miniogw: don't offset part IDs in multipart ops

Currently, if the client requests to upload part with ID=1, we upload
part with decremented ID. This patch changes it to always upload parts
with IDs passed by the client (it doesn't change the behavior of MPU
from libuplink's perspective). This enables more transparency between
what gateway and libuplink do. It also allows combining native part
uploads with gateway's (though this is an edge case). Furthermore, parts
verification during multipart upload completion will be easier.

Change-Id: I9e0cf9a0800e82f794850d8e8d6ec013cb2a9124

miniogw/multipart.go

@@ -135,7 +135,7 @@ func (layer *gatewayLayer) PutObjectPart(ctx context.Context, bucket, object, up
 		return minio.PartInfo{}, err
 	}
 
-	partUpload, err := project.UploadPart(ctx, bucket, object, uploadID, uint32(partID-1))
+	partUpload, err := project.UploadPart(ctx, bucket, object, uploadID, uint32(partID))
 	if err != nil {
 		return minio.PartInfo{}, convertMultipartError(err, bucket, object, uploadID)
 	}
@@ -161,7 +161,7 @@ func (layer *gatewayLayer) PutObjectPart(ctx context.Context, bucket, object, up
 
 	part := partUpload.Info()
 	return minio.PartInfo{
-		PartNumber:   int(part.PartNumber + 1),
+		PartNumber:   int(part.PartNumber),
 		Size:         part.Size,
 		ActualSize:   part.Size,
 		ETag:         string(part.ETag),
@@ -220,7 +220,7 @@ func (layer *gatewayLayer) ListObjectParts(ctx context.Context, bucket, object,
 	}
 
 	list := project.ListUploadParts(ctx, bucket, object, uploadID, &uplink.ListUploadPartsOptions{
-		Cursor: uint32(partNumberMarker - 1),
+		Cursor: uint32(partNumberMarker),
 	})
 
 	parts := make([]minio.PartInfo, 0, maxParts)
@@ -230,7 +230,7 @@ func (layer *gatewayLayer) ListObjectParts(ctx context.Context, bucket, object,
 		limit--
 		part := list.Item()
 		parts = append(parts, minio.PartInfo{
-			PartNumber:   int(part.PartNumber + 1),
+			PartNumber:   int(part.PartNumber),
 			LastModified: part.Modified,
 			ETag:         string(part.ETag), // Entity tag returned when the part was initially uploaded.
 			Size:         part.Size,         // Size in bytes of the part.