A function that will be called from @foreach.
buffer field will point to a the reference of the buffer at
When this function returns
true, the next buffer will be returned. When
false is returned,
@foreach will return.
buffer is set to
null, the item will be removed from the bufferlist. When
buffer has been made
writable, the new buffer reference can be assigned to
buffer. This function is responsible for unreffing the old buffer when
removing or modifying.
pointer the buffer
the index of
user data passed to @foreach