[#176] localstore: Draft storage engine structure and ops

Implement the primary structure and operation of the local object
storage engine.

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

pkg/local_object_storage/engine/head.go

@@ -0,0 +1,78 @@
+package engine
+
+import (
+	objectSDK "github.com/nspcc-dev/neofs-api-go/pkg/object"
+	"github.com/nspcc-dev/neofs-node/pkg/core/object"
+	"github.com/nspcc-dev/neofs-node/pkg/local_object_storage/shard"
+	"github.com/pkg/errors"
+	"go.uber.org/zap"
+)
+
+// HeadPrm groups the parameters of Head operation.
+type HeadPrm struct {
+	addr *objectSDK.Address
+}
+
+// HeadRes groups resulting values of Head operation.
+type HeadRes struct {
+	head *object.Object
+}
+
+// WithAddress is a Head option to set the address of the requested object.
+//
+// Option is required.
+func (p *HeadPrm) WithAddress(addr *objectSDK.Address) *HeadPrm {
+	if p != nil {
+		p.addr = addr
+	}
+
+	return p
+}
+
+// Header returns the requested object header.
+//
+// Instance has empty payload.
+func (r *HeadRes) Header() *object.Object {
+	return r.head
+}
+
+// Head reads object header from local storage.
+//
+// Returns any error encountered that
+// did not allow to completely read the object header.
+//
+// Returns ErrObjectNotFound if requested object is missing in local storage.
+func (e *StorageEngine) Head(prm *HeadPrm) (*HeadRes, error) {
+	e.mtx.RLock()
+	defer e.mtx.RUnlock()
+
+	var head *object.Object
+
+	shPrm := new(shard.GetPrm).
+		WithAddress(prm.addr)
+
+	e.iterateOverSortedShards(prm.addr, func(sh *shard.Shard) (stop bool) {
+		res, err := sh.Get(shPrm)
+		if err != nil {
+			if !errors.Is(err, shard.ErrObjectNotFound) {
+				// TODO: smth wrong with shard, need to be processed
+				e.log.Warn("could not get object from shard",
+					zap.Stringer("shard", sh.ID()),
+					zap.String("error", err.Error()),
+				)
+			}
+		} else {
+			head = res.Object()
+		}
+
+		return err == nil
+	})
+
+	if head == nil {
+		return nil, ErrObjectNotFound
+	}
+
+	return &HeadRes{
+		head: head,
+	}, nil
+}