.Id.Type)))
+	}
+
+	consistency := storage.EventualConsistency
+	if isConsistentRead(ctx) {
+		consistency = storage.StrongConsistency
+	}
+
+	resource, err := s.backend.Read(ctx, consistency, req.Id)
+	if err != nil {
+		if errors.Is(err, storage.ErrNotFound) {
+			return nil, status.Error(codes.NotFound, err.Error())
+		}
+		if errors.As(err, &storage.GroupVersionMismatchError{}) {
+			return nil, status.Error(codes.InvalidArgument, err.Error())
+		}
+		return nil, err
+	}
+	return &pbresource.ReadResponse{Resource: resource}, nil
+}
+
+func isConsistentRead(ctx context.Context) bool {
+	md, ok := metadata.FromIncomingContext(ctx)
+	if !ok {
+		return false
+	}
+
+	vals := md.Get("x-consul-consistency-mode")
+	if len(vals) == 0 {
+		return false
+	}
+
+	return vals[0] == "consistent"
+}