[#399] container: Support homomorphic hash disabling

Container could now have "__NEOFS__DISABLE_HOMOMORPHIC_HASHING" well-known
attribute. Setting that to "true" means disabling homomorphic hashing for
objects that belong to that container, any other cases mean that homomorphic
hashing is enabled.

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

container/attributes.go

@@ -12,7 +12,64 @@ const (
 
 	// SysAttributeZone is a string of zone for container name.
 	SysAttributeZone = SysAttributePrefix + "ZONE"
+
+	// SysAttributeHomomorphicHashing is a container's homomorphic hashing state.
+	SysAttributeHomomorphicHashing = SysAttributePrefix + "DISABLE_HOMOMORPHIC_HASHING"
 )
 
 // SysAttributeZoneDefault is a default value for SysAttributeZone attribute.
 const SysAttributeZoneDefault = "container"
+
+const disabledHomomorphicHashingValue = "true"
+
+// HomomorphicHashingState returns container's homomorphic
+// hashing state:
+// 	* true if hashing is enabled;
+// 	* false if hashing is disabled.
+//
+// All container's attributes must be unique, otherwise behavior
+// is undefined.
+//
+// See also SetHomomorphicHashingState.
+func (c Container) HomomorphicHashingState() bool {
+	for i := range c.attr {
+		if c.attr[i].GetKey() == SysAttributeHomomorphicHashing {
+			return c.attr[i].GetValue() != disabledHomomorphicHashingValue
+		}
+	}
+
+	return true
+}
+
+// SetHomomorphicHashingState sets homomorphic hashing state for
+// container.
+//
+// All container's attributes must be unique, otherwise behavior
+// is undefined.
+//
+// See also HomomorphicHashingState.
+func (c *Container) SetHomomorphicHashingState(enable bool) {
+	for i := range c.attr {
+		if c.attr[i].GetKey() == SysAttributeHomomorphicHashing {
+			if enable {
+				// approach without allocation/waste
+				// coping works since the attributes
+				// order is not important
+				c.attr[i] = c.attr[len(c.attr)-1]
+				c.attr = c.attr[:len(c.attr)-1]
+			} else {
+				c.attr[i].SetValue(disabledHomomorphicHashingValue)
+			}
+
+			return
+		}
+	}
+
+	if !enable {
+		attr := Attribute{}
+		attr.SetKey(SysAttributeHomomorphicHashing)
+		attr.SetValue(disabledHomomorphicHashingValue)
+
+		c.attr = append(c.attr, attr)
+	}
+}