refactor grpc-proxy backends to enhance responses from local backends with metadata in one2many mode

internal/machine/api/proxy/backend.go

@@ -0,0 +1,144 @@
+package proxy
+
+import (
+	"fmt"
+	"google.golang.org/grpc/status"
+	"google.golang.org/protobuf/encoding/protowire"
+	"google.golang.org/protobuf/proto"
+	"uncloud/internal/machine/api/pb"
+)
+
+// One2ManyResponder converts upstream responses into messages from upstreams, so that multiple
+// successful and failure responses might be returned in One2Many mode.
+type One2ManyResponder struct {
+	machine string
+}
+
+// AppendInfo is called to enhance response from the backend with additional data.
+//
+// AppendInfo enhances upstream response with machine metadata (target).
+//
+// This method depends on grpc protobuf response structure, each response should
+// look like:
+//
+//	message SomeResponse {
+//	  repeated SomeReply messages = 1; // please note field ID == 1
+//	}
+//
+//	message SomeReply {
+//	  common.Metadata metadata = 1;
+//	  <other fields go here ...>
+//	}
+//
+// As 'SomeReply' is repeated in 'SomeResponse', if we concatenate protobuf representation
+// of several 'SomeResponse' messages, we still get valid 'SomeResponse' representation but with more
+// entries (feature of protobuf binary representation).
+//
+// If we look at binary representation of any unary 'SomeResponse' message, it will always contain one
+// protobuf field with field ID 1 (see above) and type 2 (embedded message SomeReply is encoded
+// as string with length). So if we want to add fields to 'SomeReply', we can simply read field
+// header, adjust length for new 'SomeReply' representation, and prepend new field header.
+//
+// At the same time, we can add 'common.Metadata' structure to 'SomeReply' by simply
+// appending or prepending 'common.Metadata' as a single field. This requires 'metadata'
+// field to be not defined in original response. (This is due to the fact that protobuf message
+// representation is concatenation of each field representation).
+//
+// To build only single field (Metadata) we use helper message which contains exactly this
+// field with same field ID as in every other 'SomeReply':
+//
+//	message Empty {
+//	  common.Metadata metadata = 1;
+//	}
+//
+// As streaming replies are not wrapped into 'SomeResponse' with 'repeated', handling is simpler: we just
+// need to append Empty with details.
+//
+// So AppendInfo does the following: validates that response contains field ID 1 encoded as string,
+// cuts field header, rest is representation of some reply. Marshal 'Empty' as protobuf,
+// which builds 'common.Metadata' field, append it to original response message, build new header
+// for new length of some response, and add back new field header.
+func (b *One2ManyResponder) AppendInfo(streaming bool, resp []byte) ([]byte, error) {
+	payload, err := proto.Marshal(&pb.Empty{
+		Metadata: &pb.Metadata{
+			Machine: b.machine,
+		},
+	})
+
+	if streaming {
+		return append(resp, payload...), err
+	}
+
+	const (
+		metadataField = 1 // field number in proto definition for repeated response
+		metadataType  = 2 // "string" for embedded messages
+	)
+
+	// decode protobuf embedded header
+
+	typ, n1 := protowire.ConsumeVarint(resp)
+	if n1 < 0 {
+		return nil, protowire.ParseError(n1)
+	}
+
+	_, n2 := protowire.ConsumeVarint(resp[n1:]) // length
+	if n2 < 0 {
+		return nil, protowire.ParseError(n2)
+	}
+
+	if typ != (metadataField<<3)|metadataType {
+		return nil, fmt.Errorf("unexpected message format: %d", typ)
+	}
+
+	if n1+n2 > len(resp) {
+		return nil, fmt.Errorf("unexpected message size: %d", len(resp))
+	}
+
+	// cut off embedded message header
+	resp = resp[n1+n2:]
+	// build new embedded message header
+	prefix := protowire.AppendVarint(
+		protowire.AppendVarint(nil, (metadataField<<3)|metadataType),
+		uint64(len(resp)+len(payload)),
+	)
+	resp = append(prefix, resp...)
+
+	return append(resp, payload...), err
+}
+
+// BuildError converts upstream error into message from upstream, so that multiple
+// successful and failure responses might be returned.
+//
+// This simply relies on the fact that any response contains 'Empty' message.
+// So if 'Empty' is unmarshalled into any other reply message, all the fields
+// are undefined but 'Metadata':
+//
+//	message Empty {
+//	  common.Metadata metadata = 1;
+//	}
+//
+//	message EmptyResponse {
+//	  repeated Empty messages = 1;
+//	}
+//
+// Streaming responses are not wrapped into Empty, so we simply marshall EmptyResponse
+// message.
+func (b *One2ManyResponder) BuildError(streaming bool, err error) ([]byte, error) {
+	var resp proto.Message = &pb.Empty{
+		Metadata: &pb.Metadata{
+			Machine: b.machine,
+			Error:   err.Error(),
+			Status:  status.Convert(err).Proto(),
+		},
+	}
+
+	if !streaming {
+		resp = &pb.EmptyResponse{
+			Messages: []*pb.Empty{
+				resp.(*pb.Empty),
+			},
+		}
+	}
+
+	return proto.Marshal(resp)
+}