Memory leaks?

[Horusiath]

Recently some of the people using our library noticed that one of the parts is leaking memory. We decided to investigate it with the help mockalloc crate, which tracked the source of an issue within ArcSwap::rcu. I'm not sure if this is a false negative, however it may be worth taking look into.

#[global_allocator]
static ALLOCATOR: mockalloc::Mockalloc<std::alloc::System> = mockalloc::Mockalloc(std::alloc::System);

#[mockalloc::test]
fn memory_reclamation() {
    let arc = ArcSwapOption::new(Some(Arc::new(vec![1, 2])));
    for i in 1..2 {
        arc.rcu(|values| match values.as_deref() {
            None => None,
            Some(values) => {
                let mut values = values.clone();
                values.pop();
                if values.is_empty() {
                    None
                } else {
                    Some(Arc::new(values))
                }
            }
        });
    }
}

This test failed with following trace:

TracingInfo {
    leaks: [
        LeakInfo {
            ptr: 0x00007fa377804540,
            layout: Layout {
                size: 128,
                align: 64 (1 << 6),
            },
            caller:    0:        0x104ab9035 - <mockalloc::Mockalloc<T> as core::alloc::global::GlobalAlloc>::alloc::h0b6bbecde04abb09
               1:        0x104c6d720 - arc_swap::debt::list::Node::get::h1815a5c3e5a99577
               2:        0x104a1e66c - arc_swap::debt::list::LocalNode::with::ha273e1f8b7a3cb91
               3:        0x104a1acec - arc_swap::ArcSwapAny<T,S>::rcu::he96fd2eec60ac499
               4:        0x104ab97d1 - mockalloc::assert_allocs::had10960fa99e865f
               5:        0x104a49a8e - core::ops::function::FnOnce::call_once::h5d184419d27f0b52
               6:        0x104c5b6b2 - core::ops::function::FnOnce::call_once::hd55cd4785832579b
                                           at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/core/src/ops/function.rs:250:5
               7:        0x104c5b6b2 - test::__rust_begin_short_backtrace::h6138742d84faf9e0
                                           at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/test/src/lib.rs:627:18
               8:        0x104c5a5f6 - test::run_test_in_process::{{closure}}::h4bf2a1b8d80f48a4
                                           at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/test/src/lib.rs:650:60
               9:        0x104c5a5f6 - <core::panic::unwind_safe::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once::h2a6a680a145c7d41
                                           at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/core/src/panic/unwind_safe.rs:272:9
              10:        0x104c5a5f6 - std::panicking::try::do_call::hcdf53f9d5a8b5148
                                           at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/panicking.rs:552:40
              11:        0x104c5a5f6 - std::panicking::try::h9a226d31f3c36360
                                           at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/panicking.rs:516:19
              12:        0x104c5a5f6 - std::panic::catch_unwind::h5b58dc40b0f29757
                                           at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/panic.rs:142:14
              13:        0x104c5a5f6 - test::run_test_in_process::h43aabdb785ed33c8
                                           at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/test/src/lib.rs:650:27
              14:        0x104c5a5f6 - test::run_test::{{closure}}::h145bd2dd944a0cb1
                                           at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/test/src/lib.rs:573:43
              15:        0x104c22601 - test::run_test::{{closure}}::h852234e57a56a781
                                           at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/test/src/lib.rs:601:41
              16:        0x104c22601 - std::sys_common::backtrace::__rust_begin_short_backtrace::h624da82491b2ecec
                                           at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/sys_common/backtrace.rs:155:18
              17:        0x104c274a4 - std::thread::Builder::spawn_unchecked_::{{closure}}::{{closure}}::hca064782a57d6f7c
                                           at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/thread/mod.rs:529:17
              18:        0x104c274a4 - <core::panic::unwind_safe::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once::h2e1573cca99488b0
                                           at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/core/src/panic/unwind_safe.rs:272:9
              19:        0x104c274a4 - std::panicking::try::do_call::h6737b46edca0dd74
                                           at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/panicking.rs:552:40
              20:        0x104c274a4 - std::panicking::try::hfbb82914e6ff3120
                                           at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/panicking.rs:516:19
              21:        0x104c274a4 - std::panic::catch_unwind::hcbfd11b41e609449
                                           at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/panic.rs:142:14
              22:        0x104c274a4 - std::thread::Builder::spawn_unchecked_::{{closure}}::h0235d2d195c46467
                                           at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/thread/mod.rs:528:30
              23:        0x104c274a4 - core::ops::function::FnOnce::call_once{{vtable.shim}}::hcc0ed71d7fcc1527
                                           at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/core/src/ops/function.rs:250:5
              24:        0x104cf69f9 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::hef77fdfabbdc0490
                                           at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/alloc/src/boxed.rs:2015:9
              25:        0x104cf69f9 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::hd4d34ecf6438f9ac
                                           at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/alloc/src/boxed.rs:2015:9
              26:        0x104cf69f9 - std::sys::unix::thread::Thread::new::thread_start::hcdd70219a480b010
                                           at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/sys/unix/thread.rs:108:17
              27:     0x7ff800bfb202 - __pthread_start
            ,
            free_callers: [],
        },
    ],
    errors: [],
}

called `Result::unwrap()` on an `Err` value: Leak

[vorner]

Hello

The library uses some memory that it never frees (and the size depends on number of threads from which the library is used), but it should still be reachable and the library should be able to reuse it. In that sense, it would not be a true resource leak, but could cause tools to alert on it. It should not grow infinitely.

What exactly does the tool try to detect? Only the fact it was not freed, or also the fact it is reachable/not?