lancedb/python/src/runtime.rs

// SPDX-License-Identifier: Apache-2.0
// SPDX-FileCopyrightText: Copyright The LanceDB Authors

//! Fork-safe wrapper around tokio + pyo3-async-runtimes.
//!
//! `pyo3_async_runtimes::tokio` keeps its multi-threaded runtime in a
//! `OnceLock` that can never be replaced. Tokio's worker threads do not
//! survive `fork()`, so once a child inherits a "frozen" runtime, every
//! `future_into_py` call hangs forever. Normal (non-fork) process exit has
//! its own gap: nothing tells the runtime to shut down, so its worker
//! threads keep running, uncoordinated with the interpreter, right up until
//! the process ends. If one of them is mid-task exactly as `Py_Finalize`
//! starts tearing down interpreter state, it can panic on state that's
//! already gone -- and since that happens on a background thread with no
//! PyO3-wrapped call frame to catch it, Rust aborts the whole process
//! rather than failing that one call. [`shutdown`], registered as a Python
//! `atexit` callback, closes that gap by giving the runtime a coordinated,
//! bounded exit while the interpreter is still fully valid.
//!
//! Getting both of these right at once took a few tries; the design here
//! rests on three separate mechanisms, each solving one problem the others
//! cannot:
//!
//! **`OUTSTANDING`, not `Arc::strong_count`, decides when the runtime is
//! idle.** Early versions tried to infer "is anything still using this
//! runtime" from how many `Arc<Runtime>` clones existed. That signal is
//! wrong in both directions: a clone taken only for the instant a task is
//! *submitted* says nothing about whether that task has actually finished
//! running (`Runtime::shutdown_timeout` gives spawned, non-blocking tasks
//! no grace period at all -- a task "keeps running until it yields, then is
//! dropped" -- so a reclaim landing right after submission would silently
//! abandon it before it ever got to run); and a clone held for a task's
//! *whole* lifetime can end up making that task the final owner of the
//! `Runtime`, so completing it drops the `Runtime` from inside one of its
//! own worker threads, which tokio itself forbids ("cannot drop a runtime
//! in a context where blocking is not allowed") and panics. `OUTSTANDING`
//! is an explicit courtesy counter instead: every top-level `spawn`,
//! `spawn_blocking`, or `block_on` call increments it before it starts and
//! decrements it (via [`OutstandingGuard`]) when it is truly done, entirely
//! decoupled from how many `Arc` clones exist at any instant. `shutdown`
//! waits for it to reach zero before ever touching the runtime, which also
//! closes a narrower race: because the counter is incremented *before*
//! `get_runtime()` is even called, an install already in progress when
//! `shutdown` runs is never invisible to it the way an empty slot would be.
//! If the counter never reaches zero within the bound, `shutdown` stops
//! waiting and forces the retirement attempt anyway -- silently returning
//! with the runtime and its workers still fully alive would just recreate
//! the exact race this function exists to close, for any call slower than
//! the grace period.
//!
//! **Tasks never hold an `Arc<Runtime>`.** Because `OUTSTANDING` (not
//! reference counting) is what `shutdown` waits on, a top-level task only
//! needs to carry an `OutstandingGuard` -- a token whose `Drop` is a plain
//! atomic decrement -- not a clone of the runtime itself. That is what
//! makes it impossible for a task's completion to become the final,
//! worker-thread-side drop of the `Runtime`: nothing a task holds ever
//! *is* the `Runtime`.
//!
//! **`atfork_child` touches nothing but a plain counter.** `future_into_py`
//! spawns a task that, once running, spawns a second one to do the real
//! work and awaits its `JoinHandle`; if a nested call re-resolved "the
//! current runtime" independently, a reclaim landing between the two calls
//! could bind them to different instances. `spawn`/`spawn_blocking` close
//! that with `Handle::try_current`: a call already running on one of our
//! worker threads stays pinned to that instance, so only the first,
//! outermost call of a chain ever consults [`get_runtime`]. That leaves
//! fork as the other place identity can change, and it has to be handled
//! without ever calling into `ArcSwapOption` from the child handler itself
//! -- `swap`/`compare_and_swap` reconcile reader "debts" internally (via
//! thread-local state, and potentially an allocation), none of which is
//! safe to run in a forked child that may have inherited another thread's
//! lock mid-acquisition. `atfork_child` therefore does nothing but bump a
//! bare `GENERATION` counter; [`get_runtime`] compares the generation its
//! installed runtime was built in against the live counter on every call,
//! from ordinary (non-signal) context, and treats a mismatch as "stale,
//! rebuild" -- exactly the check `atfork_child` used to perform directly.

use std::future::Future;
use std::pin::Pin;
use std::sync::Arc;
use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};
use std::time::{Duration, Instant};

use arc_swap::ArcSwapOption;
use pyo3::{Bound, PyAny, PyResult, Python, conversion::IntoPyObject};
use pyo3_async_runtimes::{
    TaskLocals,
    generic::{ContextExt, JoinError, Runtime},
};
use tokio::{runtime, task};

/// A runtime tagged with the fork generation it was built in, so a stale
/// (post-fork, dead-worker-threads) instance can be told apart from a live
/// one without `atfork_child` ever having to touch it directly.
struct Tagged {
    runtime: runtime::Runtime,
    generation: u64,
}

impl std::ops::Deref for Tagged {
    type Target = runtime::Runtime;
    fn deref(&self) -> &runtime::Runtime {
        &self.runtime
    }
}

static RUNTIME: ArcSwapOption<Tagged> = ArcSwapOption::const_empty();
/// Bumped only by `atfork_child`, and only ever read elsewhere. This is the
/// entire fork-safety mechanism: no lock, no allocation, no thread-local
/// access -- just one atomic add, which is all a `pthread_atfork` child
/// handler is ever safe to do.
static GENERATION: AtomicU64 = AtomicU64::new(0);
/// Count of top-level `spawn`/`spawn_blocking`/`block_on` calls that have
/// started but not yet finished. See the module docs for why this, and not
/// `Arc::strong_count`, is what `shutdown` waits on.
static OUTSTANDING: AtomicU64 = AtomicU64::new(0);
static ATFORK_INSTALLED: AtomicBool = AtomicBool::new(false);

fn create_runtime() -> runtime::Runtime {
    runtime::Builder::new_multi_thread()
        .enable_all()
        .thread_name("lancedb-tokio-worker")
        .build()
        .expect("Failed to build tokio runtime")
}

/// Get a live, owned handle to the shared runtime, rebuilding it if the
/// installed one predates the most recent `fork()`.
fn get_runtime() -> Arc<Tagged> {
    let current_gen = GENERATION.load(Ordering::SeqCst);
    loop {
        let existing = RUNTIME.load_full();
        if let Some(existing) = &existing
            && existing.generation == current_gen
        {
            return Arc::clone(existing);
        }
        if !ATFORK_INSTALLED.fetch_or(true, Ordering::SeqCst) {
            install_atfork();
        }
        // Built optimistically, outside any lock: on the rare race where two
        // threads both find the slot empty (or stale), one candidate wins
        // the compare-and-swap below and the other is simply dropped here,
        // tearing down its own (never shared, never used) worker pool the
        // ordinary way.
        let candidate = Arc::new(Tagged {
            runtime: create_runtime(),
            generation: current_gen,
        });
        let previous = RUNTIME.compare_and_swap(&existing, Some(Arc::clone(&candidate)));
        let won = match (&*previous, &existing) {
            (None, None) => true,
            (Some(prev), Some(exist)) => Arc::ptr_eq(prev, exist),
            _ => false,
        };
        if won {
            if let Some(stale) = existing {
                // A prior generation's runtime: its worker threads are dead
                // in this process (they do not survive fork), so dropping it
                // normally would try to join them and hang. Leak it instead.
                std::mem::forget(stale);
            }
            return candidate;
        }
        // Someone else's candidate (or a concurrent shutdown) won; go around
        // and reload.
    }
}

/// RAII token tracked by [`OUTSTANDING`]. Held for the duration of a
/// top-level `block_on` call, or moved into a top-level `spawn`/
/// `spawn_blocking` task so it decrements only once that task's *entire*
/// body -- including anything it nested-spawns and awaits -- has run to
/// completion or been dropped without completing.
struct OutstandingGuard;

impl OutstandingGuard {
    fn new() -> Self {
        OUTSTANDING.fetch_add(1, Ordering::SeqCst);
        Self
    }
}

impl Drop for OutstandingGuard {
    fn drop(&mut self) {
        OUTSTANDING.fetch_sub(1, Ordering::SeqCst);
    }
}

/// Block the current thread on a future using the shared runtime.
///
/// For sync `#[pyfunction]`s that need to drive an async operation (e.g.
/// building a namespace client). Must not be called from within the runtime's
/// own worker threads.
pub fn block_on<F: std::future::Future>(fut: F) -> F::Output {
    let _guard = OutstandingGuard::new();
    get_runtime().block_on(fut)
}

/// Gracefully quiesce the shared runtime, meant to run at normal process exit.
///
/// Waits (bounded by `timeout`) for [`OUTSTANDING`] to reach zero -- i.e.
/// for every top-level call already under way to actually finish, not just
/// for `Arc::strong_count` to look low -- before ever touching the runtime.
/// If the bound elapses first, it stops waiting and attempts retirement
/// anyway: leaving the runtime and its worker threads untouched would just
/// recreate the exact race this function exists to close, for any call
/// slower than the grace period.
///
/// Retirement itself removes the runtime from the slot and calls
/// `shutdown_timeout` rather than a bare `drop`: dropping a tokio `Runtime`
/// waits (in the worst case indefinitely) for its worker threads to join,
/// whereas `shutdown_timeout` gives real in-flight work -- a connection
/// pool's keep-alive, a graceful close -- a bounded chance to finish first,
/// then forcibly ends whatever has not. Reclaiming can only proceed once
/// `Arc::try_unwrap` proves no other reference remains; if some transient
/// `get_runtime()` caller is, at that exact instant, still between loading
/// the slot and finishing its own call, this abandons the runtime instead
/// of forcing the issue -- the same trade `atfork_child` already makes.
pub fn shutdown(timeout: Duration) {
    let deadline = Instant::now() + timeout;
    loop {
        if OUTSTANDING.load(Ordering::SeqCst) == 0 {
            break;
        }
        if Instant::now() >= deadline {
            break;
        }
        std::thread::sleep(Duration::from_millis(1));
    }
    let Some(current) = RUNTIME.load_full() else {
        return;
    };
    RUNTIME.compare_and_swap(&Some(Arc::clone(&current)), None);
    if let Ok(tagged) = Arc::try_unwrap(current) {
        tagged
            .runtime
            .shutdown_timeout(deadline.saturating_duration_since(Instant::now()));
    }
}

/// Runs in async-signal context after `fork()` in the child. Touches
/// nothing but a plain atomic add -- see the module docs for why even
/// `ArcSwapOption::swap` is not safe to call here.
extern "C" fn atfork_child() {
    GENERATION.fetch_add(1, Ordering::SeqCst);
}

#[cfg(not(windows))]
fn install_atfork() {
    unsafe { libc::pthread_atfork(None, None, Some(atfork_child)) };
}

#[cfg(windows)]
fn install_atfork() {}

/// Marker type implementing [`Runtime`] over our fork-safe runtime slot.
pub struct LanceRuntime;

/// Newtype wrapper around `tokio::task::JoinError` so we can implement the
/// foreign [`JoinError`] trait without violating orphan rules.
pub struct LanceJoinError(task::JoinError);

impl JoinError for LanceJoinError {
    fn is_panic(&self) -> bool {
        self.0.is_panic()
    }
    fn into_panic(self) -> Box<dyn std::any::Any + Send + 'static> {
        self.0.into_panic()
    }
}

impl Runtime for LanceRuntime {
    type JoinError = LanceJoinError;
    type JoinHandle = Pin<Box<dyn Future<Output = Result<(), Self::JoinError>> + Send>>;

    /// `pyo3_async_runtimes::generic::future_into_py` spawns a task that,
    /// once it starts running, spawns a second one to do the real work and
    /// awaits its `JoinHandle`. `Handle::try_current` pins that nested call
    /// to whatever runtime is already executing it, so only the first,
    /// outermost call of a chain -- one running on a thread outside any
    /// runtime -- ever consults [`get_runtime`] or [`OUTSTANDING`].
    fn spawn<F>(fut: F) -> Self::JoinHandle
    where
        F: Future<Output = ()> + Send + 'static,
    {
        let handle = match tokio::runtime::Handle::try_current() {
            Ok(handle) => handle.spawn(fut),
            Err(_) => {
                let guard = OutstandingGuard::new();
                get_runtime().spawn(async move {
                    let _guard = guard;
                    fut.await;
                })
            }
        };
        Box::pin(async move { handle.await.map_err(LanceJoinError) })
    }

    fn spawn_blocking<F>(f: F) -> Self::JoinHandle
    where
        F: FnOnce() + Send + 'static,
    {
        let handle = match tokio::runtime::Handle::try_current() {
            Ok(handle) => handle.spawn_blocking(f),
            Err(_) => {
                let guard = OutstandingGuard::new();
                get_runtime().spawn_blocking(move || {
                    let _guard = guard;
                    f();
                })
            }
        };
        Box::pin(async move { handle.await.map_err(LanceJoinError) })
    }
}

tokio::task_local! {
    static TASK_LOCALS: std::cell::OnceCell<TaskLocals>;
}

impl ContextExt for LanceRuntime {
    fn scope<F, R>(locals: TaskLocals, fut: F) -> Pin<Box<dyn Future<Output = R> + Send>>
    where
        F: Future<Output = R> + Send + 'static,
    {
        let cell = std::cell::OnceCell::new();
        cell.set(locals).unwrap();
        Box::pin(TASK_LOCALS.scope(cell, fut))
    }

    fn get_task_locals() -> Option<TaskLocals> {
        TASK_LOCALS
            .try_with(|c| c.get().cloned())
            .unwrap_or_default()
    }
}

/// Drop-in replacement for `pyo3_async_runtimes::tokio::future_into_py` that
/// uses our fork-safe runtime.
pub fn future_into_py<F, T>(py: Python<'_>, fut: F) -> PyResult<Bound<'_, PyAny>>
where
    F: Future<Output = PyResult<T>> + Send + 'static,
    T: for<'py> IntoPyObject<'py> + Send + 'static,
{
    pyo3_async_runtimes::generic::future_into_py::<LanceRuntime, _, T>(py, fut)
}

#[cfg(test)]
mod tests {
    use super::*;

    // A task's own completion must never be the final drop of the shared
    // `Runtime`: tasks carry only an `OutstandingGuard` (a plain counter
    // token), never an `Arc<Runtime>`, specifically so this can't happen.
    // Getting this wrong panics ("cannot drop a runtime in a context where
    // blocking is not allowed") -- this reproduced unprompted, twice, in a
    // single run of `test_nested_spawn_survives_concurrent_shutdown` under
    // an earlier design that held the `Arc` for a task's whole lifetime.
    #[test]
    #[allow(unused_must_use)] // fire-and-forget spawn, same as future_into_py itself
    fn test_top_level_task_survives_concurrent_shutdown_reclaim() {
        use std::sync::mpsc;

        for _ in 0..50 {
            let (tx, rx) = mpsc::channel::<()>();

            LanceRuntime::spawn(async move {
                for _ in 0..5 {
                    task::yield_now().await;
                }
                let _ = tx.send(());
            });

            shutdown(Duration::from_secs(2));

            rx.recv_timeout(Duration::from_secs(5))
                .expect("top-level task was abandoned by a concurrent shutdown reclaim");
        }
    }

    #[test]
    fn test_shutdown_stops_and_the_runtime_rebuilds_lazily_after() {
        // No runtime created yet in this process: shutdown must be a no-op,
        // not a null-pointer dereference.
        shutdown(Duration::from_secs(1));

        // Force the runtime into existence, then shut it down. Bounded by
        // the timeout, so a hang here means shutdown itself is broken, not
        // that the test is slow.
        assert_eq!(block_on(async { 1 + 1 }), 2);
        shutdown(Duration::from_secs(5));

        // A caller after shutdown -- e.g. a stray call racing with the
        // atexit callback -- must get a fresh, working runtime rather than
        // a dangling reference into the one just torn down.
        assert_eq!(block_on(async { 2 + 2 }), 4);

        // Shutting down twice in a row (e.g. atexit firing more than once)
        // must not panic or double-free.
        shutdown(Duration::from_secs(5));
        shutdown(Duration::from_secs(5));
    }

    // Adapted from the concurrent reproducer that found the original bug:
    // many threads hammering the runtime while shutdown races them, not
    // just the sequential rebuild path above. Before every reader held its
    // own `Arc`, this could dereference a runtime `shutdown` had already
    // freed, or hang forever. Repeated, since a race is not guaranteed to
    // show up on any single attempt.
    #[test]
    fn test_shutdown_is_safe_concurrently_with_live_callers() {
        use std::sync::Barrier;
        use std::sync::atomic::AtomicBool as StopFlag;

        for _ in 0..50 {
            let barrier = Arc::new(Barrier::new(9));
            let stop = Arc::new(StopFlag::new(false));
            let workers: Vec<_> = (0..8)
                .map(|_| {
                    let barrier = Arc::clone(&barrier);
                    let stop = Arc::clone(&stop);
                    std::thread::spawn(move || {
                        barrier.wait();
                        while !stop.load(Ordering::Relaxed) {
                            assert_eq!(block_on(async { 1 + 1 }), 2);
                        }
                    })
                })
                .collect();

            barrier.wait();
            shutdown(Duration::from_millis(50));
            stop.store(true, Ordering::Relaxed);
            for worker in workers {
                worker.join().unwrap();
            }
        }
    }

    // Reproduces the actual bug mechanism, not just concurrent `block_on`
    // traffic: `future_into_py` spawns an outer task that, once running,
    // spawns a second (inner) one for the real work and awaits its
    // `JoinHandle` (see `pyo3_async_runtimes::generic::future_into_py_with_locals`).
    // Before `spawn`/`spawn_blocking` pinned nested calls to whatever
    // runtime is already executing them, a reclaim landing between the
    // outer and inner spawn could bind them to two different runtime
    // instances -- and if the outer task's own runtime was the one torn
    // down while it awaited the inner task, it never resumed. Every wait
    // here is bounded so a reintroduced bug fails this test instead of
    // hanging the suite.
    #[test]
    #[allow(unused_must_use)] // fire-and-forget outer spawn, same as future_into_py itself
    fn test_nested_spawn_survives_concurrent_shutdown() {
        use std::sync::mpsc;

        for _ in 0..50 {
            let stop = Arc::new(AtomicBool::new(false));
            let (done_tx, done_rx) = mpsc::channel::<()>();

            let workers: Vec<_> = (0..8)
                .map(|_| {
                    let stop = Arc::clone(&stop);
                    let done_tx = done_tx.clone();
                    std::thread::spawn(move || {
                        while !stop.load(Ordering::Relaxed) {
                            let (tx, rx) = mpsc::sync_channel::<()>(1);
                            LanceRuntime::spawn(async move {
                                let inner = LanceRuntime::spawn(async {
                                    let _ = 1 + 1;
                                });
                                let _ = inner.await;
                                let _ = tx.send(());
                            });
                            // A hang here across a concurrent shutdown is
                            // exactly the bug this test exists to catch.
                            let _ = rx.recv_timeout(Duration::from_secs(2));
                        }
                        let _ = done_tx.send(());
                    })
                })
                .collect();

            std::thread::sleep(Duration::from_millis(10));
            shutdown(Duration::from_millis(50));
            stop.store(true, Ordering::Relaxed);

            for _ in 0..8 {
                done_rx
                    .recv_timeout(Duration::from_secs(10))
                    .expect("worker hung after shutdown raced a nested spawn");
            }
            for worker in workers {
                worker.join().unwrap();
            }
        }
    }

    // Forces the exact interleaving the install-race finding described: an
    // installer registers as outstanding (as `spawn`'s outermost branch
    // does, before it ever calls `get_runtime()`) and then pauses, while a
    // concurrent `shutdown()` must not decide "nothing here" and return
    // before that install actually completes and is retired in turn.
    #[test]
    fn test_shutdown_waits_for_a_racing_install() {
        use std::sync::mpsc;

        // Clean slate: no runtime installed, OUTSTANDING at zero.
        shutdown(Duration::from_secs(5));

        let (installer_ready_tx, installer_ready_rx) = mpsc::channel::<()>();
        let (proceed_tx, proceed_rx) = mpsc::channel::<()>();

        let installer = std::thread::spawn(move || {
            let _guard = OutstandingGuard::new();
            installer_ready_tx.send(()).unwrap();
            proceed_rx.recv().unwrap();
            assert_eq!(get_runtime().block_on(async { 1 + 1 }), 2);
        });

        installer_ready_rx.recv().unwrap();
        let shutdown_thread = std::thread::spawn(|| shutdown(Duration::from_secs(5)));
        // Give shutdown's polling loop several chances to (wrongly) observe
        // an idle runtime before the installer is allowed to proceed.
        std::thread::sleep(Duration::from_millis(50));
        proceed_tx.send(()).unwrap();

        installer.join().unwrap();
        shutdown_thread.join().unwrap();

        // shutdown must have waited for the install to finish and then
        // retired it, not returned early and left it stranded.
        assert!(RUNTIME.load_full().is_none());
        assert_eq!(OUTSTANDING.load(Ordering::SeqCst), 0);
    }
}