[#1295] getSvc: Assemble complex EC object headers without linking object

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

pkg/services/object/get/assembler.go

@@ -19,6 +19,7 @@ type assembler struct {
 	splitInfo *objectSDK.SplitInfo
 	rng       *objectSDK.Range
 	objGetter objectGetter
+	head      bool
 
 	currentOffset uint64
 
@@ -30,18 +31,23 @@ func newAssembler(
 	splitInfo *objectSDK.SplitInfo,
 	rng *objectSDK.Range,
 	objGetter objectGetter,
+	head bool,
 ) *assembler {
 	return &assembler{
 		addr:      addr,
 		rng:       rng,
 		splitInfo: splitInfo,
 		objGetter: objGetter,
+		head:      head,
 	}
 }
 
 // Assemble assembles splitted large object and writes it's content to ObjectWriter.
 // It returns parent object.
 func (a *assembler) Assemble(ctx context.Context, writer ObjectWriter) (*objectSDK.Object, error) {
+	if a.head {
+		return a.assembleHeader(ctx, writer)
+	}
 	sourceObjectID, ok := a.getLastPartOrLinkObjectID()
 	if !ok {
 		return nil, objectSDK.NewSplitInfoError(a.splitInfo)
@@ -65,6 +71,43 @@ func (a *assembler) Assemble(ctx context.Context, writer ObjectWriter) (*objectS
 	return a.parentObject, nil
 }
 
+func (a *assembler) assembleHeader(ctx context.Context, writer ObjectWriter) (*objectSDK.Object, error) {
+	var sourceObjectIDs []oid.ID
+	sourceObjectID, ok := a.splitInfo.Link()
+	if ok {
+		sourceObjectIDs = append(sourceObjectIDs, sourceObjectID)
+	}
+	sourceObjectID, ok = a.splitInfo.LastPart()
+	if ok {
+		sourceObjectIDs = append(sourceObjectIDs, sourceObjectID)
+	}
+	if len(sourceObjectIDs) == 0 {
+		return nil, objectSDK.NewSplitInfoError(a.splitInfo)
+	}
+	for _, sourceObjectID = range sourceObjectIDs {
+		obj, err := a.getParent(ctx, sourceObjectID, writer)
+		if err == nil {
+			return obj, nil
+		}
+	}
+	return nil, objectSDK.NewSplitInfoError(a.splitInfo)
+}
+
+func (a *assembler) getParent(ctx context.Context, sourceObjectID oid.ID, writer ObjectWriter) (*objectSDK.Object, error) {
+	obj, err := a.objGetter.HeadObject(ctx, sourceObjectID)
+	if err != nil {
+		return nil, err
+	}
+	parent := obj.Parent()
+	if parent == nil {
+		return nil, objectSDK.NewSplitInfoError(a.splitInfo)
+	}
+	if err := writer.WriteHeader(ctx, parent); err != nil {
+		return nil, err
+	}
+	return obj, nil
+}
+
 func (a *assembler) getLastPartOrLinkObjectID() (oid.ID, bool) {
 	sourceObjectID, ok := a.splitInfo.Link()
 	if ok {