Method ReadAt

api-get-object.go:463–536

ReadAt reads len(b) bytes from the File starting at byte offset off. It returns the number of bytes read and the error, if any. ReadAt always returns a non-nil error when n < len(b). At end of file, that error is io.EOF.

(b []byte, offset int64)

// ReadAt always returns a non-nil error when n < len(b). At end of
// file, that error is io.EOF.
func (o *Object) ReadAt(b []byte, offset int64) (n int, err error) {
	if o == nil {
		return 0, errInvalidArgument("Object is nil")
	}

	// Locking.
	o.mutex.Lock()
	defer o.mutex.Unlock()

	// prevErr is error which was saved in previous operation.
	if o.prevErr != nil && o.prevErr != io.EOF || o.isClosed {
		return 0, o.prevErr
	}

	// Save and restore currOffset so ReadAt doesn't affect sequential Read operations.
	// Per io.ReaderAt: "ReadAt should not affect nor be affected by the underlying seek offset."
	savedOffset := o.currOffset
	defer func() {
		o.currOffset = savedOffset
		o.seekData = true // Force next Read to re-establish stream at correct position
	}()

	o.currOffset = offset

	// Can only compare offsets to size when size has been set.
	if o.objectInfoSet {
		// If offset is negative than we return io.EOF.
		// If offset is greater than or equal to object size we return io.EOF.
		if (o.objectInfo.Size > -1 && offset >= o.objectInfo.Size) || offset < 0 {
			return 0, io.EOF
		}
	}

	// Create the new readAt request.
	readAtReq := getRequest{
		isReadOp:        true,
		isReadAt:        true,
		DidOffsetChange: true,       // Offset always changes.
		beenRead:        o.beenRead, // Set if this is the first request to try and read.
		Offset:          offset,     // Set the offset.
		Buffer:          b,
	}

	// Alert that this is the first request.
	if !o.isStarted {
		readAtReq.isFirstReq = true
	}

	// Send and receive from the first request.
	response, err := o.doGetRequest(readAtReq)
	if err != nil && err != io.EOF {
		// Save the error.
		o.prevErr = err
		return response.Size, err
	}
	// Bytes read.
	bytesRead := int64(response.Size)
	// There is no valid objectInfo yet