Await until at least desiredSize is available for read or EOF and invoke block function. The block function should never capture a provided Memory instance outside otherwise an undefined behaviour may occur including accidental crash or data corruption. Block function should return number of bytes consumed or 0.
Specifying desiredSize larger than the channel's capacity leads to block function invocation earlier when the channel is full. So specifying too big desiredSize is identical to specifying desiredSize equal to the channel's capacity. The other case when a provided memory range could be less than desiredSize is that all the requested bytes couldn't be represented as a single memory range due to internal implementation reasons.
number of bytes consumed, possibly 0