[pbs-devel] parallelize restore.rs fn restore_image: problems in async move

[Niko Fellner]

> first thing: the error you get is simply a return type mismatch, the 'async move' block expects to return a Result<(), Error> because there is a bail!() and multiple '?' calls.
> the simplest way to fix this is to write
> Ok(())
> at the end of the async move block
> the next problem you will run into is that there are multiple uses of variables that get moved in a loop (which does not work in rust) so you have to pull out some things, and clone some others

@Dominik: Thanks a lot for your help! Yes, and after following your instructions I've discovered more and more similarities with "pull.rs"... 

> a much bigger problem is the following:
> the callbacks that get called (write_data_callback/write_zero_callback)
> are not 'Send' meaning they cannot send between threads safely (needed by tokio::spawn) because they both are closures that use a '*mut T'
> in that code this would not actually be a problem, since the restored blocks never overlap, but rust does not know that
> without digging further, i think this is not easily solvable in rust without either writing unsafe code or changing our c <-> rust api (idk if that is possible)

So that's not only a restriction of tokio::spawn, but also anything that Rayon provides (including its ParallelBridge?), and the code in pull.rs, too? (ParallelHandler, futures::stream, ...) ?

> as pointed out in the bug report, we already have such code in pull.rs
> Why not use that instead of something new?

@Dietmar: You're right, that looks like a gold mine - but unfortunately one that I cannot mine due to my limited knowledge of Rust...  Maybe you can help out to apply its magic on restore.rs? Most importantly, will it work with that write_data_callback and write_zero_callback which are not Send?

> this is not easily solvable in rust without either writing unsafe code or changing our c <-> rust api (idk if that is possible)

"Unsafe" seems to be the right keyword here. In proxmox-backup-qemu the CallbackPointers (capi_types.rs) do this: "unsafe impl std::marker::Send". Those CallbackPointers are used by lots of "_async" functions within the c <-> rust api. So I guess a hypothetic proxmox_restore_image_async needs to use CallbackPointers, too?