Return the amount of bytes read from Memory::read

[ranile]

Memory::read takes in an offset and buffer but does not return how much data has been written to the buffer.

Consider the following scenario:

let memory_manger = MemoryManager::init(DefaultMemoryImpl::default());
let mem = memory_manger.get(1);


let mut data = vec![];
let mut pos = 0;

loop {
    let mut buf = [0; 1028];
    mem.read(pos, &mut buf);
    pos = buf.len()
    vec.extend(buf);
    if ??? { break } // ??? here would be checking if buf.len() is more than amount of bytes returned, suggesting that the buffer has been memory has been all read and there's nothing more to read. If the number of bytes read is less than buf.len() than it should also tell that since bunch of 0s at the end of a buffer aren't helpful 
}

Use case

Serializing data to bytes to be stored in memory and deserializing those bytes later into a rust type. This is needed for data that can't be morphed into any of the Stable* structures.
While StableVec of u8 can get the job done, it requires all the data be read into memory and then deserialized, which is not ideal for large amount of data:

let memory_manger = MemoryManager::init(DefaultMemoryImpl::default());
let mem = memory_manger.get(1);
let stable_vec = StableVec::<u8, _>::init(mem).expect("memory was not for stable vec");
let bytes = stable_vec.iter().collect::<Vec<_>>();
// deserialize here

[adambratschikaye]

Well it'll always fill up the buffer, unless you try to read beyond the end of the memory (in which case it panics). So returning the length probably isn't necessary. But I think it would make sense for us to document this behavior.

[maksymar]

I've updated the documentation, will be available in the next release.

#352

[maksymar]

I suppose this ticket can be closed, since the read/write methods are mirroring the WebAssembly semantics and there are no plans to change that at the moment.