[#762] storage/shard: Try to read headers from write-cache

Shard should try to read object headers from write-cache if it is enabled.

Extend `writecache.Cache` interface with `Head` method. Call the method in
`Shard.Head` if `Shard.hasWriteCache` returns true.

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

pkg/local_object_storage/shard/head.go

@@ -1,6 +1,9 @@
 package shard
 
 import (
+	"errors"
+	"fmt"
+
 	objectSDK "github.com/nspcc-dev/neofs-api-go/pkg/object"
 	"github.com/nspcc-dev/neofs-node/pkg/core/object"
 	meta "github.com/nspcc-dev/neofs-node/pkg/local_object_storage/metabase"
@@ -48,13 +51,33 @@ func (r *HeadRes) Object() *object.Object {
 //
 // Returns any error encountered.
 func (s *Shard) Head(prm *HeadPrm) (*HeadRes, error) {
+	// object can be saved in write-cache (if enabled) or in metabase
+
+	if s.hasWriteCache() {
+		// try to read header from write-cache
+		header, err := s.writeCache.Head(prm.addr)
+		if err == nil {
+			return &HeadRes{
+				obj: header,
+			}, nil
+		} else if !errors.Is(err, object.ErrNotFound) {
+			// in this case we think that object is presented in write-cache, but corrupted
+			return nil, fmt.Errorf("could not read header from write-cache: %w", err)
+		}
+
+		// otherwise object seems to be flushed to metabase
+	}
+
 	headParams := new(meta.GetPrm).
 		WithAddress(prm.addr).
 		WithRaw(prm.raw)
 
-	head, err := s.metaBase.Get(headParams)
+	res, err := s.metaBase.Get(headParams)
+	if err != nil {
+		return nil, err
+	}
 
 	return &HeadRes{
-		obj: head.Header(),
+		obj: res.Header(),
-	}, err
+	}, nil
 }

pkg/local_object_storage/writecache/writecache.go

@@ -13,6 +13,7 @@ import (
 // Cache represents write-cache for objects.
 type Cache interface {
 	Get(*objectSDK.Address) (*object.Object, error)
+	Head(*objectSDK.Address) (*object.Object, error)
 	Delete(*objectSDK.Address) error
 	Put(*object.Object) error