[#372] object: Add object notification attributes

Also, add functions for parsing and setting object notifications.

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

object/attributes.go

@@ -1,5 +1,11 @@
 package object
 
+import (
+	"errors"
+	"fmt"
+	"strconv"
+)
+
 // SysAttributePrefix is a prefix of key to system attribute.
 const SysAttributePrefix = "__NEOFS__"
 
@@ -9,4 +15,155 @@ const (
 
 	// SysAttributeExpEpoch tells GC to delete object after that epoch.
 	SysAttributeExpEpoch = SysAttributePrefix + "EXPIRATION_EPOCH"
+
+	// SysAttributeTickEpoch defines what epoch must produce object
+	// notification.
+	SysAttributeTickEpoch = SysAttributePrefix + "TICK_EPOCH"
+
+	// SysAttributeTickTopic defines what topic object notification
+	// must be sent to.
+	SysAttributeTickTopic = SysAttributePrefix + "TICK_TOPIC"
 )
+
+// NotificationInfo groups information about object notification
+// that can be written to object.
+//
+// Topic is an optional field.
+type NotificationInfo struct {
+	epoch uint64
+	topic string
+}
+
+// Epoch returns object notification tick
+// epoch.
+func (n NotificationInfo) Epoch() uint64 {
+	return n.epoch
+}
+
+// SetEpoch sets object notification tick
+// epoch.
+func (n *NotificationInfo) SetEpoch(epoch uint64) {
+	n.epoch = epoch
+}
+
+// Topic return optional object notification
+// topic.
+func (n NotificationInfo) Topic() string {
+	return n.topic
+}
+
+// SetTopic sets optional object notification
+// topic.
+func (n *NotificationInfo) SetTopic(topic string) {
+	n.topic = topic
+}
+
+// WriteNotificationInfo writes NotificationInfo to the Object via attributes. Object must not be nil.
+//
+// Existing notification attributes are expected to be key-unique, otherwise undefined behavior.
+func WriteNotificationInfo(o *Object, ni NotificationInfo) {
+	h := o.GetHeader()
+	if h == nil {
+		h = new(Header)
+		o.SetHeader(h)
+	}
+
+	var (
+		attrs = h.GetAttributes()
+
+		epoch = strconv.FormatUint(ni.Epoch(), 10)
+		topic = ni.Topic()
+
+		changedEpoch bool
+		changedTopic bool
+		deleteIndex  = -1
+	)
+
+	for i := range attrs {
+		switch attrs[i].GetKey() {
+		case SysAttributeTickEpoch:
+			attrs[i].SetValue(epoch)
+			changedEpoch = true
+		case SysAttributeTickTopic:
+			changedTopic = true
+
+			if topic == "" {
+				deleteIndex = i
+				break
+			}
+
+			attrs[i].SetValue(topic)
+		}
+
+		if changedEpoch && changedTopic {
+			break
+		}
+	}
+
+	if deleteIndex != -1 {
+		// approach without allocation/waste
+		// coping works since the attributes
+		// order is not important
+		attrs[deleteIndex] = attrs[len(attrs)-1]
+		attrs = attrs[:len(attrs)-1]
+	}
+
+	notifyAttrs := make([]*Attribute, 0, 2)
+
+	if !changedEpoch {
+		eAttr := new(Attribute)
+		eAttr.SetKey(SysAttributeTickEpoch)
+		eAttr.SetValue(epoch)
+
+		notifyAttrs = append(notifyAttrs, eAttr)
+	}
+
+	if !changedTopic && topic != "" {
+		tAttr := new(Attribute)
+		tAttr.SetKey(SysAttributeTickTopic)
+		tAttr.SetValue(topic)
+
+		notifyAttrs = append(notifyAttrs, tAttr)
+	}
+
+	attrs = append(attrs, notifyAttrs...)
+
+	h.SetAttributes(attrs)
+}
+
+// ErrNotificationNotSet means that object does not have notification.
+var ErrNotificationNotSet = errors.New("notification for object is not set")
+
+// GetNotificationInfo looks for object notification attributes. Object must not be nil.
+// Returns ErrNotificationNotSet if no corresponding attributes
+// were found.
+//
+// Existing notification attributes are expected to be key-unique, otherwise undefined behavior.
+func GetNotificationInfo(o *Object) (*NotificationInfo, error) {
+	var (
+		foundEpoch bool
+		ni         = new(NotificationInfo)
+	)
+
+	for _, attr := range o.GetHeader().GetAttributes() {
+		switch key := attr.GetKey(); key {
+		case SysAttributeTickEpoch:
+			epoch, err := strconv.ParseUint(attr.GetValue(), 10, 64)
+			if err != nil {
+				return nil, fmt.Errorf("could not parse epoch: %w", err)
+			}
+
+			ni.SetEpoch(epoch)
+
+			foundEpoch = true
+		case SysAttributeTickTopic:
+			ni.SetTopic(attr.GetValue())
+		}
+	}
+
+	if !foundEpoch {
+		return nil, ErrNotificationNotSet
+	}
+
+	return ni, nil
+}