[#11] accounting: Define service executor

Defines ServiceExecutor interface of the executor that handles service
request bodies and returns service response bodies. Adds Service
implementation that writes the result of ServiceExecutor to response body
and attaches the internal ResponseMetaHeader.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/services/accounting/executor.go

@@ -0,0 +1,42 @@
+package accounting
+
+import (
+	"context"
+
+	"github.com/nspcc-dev/neofs-api-go/v2/accounting"
+	"github.com/nspcc-dev/neofs-api-go/v2/session"
+	"github.com/pkg/errors"
+)
+
+type ServiceExecutor interface {
+	Balance(context.Context, *accounting.BalanceRequestBody) (*accounting.BalanceResponseBody, error)
+}
+
+type executorSvc struct {
+	exec ServiceExecutor
+
+	metaHeader *session.ResponseMetaHeader
+}
+
+// NewExecutionService wraps ServiceExecutor and returns Accounting Service interface.
+//
+// Passed meta header is attached to all responses.
+func NewExecutionService(exec ServiceExecutor, metaHdr *session.ResponseMetaHeader) accounting.Service {
+	return &executorSvc{
+		exec:       exec,
+		metaHeader: metaHdr,
+	}
+}
+
+func (s *executorSvc) Balance(ctx context.Context, req *accounting.BalanceRequest) (*accounting.BalanceResponse, error) {
+	respBody, err := s.exec.Balance(ctx, req.GetBody())
+	if err != nil {
+		return nil, errors.Wrap(err, "could not execute Balance request")
+	}
+
+	resp := new(accounting.BalanceResponse)
+	resp.SetBody(respBody)
+	resp.SetMetaHeader(s.metaHeader)
+
+	return resp, nil
+}