wasi-common: add preopened_handle, to be able to open any handle

[Ekleog]

This is a follow-up to #1600 ; that also makes it possible to write custom Handles for directories. For my specific case, it is so as to be able to have a wasmtime-async-compatible WASI.

I've raised the question on zulip #wasmtime without answer yet (not sure whether #general should be preferred?), so I've just decided to go on with this PR, and will start using my fork and checking whether I can make things work properly with it starting now :)

[Ekleog]

Side-note: preopened_dir and preopened_virt could both be refactored to lower to preopened_handle if making it take a function instead of directly a handle. I'm not yet sure whether that's something you'd want so restrained myself for now but it would be possible too, though I'm not really sure what PendingPreopen actually brings in practice compared to just opening ahead-of-time (and thus seeing potential errors happen as soon as possible)

[pchickey]

Hi. Sorry I missed your message on the bytecode alliance discord.

Your use case is important to me. I'm nearing completion of a complete rewrite of wasi-common that, among other things, redesigns the traits so that files and directories have distinct sets of methods. #2487 I expect this PR will land within a week or so. (When it is ready to land, i'll rename the wasi-c2 crate back to wasi-common, thats just a name under development.)

The new design of the ctx builder should support your use case possible - https://github.com/bytecodealliance/wasmtime/blob/pch/wasi_common_cap_std/crates/wasi-c2/src/ctx.rs#L104

Is it OK to close this PR and you can re-target your work at the rewritten wasi-common?

[Ekleog]

That sounds awesome, thank you! Is there a branch somewhere where wasmtime-wasi depends on this version of wasi-common so I could take a git-dependency and start developing against that version?

[pchickey]

If you're on that branch and depend on wasi-c2-wasmtime it should work in the place of wasmtime-wasi

[Ekleog]

Great, I'll try doing that. Thank you!