Function take_read_buffer 

pub unsafe fn take_read_buffer(
    stash: &mut BytesMut,
    read_size: usize,
) -> BytesMut

Take a read-sized scratch buffer from the front of stash, leaving the remaining tail in stash to hand out on the next call.

This replaces the old bump-pointer read arena: successive reads carve read_size chunks off one READ_SLAB_SIZE allocation until it is used up, then a fresh slab is allocated. Freezing a returned buffer shares that slab’s allocation (via bytes refcounting), so a lagging consumer pins the slab exactly as the arena page did.

Safety

The returned buffer reports read_size initialized bytes that are in fact uninitialized, so it can be handed to an owned-buffer read without zero-filling first. The caller must pass it straight to a read and truncate it to the number of bytes actually read before freezing, inspecting, or otherwise exposing its contents.