Add support for futex_waitv

[alex]

Currently if you attempt to use futex_waitv on linux, it fails with:

error: unsupported operation: syscall: unsupported syscall number 449
   --> /home/alex/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rustix-1.0.7/src/backend/libc/thread/syscalls.rs:659:5
    |
659 | /     syscall! {
660 | |         fn futex_waitv(
661 | |             waiters: *const FutexWait,
662 | |             nr_futexes: c::c_uint,
...   |
666 | |         ) via SYS_futex_waitv -> c::c_int
667 | |     }
    | |_____^ unsupported operation occurred here
    |
    = help: this is likely not a bug in the program; it indicates that the program performed an operation that Miri does not support
    = note: BACKTRACE on thread `unnamed-13`:
    = note: inside `rustix::backend::thread::syscalls::futex_waitv::futex_waitv` at /home/alex/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rustix-1.0.7/src/weak.rs:249:13: 249:66
    = note: inside `rustix::backend::thread::syscalls::futex_waitv` at /home/alex/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rustix-1.0.7/src/backend/libc/thread/syscalls.rs:672:19: 678:10
    = note: inside `rustix::thread::futex::waitv` at /home/alex/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rustix-1.0.7/src/thread/futex.rs:574:5: 574:77

[RalfJung]

Thanks for the report! Generally, we focus on supporting what the standard library needs and things that are very widely used in the ecosystem. But AFAIK the syscall is still fairly new -- so it's probably not picked up a lot of usage yet?

[alex]

That's almost certainly right that it's use is very rare ATM, so it completely makes sense that it's not implemented yet :-)

(There might be a chicken-and-egg situation where people don't want to write scary unsafe concurrency code until they can verify it with miri :-))

[RalfJung]

To clarify, Miri can't verify anything, it's just a testing tool. :) So at best one could call it validation.