[Bug Report] AsyncPoolSink may lose the last log records and panic on program exit

[SpriteOvO]

Context: #63

---

Minimum reproducible example

use std::sync::Arc;

use spdlog::{
    prelude::*,
    sink::{AsyncPoolSink, FileSink},
};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    const LOG_FILE: &str = "/tmp/async_file_sink.log";

    let file_sink = Arc::new(FileSink::builder().path(LOG_FILE).truncate(true).build()?);
    let async_pool_sink = Arc::new(AsyncPoolSink::builder().sink(file_sink).build()?);

    let logger = Arc::new(Logger::builder().sink(async_pool_sink).build()?);
    logger.set_flush_level_filter(LevelFilter::All);
    spdlog::set_default_logger(logger);

    info!("hello async_pool_sink");

    // Workaround, adding these 2 lines at the end of `main` function
    //
    // std::thread::sleep(std::time::Duration::from_millis(100));
    // spdlog::default_logger().flush();

    Ok(())
}

Expected to see a log record in /tmp/async_file_sink.log but nothing there.

A guess is because the main function returns before AsyncPoolSink receives the last log task.

---

Another problem is that if we don't explicitly flush at the end, flush will be called in the atexit callback on program exit then there is a probability of panic.

thread '<unnamed>' panicked at library/std/src/thread/mod.rs:713:35:
use of std::thread::current() is not possible after the thread's local data has been destroyed
stack backtrace:
   0: rust_begin_unwind
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/std/src/panicking.rs:645:5
   1: core::panicking::panic_fmt
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/core/src/panicking.rs:72:14
   2: core::panicking::panic_display
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/core/src/panicking.rs:178:5
   3: core::panicking::panic_str
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/core/src/panicking.rs:152:5
   4: core::option::expect_failed
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/core/src/option.rs:1985:5
   5: core::option::Option<T>::expect
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/core/src/option.rs:894:21
   6: std::thread::current
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/std/src/thread/mod.rs:713:35
   7: crossbeam_channel::waker::current_thread_id::THREAD_ID::__init
             at ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/crossbeam-channel-0.5.11/src/waker.rs:280:38
   8: crossbeam_channel::waker::current_thread_id::THREAD_ID::__getit::{{closure}}
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/std/src/sys/common/thread_local/fast_local.rs:99:25
   9: std::sys::common::thread_local::lazy::LazyKeyInner<T>::initialize
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/std/src/sys/common/thread_local/mod.rs:54:25
  10: std::sys::common::thread_local::fast_local::Key<T>::try_initialize
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/std/src/sys/common/thread_local/fast_local.rs:190:27
  11: std::sys::common::thread_local::fast_local::Key<T>::get
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/std/src/sys/common/thread_local/fast_local.rs:173:25
  12: crossbeam_channel::waker::current_thread_id::THREAD_ID::__getit
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/std/src/sys/common/thread_local/fast_local.rs:91:21
  13: std::thread::local::LocalKey<T>::try_with
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/std/src/thread/local.rs:269:32
  14: crossbeam_channel::waker::current_thread_id
             at ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/crossbeam-channel-0.5.11/src/waker.rs:283:5
  15: crossbeam_channel::waker::Waker::try_select
             at ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/crossbeam-channel-0.5.11/src/waker.rs:83:29
  16: crossbeam_channel::waker::SyncWaker::notify
             at ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/crossbeam-channel-0.5.11/src/waker.rs:224:17
  17: crossbeam_channel::flavors::array::Channel<T>::write
             at ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/crossbeam-channel-0.5.11/src/flavors/array.rs:226:9
  18: crossbeam_channel::flavors::array::Channel<T>::send
             at ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/crossbeam-channel-0.5.11/src/flavors/array.rs:343:40
  19: crossbeam_channel::channel::Sender<T>::send
             at ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/crossbeam-channel-0.5.11/src/channel.rs:437:42
  20: spdlog::thread_pool::ThreadPool::assign_task
             at ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/spdlog-rs-0.3.13/src/thread_pool.rs:68:38
  21: spdlog::sink::async_sink::async_pool_sink::AsyncPoolSink::assign_task
             at ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/spdlog-rs-0.3.13/src/sink/async_sink/async_pool_sink.rs:60:9
  22: <spdlog::sink::async_sink::async_pool_sink::AsyncPoolSink as spdlog::sink::Sink>::flush
             at ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/spdlog-rs-0.3.13/src/sink/async_sink/async_pool_sink.rs:81:9
  23: spdlog::logger::Logger::flush_sinks::{{closure}}
             at ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/spdlog-rs-0.3.13/src/logger.rs:404:31
  24: <core::slice::iter::Iter<T> as core::iter::traits::iterator::Iterator>::for_each
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/core/src/slice/iter/macros.rs:254:21
  25: spdlog::logger::Logger::flush_sinks
             at ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/spdlog-rs-0.3.13/src/logger.rs:403:9
  26: spdlog::logger::Logger::flush
             at ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/spdlog-rs-0.3.13/src/logger.rs:160:9
  27: spdlog::flush_default_logger_at_exit::handler
             at ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/spdlog-rs-0.3.13/src/lib.rs:656:13
  28: <unknown>
  29: exit
  30: <unknown>
  31: __libc_start_main
  32: _start
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
fatal runtime error: failed to initiate panic, error 5
fish: Job 1, 'RUST_BACKTRACE=1 cargo run' terminated by signal SIGABRT (Abort)

This might be a bit hard to fix (the reason may be related to signal-safety, see #18 (comment)), but it shouldn't happen and is worth mentioning here and giving it a try.

[SpriteOvO]

Fixed in PR #66.