Occasional memory leak in mpsc channels

[RalfJung]

Running this test:

use std::sync::mpsc::channel;
use std::thread;

fn main() {
    let (tx, rx) = channel::<i32>();
    let _t = thread::spawn(move || {
        drop(rx);
    });
    let _ = thread::spawn(move || {
        tx.send(1).unwrap();
    })
    .join();
}

with many different seeds in Miri eventually shows:

error: memory leaked: alloc2567 (Rust heap, size: 504, align: 8), allocated here:
  --> /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/library/alloc/src/alloc.rs:98:9
   |
98 |         __rust_alloc(layout.size(), layout.align())
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: inside `std::alloc::alloc` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/library/alloc/src/alloc.rs:98:9: 98:52
   = note: inside `std::alloc::Global::alloc_impl` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/library/alloc/src/alloc.rs:181:73: 181:86
   = note: inside `<std::alloc::Global as std::alloc::Allocator>::allocate` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/library/alloc/src/alloc.rs:241:9: 241:39
   = note: inside `alloc::alloc::exchange_malloc` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/library/alloc/src/alloc.rs:330:11: 330:34
   = note: inside `std::boxed::Box::<std::sync::mpmc::list::Block<i32>>::new` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/library/alloc/src/boxed.rs:218:9: 218:20
   = note: inside `std::sync::mpmc::list::Channel::<i32>::start_send` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/sync/mpmc/list.rs:209:41: 209:68
   = note: inside `std::sync::mpmc::list::Channel::<i32>::send` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/sync/mpmc/list.rs:402:17: 402:39
   = note: inside `std::sync::mpmc::Sender::<i32>::send` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/sync/mpmc/mod.rs:128:41: 128:61
   = note: inside `std::sync::mpsc::Sender::<i32>::send` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/sync/mpsc/mod.rs:613:9: 613:27
note: inside closure
  --> leak.rs:10:9
   |
10 |         tx.send(1).unwrap();
   |         ^^^^^^^^^^

This indicates that the allocation made here does not get freed properly:

rust/library/std/src/sync/mpmc/list.rs

Line 209 in f0d487d

let new = Box::into_raw(Box::new(Block::<T>::new()));

Specifically when running this with Miri 3fe10973bb6e9a01b280686534d0242da07f3ede, seed 139 causes the issue.

I don't think there is any way for this to be a false positive.

Cc @ibraheemdev @taiki-e @Amanieu

[RalfJung]

I think this is not recent, it's just hard to trigger. I'll try reproducing on some older versions of rustc+Miri:

• 1244dca7fd35293b5c88d51efea781cb566d8cfe (Jan 24th): seed 1870
• cad4f40616ccf4ada87506ec5437b2ff41af2671 (Nov 22nd, 2023): seed 1020

We can't go back much further since it seems we're not keeping the artifacts long enough for that?

So probably this existed ever since #93563 landed.

[ibraheemdev]

Reproduced this with crossbeam-channel 0.5.11 and miri 381d699`.