monarch_hyperactor/

runtime.rs

/*
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 * All rights reserved.
 *
 * This source code is licensed under the BSD-style license found in the
 * LICENSE file in the root directory of this source tree.
 */

use std::future::Future;
use std::pin::Pin;
use std::sync::OnceLock;
use std::sync::RwLock;
use std::sync::RwLockReadGuard;
use std::sync::atomic::AtomicUsize;
use std::sync::atomic::Ordering;
use std::time::Duration;

use anyhow::Result;
use hyperactor::Proc;
use hyperactor::channel::ChannelAddr;
use hyperactor::channel::ChannelTransport;
use hyperactor::mailbox::BoxedMailboxSender;
use hyperactor::mailbox::PanickingMailboxSender;
use hyperactor::reference;
use once_cell::sync::Lazy;
use once_cell::unsync::OnceCell as UnsyncOnceCell;
use pyo3::PyResult;
use pyo3::Python;
use pyo3::exceptions::PyRuntimeError;
use pyo3::prelude::*;
use pyo3::types::PyAnyMethods;
use pyo3_async_runtimes::TaskLocals;
use tokio::task;

// this must be a RwLock and only return a guard for reading the runtime.
// Otherwise multiple threads can deadlock fighting for the Runtime object if they hold it
// while blocking on something.
static INSTANCE: std::sync::LazyLock<RwLock<Option<tokio::runtime::Runtime>>> =
    std::sync::LazyLock::new(|| RwLock::new(None));

pub fn get_tokio_runtime<'l>() -> std::sync::MappedRwLockReadGuard<'l, tokio::runtime::Runtime> {
    // First try to get a read lock and check if runtime exists
    {
        let read_guard = INSTANCE.read().unwrap();
        if read_guard.is_some() {
            return RwLockReadGuard::map(read_guard, |lock: &Option<tokio::runtime::Runtime>| {
                lock.as_ref().unwrap()
            });
        }
        // Drop the read lock by letting it go out of scope
    }

    // Runtime doesn't exist, upgrade to write lock to initialize
    let mut write_guard = INSTANCE.write().unwrap();
    if write_guard.is_none() {
        *write_guard = Some(
            tokio::runtime::Builder::new_multi_thread()
                .thread_name_fn(|| {
                    static ATOMIC_ID: AtomicUsize = AtomicUsize::new(0);
                    let id = ATOMIC_ID.fetch_add(1, Ordering::SeqCst);
                    format!("monarch-pytokio-worker-{}", id)
                })
                .enable_all()
                .build()
                .unwrap(),
        );
    }

    // Downgrade write lock to read lock and return the reference
    let read_guard = std::sync::RwLockWriteGuard::downgrade(write_guard);
    RwLockReadGuard::map(read_guard, |lock: &Option<tokio::runtime::Runtime>| {
        lock.as_ref().unwrap()
    })
}

#[pyfunction]
pub fn shutdown_tokio_runtime(py: Python<'_>) {
    // Called from Python's atexit, which holds the GIL. Release it so tokio
    // worker threads can acquire it to complete their Python work.
    py.detach(|| {
        if let Some(x) = INSTANCE.write().unwrap().take() {
            x.shutdown_timeout(Duration::from_secs(1));
        }
    });
}

/// A global runtime proc used by this crate.
pub(crate) fn get_proc_runtime() -> &'static Proc {
    static RUNTIME_PROC: OnceLock<Proc> = OnceLock::new();
    RUNTIME_PROC.get_or_init(|| {
        let addr = ChannelAddr::any(ChannelTransport::Local);
        let proc_id = reference::ProcId::unique(addr, "monarch_hyperactor_runtime");
        Proc::configured(proc_id, BoxedMailboxSender::new(PanickingMailboxSender))
    })
}

/// Stores the native thread ID of the main Python thread.
/// This is lazily initialized on first call to `is_main_thread`.
static MAIN_THREAD_NATIVE_ID: OnceLock<i64> = OnceLock::new();

/// Returns the native thread ID of the main Python thread.
/// On first call, looks it up via `threading.main_thread().native_id`.
fn get_main_thread_native_id() -> i64 {
    *MAIN_THREAD_NATIVE_ID.get_or_init(|| {
        Python::attach(|py| {
            let threading = py.import("threading").expect("failed to import threading");
            let main_thread = threading
                .call_method0("main_thread")
                .expect("failed to get main_thread");
            main_thread
                .getattr("native_id")
                .expect("failed to get native_id")
                .extract::<i64>()
                .expect("native_id is not an i64")
        })
    })
}

/// Returns the current thread's native ID in a cross-platform way.
#[cfg(target_os = "linux")]
fn get_current_thread_id() -> i64 {
    nix::unistd::gettid().as_raw() as i64
}

/// Returns the current thread's native ID in a cross-platform way.
#[cfg(target_os = "macos")]
fn get_current_thread_id() -> i64 {
    let mut tid: u64 = 0;
    // pthread_threadid_np with thread=0 (null pthread_t) gets the current thread's ID.
    unsafe {
        let ret = libc::pthread_threadid_np(0, &mut tid);
        debug_assert_eq!(
            ret, 0,
            "pthread_threadid_np failed with error code: {}",
            ret
        );
    }
    // macOS thread IDs are u64 so we need to convert to i64.
    debug_assert!(tid <= i64::MAX as u64, "thread ID {} exceeds i64::MAX", tid);
    tid as i64
}

/// Returns the current thread's native ID in a cross-platform way.
#[cfg(not(any(target_os = "linux", target_os = "macos")))]
compile_error!("get_current_thread_id is only implemented for Linux and macOS");

/// Returns true if the current thread is the main Python thread.
/// Compares the current thread's native ID against the main Python thread's native ID.
pub fn is_main_thread() -> bool {
    let current_tid = get_current_thread_id();
    current_tid == get_main_thread_native_id()
}

pub fn initialize(py: Python) -> Result<()> {
    // Eagerly initialize the main thread ID while we're on the main thread
    // with the GIL held. If this were lazily initialized on a background
    // tokio thread during shutdown, the `py.import("threading")` call inside
    // get_main_thread_native_id() would trigger module_from_spec on a
    // partially-finalized interpreter, causing a segfault.
    let _ = get_main_thread_native_id();

    let atexit = py.import("atexit")?;
    let shutdown_fn = wrap_pyfunction!(shutdown_tokio_runtime, py)?;
    atexit.call_method1("register", (shutdown_fn,))?;
    Ok(())
}

/// Block the current thread on a future, but make sure to check for signals
/// originating from the Python signal handler.
///
/// Python's signal handler just sets a flag that it expects the Python
/// interpreter to handle later via a call to `PyErr_CheckSignals`. When we
/// enter into potentially long-running native code, we need to make sure to be
/// checking for signals frequently, otherwise we will ignore them. This will
/// manifest as `ctrl-C` not doing anything.
///
/// One additional wrinkle is that `PyErr_CheckSignals` only works on the main
/// Python thread; if it's called on any other thread it silently does nothing.
/// So, we check if we're on the main thread by comparing native thread IDs.
pub fn signal_safe_block_on<F>(py: Python, future: F) -> PyResult<F::Output>
where
    F: Future + Send + 'static,
    F::Output: Send + 'static,
{
    let runtime = get_tokio_runtime();
    // Release the GIL, otherwise the work in `future` that tries to acquire the
    // GIL on another thread may deadlock.
    py.detach(|| {
        if is_main_thread() {
            // Spawn the future onto the tokio runtime
            let handle = runtime.spawn(future);
            // Block the current thread on waiting for *either* the future to
            // complete or a signal.
            runtime.block_on(async {
                tokio::select! {
                    result = handle => result.map_err(|e| PyRuntimeError::new_err(format!("JoinErr: {:?}", e))),
                    signal = async {
                        let sleep_for = std::time::Duration::from_millis(100);
                        loop {
                            // Acquiring the GIL in a loop is sad, hopefully once
                            // every 100ms is fine.
                            Python::attach(|py| py.check_signals())?;
                            tokio::time::sleep(sleep_for).await;
                        }
                    } => signal
                }
            })
        } else {
            // If we're not on the main thread, we can just block it. We've
            // released the GIL, so the Python main thread will continue on, and
            // `PyErr_CheckSignals` doesn't do anything anyway.
            Ok(runtime.block_on(future))
        }
    })
}

/// A test function that sleeps indefinitely in a loop.
/// This is used for testing signal handling in signal_safe_block_on.
/// The function will sleep forever until interrupted by a signal.
#[pyfunction]
pub fn sleep_indefinitely_for_unit_tests(py: Python) -> PyResult<()> {
    // Create a future that sleeps indefinitely
    let future = async {
        loop {
            tracing::info!("idef sleeping for 100ms");
            tokio::time::sleep(Duration::from_millis(100)).await;
        }
    };

    // Use signal_safe_block_on to run the future, which should make it
    // interruptible by signals like SIGINT
    signal_safe_block_on(py, future)
}

/// Initialize the runtime module and expose Python functions
pub fn register_python_bindings(runtime_mod: &Bound<'_, PyModule>) -> PyResult<()> {
    let sleep_indefinitely_fn =
        wrap_pyfunction!(sleep_indefinitely_for_unit_tests, runtime_mod.py())?;
    sleep_indefinitely_fn.setattr(
        "__module__",
        "monarch._rust_bindings.monarch_hyperactor.runtime",
    )?;
    runtime_mod.add_function(sleep_indefinitely_fn)?;
    Ok(())
}

struct SimpleRuntime;

impl pyo3_async_runtimes::generic::Runtime for SimpleRuntime {
    type JoinError = task::JoinError;
    type JoinHandle = task::JoinHandle<()>;

    fn spawn<F>(fut: F) -> Self::JoinHandle
    where
        F: Future<Output = ()> + Send + 'static,
    {
        get_tokio_runtime().spawn(async move {
            fut.await;
        })
    }
}

tokio::task_local! {
    static TASK_LOCALS: UnsyncOnceCell<TaskLocals>;
}

impl pyo3_async_runtimes::generic::ContextExt for SimpleRuntime {
    fn scope<F, R>(locals: TaskLocals, fut: F) -> Pin<Box<dyn Future<Output = R> + Send>>
    where
        F: Future<Output = R> + Send + 'static,
    {
        let cell = UnsyncOnceCell::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()
                    .map(|locals| monarch_with_gil_blocking(|py| locals.clone_ref(py)))
            })
            .unwrap_or_default()
    }
}

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>,
{
    pyo3_async_runtimes::generic::future_into_py::<SimpleRuntime, F, T>(py, fut)
}

/// Global lock to serialize GIL acquisition from Rust threads in async contexts.
///
/// Under high concurrency, many async tasks can simultaneously try to acquire the GIL.
/// Each call blocks the current tokio worker thread, which can cause runtime starvation
/// and apparent deadlocks (nothing else gets polled).
///
/// This wrapper serializes GIL acquisition among callers that opt in, so at most one
/// tokio task is blocked in `Python::attach` at a time, improving fairness under
/// contention.
///
/// Note: this does not globally prevent other sync code from calling `Python::attach`
/// directly. Use `monarch_with_gil` or `monarch_with_gil_blocking` for Python interaction
/// that occurs on async hot paths.
static GIL_LOCK: Lazy<tokio::sync::Mutex<()>> = Lazy::new(|| tokio::sync::Mutex::new(()));

// Thread-local depth counter for re-entrant GIL acquisition.
//
// This tracks when we're already inside a `monarch_with_gil` or `monarch_with_gil_blocking`
// call. On re-entry (e.g., when Python calls back into Rust while we're already executing
// under `Python::attach`), we bypass the `GIL_LOCK` to avoid deadlocks.
//
// Without this, the following scenario would deadlock:
// 1. Rust async code calls `monarch_with_gil`, acquires `GIL_LOCK`
// 2. Inside the closure, Python code is executed
// 3. Python code calls back into Rust (e.g., via a PyO3 callback)
// 4. The callback tries to call `monarch_with_gil` again
// 5. DEADLOCK: waiting for `GIL_LOCK` which is held by the same logical call chain
thread_local! {
    static GIL_DEPTH: std::cell::Cell<u32> = const { std::cell::Cell::new(0) };
}

/// RAII guard that decrements the GIL depth counter when dropped.
struct GilDepthGuard {
    prev_depth: u32,
}

impl Drop for GilDepthGuard {
    fn drop(&mut self) {
        GIL_DEPTH.with(|d| d.set(self.prev_depth));
    }
}

/// Increments the GIL depth counter and returns a guard that restores it on drop.
fn increment_gil_depth() -> GilDepthGuard {
    let prev_depth = GIL_DEPTH.with(|d| {
        let current = d.get();
        d.set(current + 1);
        current
    });
    GilDepthGuard { prev_depth }
}

/// Returns true if we're already inside a `monarch_with_gil` call (re-entrant).
fn is_reentrant() -> bool {
    GIL_DEPTH.with(|d| d.get() > 0)
}

/// Async wrapper around `Python::attach` intended for async call sites.
///
/// Why: under high concurrency, many async tasks can simultaneously
/// try to acquire the GIL. Each call blocks the current tokio worker
/// thread, which can cause runtime starvation / apparent deadlocks
/// (nothing else gets polled).
///
/// This wrapper serializes GIL acquisition among async callers so at most one tokio
/// task is blocked in `Python::attach` at a time, preventing runtime starvation
/// under GIL contention.
///
/// Note: this does not globally prevent other sync code from calling
/// `Python::attach` directly. Use this wrapper for Python
/// interaction that occurs on async hot paths.
///
/// # Re-entrancy Safety
///
/// This function is re-entrant safe. If called while already inside a `monarch_with_gil`
/// or `monarch_with_gil_blocking` call (e.g., from a Python→Rust callback), it bypasses
/// the `GIL_LOCK` to avoid deadlocks.
///
/// # Example
/// ```ignore
/// let result = monarch_with_gil(|py| {
///     // Do work with Python GIL
///     Ok(42)
/// })
/// .await?;
/// ```
pub async fn monarch_with_gil<F, R>(f: F) -> R
where
    F: for<'py> FnOnce(Python<'py>) -> R + Send,
{
    // If we're already inside a monarch_with_gil call (re-entrant), skip the lock
    // to avoid deadlock from Python→Rust callbacks
    if is_reentrant() {
        let _depth_guard = increment_gil_depth();
        return Python::attach(f);
    }

    // Not re-entrant: acquire the serialization lock
    let _lock_guard = GIL_LOCK.lock().await;
    let _depth_guard = increment_gil_depth();
    Python::attach(f)
}

/// Blocking wrapper around `Python::with_gil` for use in synchronous contexts.
///
/// Unlike `monarch_with_gil`, this function does NOT use the `GIL_LOCK` async mutex.
/// Since it is blocking call, it simply acquires the GIL and releases it when the
/// closure returns.
///
/// # Example
/// ```ignore
/// let result = monarch_with_gil_blocking(|py| {
///     // Do work with Python GIL
///     Ok(42)
/// })?;
/// ```
pub fn monarch_with_gil_blocking<F, R>(f: F) -> R
where
    F: for<'py> FnOnce(Python<'py>) -> R + Send,
{
    let _depth_guard = increment_gil_depth();
    Python::attach(f)
}