client/object: Allow zero range in payload range and hash ops (#685)

client/object_get.go

@@ -671,6 +671,9 @@ func (x *ObjectRangeReader) Read(p []byte) (int, error) {
 // ObjectRangeInit initiates reading an object's payload range through a remote
 // server using NeoFS API protocol.
 //
+// To get full payload, set both offset and length to zero. Otherwise, length
+// must not be zero.
+//
 // The call only opens the transmission channel, explicit fetching is done using the ObjectRangeReader.
 // Exactly one return value is non-nil. Resulting reader must be finally closed.
 //
@@ -692,7 +695,7 @@ func (c *Client) ObjectRangeInit(ctx context.Context, containerID cid.ID, object
 		}()
 	}
 
-	if length == 0 {
+	if length == 0 && offset != 0 {
 		err = ErrZeroRangeLength
 		return nil, err
 	}

client/object_get_test.go

@@ -284,7 +284,7 @@ func (x *testGetObjectPayloadRangeServer) verifyRequest(req *protoobject.GetRang
 	if body.Range == nil {
 		return newErrMissingRequestBodyField("range")
 	}
-	if body.Range.Length == 0 {
+	if body.Range.Length == 0 && body.Range.Offset != 0 {
 		return newErrInvalidRequestField("range", errors.New("zero length"))
 	}
 	if x.reqRng != nil {

client/object_hash.go

@@ -75,6 +75,9 @@ func (x *PrmObjectHash) UseSalt(salt []byte) {
 // ObjectHash requests checksum of the range list of the object payload using
 // NeoFS API protocol.
 //
+// To hash full payload, set both offset and length to zero. Otherwise, length
+// must not be zero.
+//
 // Returns a list of checksums in raw form: the format of hashes and their number
 // is left for the caller to check. Client preserves the order of the server's response.
 //