func (f *fragmentBuffer) size() int {
	size := 0
	for i := range f.cache {
		for j := range f.cache[i] {
			size += len(f.cache[i][j].data)
		}
	}
	return size
}
The program copies an input buffer to an output buffer without verifying that the size of the input buffer is less than the size of the output buffer, leading to a buffer overflow.