Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1236 insertions, 0 deletions

diff --git a/third_party/rust/tokio/src/runtime/builder.rs b/third_party/rust/tokio/src/runtime/builder.rs
new file mode 100644
index 0000000000..af9e0e172f
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/builder.rs
@@ -0,0 +1,1236 @@
+use crate::runtime::handle::Handle;
+use crate::runtime::{blocking, driver, Callback, HistogramBuilder, Runtime};
+use crate::util::rand::{RngSeed, RngSeedGenerator};
+
+use std::fmt;
+use std::io;
+use std::time::Duration;
+
+/// Builds Tokio Runtime with custom configuration values.
+///
+/// Methods can be chained in order to set the configuration values. The
+/// Runtime is constructed by calling [`build`].
+///
+/// New instances of `Builder` are obtained via [`Builder::new_multi_thread`]
+/// or [`Builder::new_current_thread`].
+///
+/// See function level documentation for details on the various configuration
+/// settings.
+///
+/// [`build`]: method@Self::build
+/// [`Builder::new_multi_thread`]: method@Self::new_multi_thread
+/// [`Builder::new_current_thread`]: method@Self::new_current_thread
+///
+/// # Examples
+///
+/// ```
+/// use tokio::runtime::Builder;
+///
+/// fn main() {
+///     // build runtime
+///     let runtime = Builder::new_multi_thread()
+///         .worker_threads(4)
+///         .thread_name("my-custom-name")
+///         .thread_stack_size(3 * 1024 * 1024)
+///         .build()
+///         .unwrap();
+///
+///     // use runtime ...
+/// }
+/// ```
+pub struct Builder {
+    /// Runtime type
+    kind: Kind,
+
+    /// Whether or not to enable the I/O driver
+    enable_io: bool,
+    nevents: usize,
+
+    /// Whether or not to enable the time driver
+    enable_time: bool,
+
+    /// Whether or not the clock should start paused.
+    start_paused: bool,
+
+    /// The number of worker threads, used by Runtime.
+    ///
+    /// Only used when not using the current-thread executor.
+    worker_threads: Option<usize>,
+
+    /// Cap on thread usage.
+    max_blocking_threads: usize,
+
+    /// Name fn used for threads spawned by the runtime.
+    pub(super) thread_name: ThreadNameFn,
+
+    /// Stack size used for threads spawned by the runtime.
+    pub(super) thread_stack_size: Option<usize>,
+
+    /// Callback to run after each thread starts.
+    pub(super) after_start: Option<Callback>,
+
+    /// To run before each worker thread stops
+    pub(super) before_stop: Option<Callback>,
+
+    /// To run before each worker thread is parked.
+    pub(super) before_park: Option<Callback>,
+
+    /// To run after each thread is unparked.
+    pub(super) after_unpark: Option<Callback>,
+
+    /// Customizable keep alive timeout for BlockingPool
+    pub(super) keep_alive: Option<Duration>,
+
+    /// How many ticks before pulling a task from the global/remote queue?
+    ///
+    /// When `None`, the value is unspecified and behavior details are left to
+    /// the scheduler. Each scheduler flavor could choose to either pick its own
+    /// default value or use some other strategy to decide when to poll from the
+    /// global queue. For example, the multi-threaded scheduler uses a
+    /// self-tuning strategy based on mean task poll times.
+    pub(super) global_queue_interval: Option<u32>,
+
+    /// How many ticks before yielding to the driver for timer and I/O events?
+    pub(super) event_interval: u32,
+
+    /// When true, the multi-threade scheduler LIFO slot should not be used.
+    ///
+    /// This option should only be exposed as unstable.
+    pub(super) disable_lifo_slot: bool,
+
+    /// Specify a random number generator seed to provide deterministic results
+    pub(super) seed_generator: RngSeedGenerator,
+
+    /// When true, enables task poll count histogram instrumentation.
+    pub(super) metrics_poll_count_histogram_enable: bool,
+
+    /// Configures the task poll count histogram
+    pub(super) metrics_poll_count_histogram: HistogramBuilder,
+
+    #[cfg(tokio_unstable)]
+    pub(super) unhandled_panic: UnhandledPanic,
+}
+
+cfg_unstable! {
+    /// How the runtime should respond to unhandled panics.
+    ///
+    /// Instances of `UnhandledPanic` are passed to `Builder::unhandled_panic`
+    /// to configure the runtime behavior when a spawned task panics.
+    ///
+    /// See [`Builder::unhandled_panic`] for more details.
+    #[derive(Debug, Clone)]
+    #[non_exhaustive]
+    pub enum UnhandledPanic {
+        /// The runtime should ignore panics on spawned tasks.
+        ///
+        /// The panic is forwarded to the task's [`JoinHandle`] and all spawned
+        /// tasks continue running normally.
+        ///
+        /// This is the default behavior.
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// use tokio::runtime::{self, UnhandledPanic};
+        ///
+        /// # pub fn main() {
+        /// let rt = runtime::Builder::new_current_thread()
+        ///     .unhandled_panic(UnhandledPanic::Ignore)
+        ///     .build()
+        ///     .unwrap();
+        ///
+        /// let task1 = rt.spawn(async { panic!("boom"); });
+        /// let task2 = rt.spawn(async {
+        ///     // This task completes normally
+        ///     "done"
+        /// });
+        ///
+        /// rt.block_on(async {
+        ///     // The panic on the first task is forwarded to the `JoinHandle`
+        ///     assert!(task1.await.is_err());
+        ///
+        ///     // The second task completes normally
+        ///     assert!(task2.await.is_ok());
+        /// })
+        /// # }
+        /// ```
+        ///
+        /// [`JoinHandle`]: struct@crate::task::JoinHandle
+        Ignore,
+
+        /// The runtime should immediately shutdown if a spawned task panics.
+        ///
+        /// The runtime will immediately shutdown even if the panicked task's
+        /// [`JoinHandle`] is still available. All further spawned tasks will be
+        /// immediately dropped and call to [`Runtime::block_on`] will panic.
+        ///
+        /// # Examples
+        ///
+        /// ```should_panic
+        /// use tokio::runtime::{self, UnhandledPanic};
+        ///
+        /// # pub fn main() {
+        /// let rt = runtime::Builder::new_current_thread()
+        ///     .unhandled_panic(UnhandledPanic::ShutdownRuntime)
+        ///     .build()
+        ///     .unwrap();
+        ///
+        /// rt.spawn(async { panic!("boom"); });
+        /// rt.spawn(async {
+        ///     // This task never completes.
+        /// });
+        ///
+        /// rt.block_on(async {
+        ///     // Do some work
+        /// # loop { tokio::task::yield_now().await; }
+        /// })
+        /// # }
+        /// ```
+        ///
+        /// [`JoinHandle`]: struct@crate::task::JoinHandle
+        ShutdownRuntime,
+    }
+}
+
+pub(crate) type ThreadNameFn = std::sync::Arc<dyn Fn() -> String + Send + Sync + 'static>;
+
+#[derive(Clone, Copy)]
+pub(crate) enum Kind {
+    CurrentThread,
+    #[cfg(all(feature = "rt-multi-thread", not(tokio_wasi)))]
+    MultiThread,
+}
+
+impl Builder {
+    /// Returns a new builder with the current thread scheduler selected.
+    ///
+    /// Configuration methods can be chained on the return value.
+    ///
+    /// To spawn non-`Send` tasks on the resulting runtime, combine it with a
+    /// [`LocalSet`].
+    ///
+    /// [`LocalSet`]: crate::task::LocalSet
+    pub fn new_current_thread() -> Builder {
+        #[cfg(loom)]
+        const EVENT_INTERVAL: u32 = 4;
+        // The number `61` is fairly arbitrary. I believe this value was copied from golang.
+        #[cfg(not(loom))]
+        const EVENT_INTERVAL: u32 = 61;
+
+        Builder::new(Kind::CurrentThread, EVENT_INTERVAL)
+    }
+
+    cfg_not_wasi! {
+        /// Returns a new builder with the multi thread scheduler selected.
+        ///
+        /// Configuration methods can be chained on the return value.
+        #[cfg(feature = "rt-multi-thread")]
+        #[cfg_attr(docsrs, doc(cfg(feature = "rt-multi-thread")))]
+        pub fn new_multi_thread() -> Builder {
+            // The number `61` is fairly arbitrary. I believe this value was copied from golang.
+            Builder::new(Kind::MultiThread, 61)
+        }
+    }
+
+    /// Returns a new runtime builder initialized with default configuration
+    /// values.
+    ///
+    /// Configuration methods can be chained on the return value.
+    pub(crate) fn new(kind: Kind, event_interval: u32) -> Builder {
+        Builder {
+            kind,
+
+            // I/O defaults to "off"
+            enable_io: false,
+            nevents: 1024,
+
+            // Time defaults to "off"
+            enable_time: false,
+
+            // The clock starts not-paused
+            start_paused: false,
+
+            // Read from environment variable first in multi-threaded mode.
+            // Default to lazy auto-detection (one thread per CPU core)
+            worker_threads: None,
+
+            max_blocking_threads: 512,
+
+            // Default thread name
+            thread_name: std::sync::Arc::new(|| "tokio-runtime-worker".into()),
+
+            // Do not set a stack size by default
+            thread_stack_size: None,
+
+            // No worker thread callbacks
+            after_start: None,
+            before_stop: None,
+            before_park: None,
+            after_unpark: None,
+
+            keep_alive: None,
+
+            // Defaults for these values depend on the scheduler kind, so we get them
+            // as parameters.
+            global_queue_interval: None,
+            event_interval,
+
+            seed_generator: RngSeedGenerator::new(RngSeed::new()),
+
+            #[cfg(tokio_unstable)]
+            unhandled_panic: UnhandledPanic::Ignore,
+
+            metrics_poll_count_histogram_enable: false,
+
+            metrics_poll_count_histogram: Default::default(),
+
+            disable_lifo_slot: false,
+        }
+    }
+
+    /// Enables both I/O and time drivers.
+    ///
+    /// Doing this is a shorthand for calling `enable_io` and `enable_time`
+    /// individually. If additional components are added to Tokio in the future,
+    /// `enable_all` will include these future components.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::runtime;
+    ///
+    /// let rt = runtime::Builder::new_multi_thread()
+    ///     .enable_all()
+    ///     .build()
+    ///     .unwrap();
+    /// ```
+    pub fn enable_all(&mut self) -> &mut Self {
+        #[cfg(any(
+            feature = "net",
+            all(unix, feature = "process"),
+            all(unix, feature = "signal")
+        ))]
+        self.enable_io();
+        #[cfg(feature = "time")]
+        self.enable_time();
+
+        self
+    }
+
+    /// Sets the number of worker threads the `Runtime` will use.
+    ///
+    /// This can be any number above 0 though it is advised to keep this value
+    /// on the smaller side.
+    ///
+    /// This will override the value read from environment variable `TOKIO_WORKER_THREADS`.
+    ///
+    /// # Default
+    ///
+    /// The default value is the number of cores available to the system.
+    ///
+    /// When using the `current_thread` runtime this method has no effect.
+    ///
+    /// # Examples
+    ///
+    /// ## Multi threaded runtime with 4 threads
+    ///
+    /// ```
+    /// use tokio::runtime;
+    ///
+    /// // This will spawn a work-stealing runtime with 4 worker threads.
+    /// let rt = runtime::Builder::new_multi_thread()
+    ///     .worker_threads(4)
+    ///     .build()
+    ///     .unwrap();
+    ///
+    /// rt.spawn(async move {});
+    /// ```
+    ///
+    /// ## Current thread runtime (will only run on the current thread via `Runtime::block_on`)
+    ///
+    /// ```
+    /// use tokio::runtime;
+    ///
+    /// // Create a runtime that _must_ be driven from a call
+    /// // to `Runtime::block_on`.
+    /// let rt = runtime::Builder::new_current_thread()
+    ///     .build()
+    ///     .unwrap();
+    ///
+    /// // This will run the runtime and future on the current thread
+    /// rt.block_on(async move {});
+    /// ```
+    ///
+    /// # Panics
+    ///
+    /// This will panic if `val` is not larger than `0`.
+    #[track_caller]
+    pub fn worker_threads(&mut self, val: usize) -> &mut Self {
+        assert!(val > 0, "Worker threads cannot be set to 0");
+        self.worker_threads = Some(val);
+        self
+    }
+
+    /// Specifies the limit for additional threads spawned by the Runtime.
+    ///
+    /// These threads are used for blocking operations like tasks spawned
+    /// through [`spawn_blocking`]. Unlike the [`worker_threads`], they are not
+    /// always active and will exit if left idle for too long. You can change
+    /// this timeout duration with [`thread_keep_alive`].
+    ///
+    /// The default value is 512.
+    ///
+    /// # Panics
+    ///
+    /// This will panic if `val` is not larger than `0`.
+    ///
+    /// # Upgrading from 0.x
+    ///
+    /// In old versions `max_threads` limited both blocking and worker threads, but the
+    /// current `max_blocking_threads` does not include async worker threads in the count.
+    ///
+    /// [`spawn_blocking`]: fn@crate::task::spawn_blocking
+    /// [`worker_threads`]: Self::worker_threads
+    /// [`thread_keep_alive`]: Self::thread_keep_alive
+    #[track_caller]
+    #[cfg_attr(docsrs, doc(alias = "max_threads"))]
+    pub fn max_blocking_threads(&mut self, val: usize) -> &mut Self {
+        assert!(val > 0, "Max blocking threads cannot be set to 0");
+        self.max_blocking_threads = val;
+        self
+    }
+
+    /// Sets name of threads spawned by the `Runtime`'s thread pool.
+    ///
+    /// The default name is "tokio-runtime-worker".
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use tokio::runtime;
+    ///
+    /// # pub fn main() {
+    /// let rt = runtime::Builder::new_multi_thread()
+    ///     .thread_name("my-pool")
+    ///     .build();
+    /// # }
+    /// ```
+    pub fn thread_name(&mut self, val: impl Into<String>) -> &mut Self {
+        let val = val.into();
+        self.thread_name = std::sync::Arc::new(move || val.clone());
+        self
+    }
+
+    /// Sets a function used to generate the name of threads spawned by the `Runtime`'s thread pool.
+    ///
+    /// The default name fn is `|| "tokio-runtime-worker".into()`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use tokio::runtime;
+    /// # use std::sync::atomic::{AtomicUsize, Ordering};
+    /// # pub fn main() {
+    /// let rt = 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!("my-pool-{}", id)
+    ///     })
+    ///     .build();
+    /// # }
+    /// ```
+    pub fn thread_name_fn<F>(&mut self, f: F) -> &mut Self
+    where
+        F: Fn() -> String + Send + Sync + 'static,
+    {
+        self.thread_name = std::sync::Arc::new(f);
+        self
+    }
+
+    /// Sets the stack size (in bytes) for worker threads.
+    ///
+    /// The actual stack size may be greater than this value if the platform
+    /// specifies minimal stack size.
+    ///
+    /// The default stack size for spawned threads is 2 MiB, though this
+    /// particular stack size is subject to change in the future.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use tokio::runtime;
+    ///
+    /// # pub fn main() {
+    /// let rt = runtime::Builder::new_multi_thread()
+    ///     .thread_stack_size(32 * 1024)
+    ///     .build();
+    /// # }
+    /// ```
+    pub fn thread_stack_size(&mut self, val: usize) -> &mut Self {
+        self.thread_stack_size = Some(val);
+        self
+    }
+
+    /// Executes function `f` after each thread is started but before it starts
+    /// doing work.
+    ///
+    /// This is intended for bookkeeping and monitoring use cases.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use tokio::runtime;
+    /// # pub fn main() {
+    /// let runtime = runtime::Builder::new_multi_thread()
+    ///     .on_thread_start(|| {
+    ///         println!("thread started");
+    ///     })
+    ///     .build();
+    /// # }
+    /// ```
+    #[cfg(not(loom))]
+    pub fn on_thread_start<F>(&mut self, f: F) -> &mut Self
+    where
+        F: Fn() + Send + Sync + 'static,
+    {
+        self.after_start = Some(std::sync::Arc::new(f));
+        self
+    }
+
+    /// Executes function `f` before each thread stops.
+    ///
+    /// This is intended for bookkeeping and monitoring use cases.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use tokio::runtime;
+    /// # pub fn main() {
+    /// let runtime = runtime::Builder::new_multi_thread()
+    ///     .on_thread_stop(|| {
+    ///         println!("thread stopping");
+    ///     })
+    ///     .build();
+    /// # }
+    /// ```
+    #[cfg(not(loom))]
+    pub fn on_thread_stop<F>(&mut self, f: F) -> &mut Self
+    where
+        F: Fn() + Send + Sync + 'static,
+    {
+        self.before_stop = Some(std::sync::Arc::new(f));
+        self
+    }
+
+    /// Executes function `f` just before a thread is parked (goes idle).
+    /// `f` is called within the Tokio context, so functions like [`tokio::spawn`](crate::spawn)
+    /// can be called, and may result in this thread being unparked immediately.
+    ///
+    /// This can be used to start work only when the executor is idle, or for bookkeeping
+    /// and monitoring purposes.
+    ///
+    /// Note: There can only be one park callback for a runtime; calling this function
+    /// more than once replaces the last callback defined, rather than adding to it.
+    ///
+    /// # Examples
+    ///
+    /// ## Multithreaded executor
+    /// ```
+    /// # use std::sync::Arc;
+    /// # use std::sync::atomic::{AtomicBool, Ordering};
+    /// # use tokio::runtime;
+    /// # use tokio::sync::Barrier;
+    /// # pub fn main() {
+    /// let once = AtomicBool::new(true);
+    /// let barrier = Arc::new(Barrier::new(2));
+    ///
+    /// let runtime = runtime::Builder::new_multi_thread()
+    ///     .worker_threads(1)
+    ///     .on_thread_park({
+    ///         let barrier = barrier.clone();
+    ///         move || {
+    ///             let barrier = barrier.clone();
+    ///             if once.swap(false, Ordering::Relaxed) {
+    ///                 tokio::spawn(async move { barrier.wait().await; });
+    ///            }
+    ///         }
+    ///     })
+    ///     .build()
+    ///     .unwrap();
+    ///
+    /// runtime.block_on(async {
+    ///    barrier.wait().await;
+    /// })
+    /// # }
+    /// ```
+    /// ## Current thread executor
+    /// ```
+    /// # use std::sync::Arc;
+    /// # use std::sync::atomic::{AtomicBool, Ordering};
+    /// # use tokio::runtime;
+    /// # use tokio::sync::Barrier;
+    /// # pub fn main() {
+    /// let once = AtomicBool::new(true);
+    /// let barrier = Arc::new(Barrier::new(2));
+    ///
+    /// let runtime = runtime::Builder::new_current_thread()
+    ///     .on_thread_park({
+    ///         let barrier = barrier.clone();
+    ///         move || {
+    ///             let barrier = barrier.clone();
+    ///             if once.swap(false, Ordering::Relaxed) {
+    ///                 tokio::spawn(async move { barrier.wait().await; });
+    ///            }
+    ///         }
+    ///     })
+    ///     .build()
+    ///     .unwrap();
+    ///
+    /// runtime.block_on(async {
+    ///    barrier.wait().await;
+    /// })
+    /// # }
+    /// ```
+    #[cfg(not(loom))]
+    pub fn on_thread_park<F>(&mut self, f: F) -> &mut Self
+    where
+        F: Fn() + Send + Sync + 'static,
+    {
+        self.before_park = Some(std::sync::Arc::new(f));
+        self
+    }
+
+    /// Executes function `f` just after a thread unparks (starts executing tasks).
+    ///
+    /// This is intended for bookkeeping and monitoring use cases; note that work
+    /// in this callback will increase latencies when the application has allowed one or
+    /// more runtime threads to go idle.
+    ///
+    /// Note: There can only be one unpark callback for a runtime; calling this function
+    /// more than once replaces the last callback defined, rather than adding to it.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use tokio::runtime;
+    /// # pub fn main() {
+    /// let runtime = runtime::Builder::new_multi_thread()
+    ///     .on_thread_unpark(|| {
+    ///         println!("thread unparking");
+    ///     })
+    ///     .build();
+    ///
+    /// runtime.unwrap().block_on(async {
+    ///    tokio::task::yield_now().await;
+    ///    println!("Hello from Tokio!");
+    /// })
+    /// # }
+    /// ```
+    #[cfg(not(loom))]
+    pub fn on_thread_unpark<F>(&mut self, f: F) -> &mut Self
+    where
+        F: Fn() + Send + Sync + 'static,
+    {
+        self.after_unpark = Some(std::sync::Arc::new(f));
+        self
+    }
+
+    /// Creates the configured `Runtime`.
+    ///
+    /// The returned `Runtime` instance is ready to spawn tasks.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::runtime::Builder;
+    ///
+    /// let rt  = Builder::new_multi_thread().build().unwrap();
+    ///
+    /// rt.block_on(async {
+    ///     println!("Hello from the Tokio runtime");
+    /// });
+    /// ```
+    pub fn build(&mut self) -> io::Result<Runtime> {
+        match &self.kind {
+            Kind::CurrentThread => self.build_current_thread_runtime(),
+            #[cfg(all(feature = "rt-multi-thread", not(tokio_wasi)))]
+            Kind::MultiThread => self.build_threaded_runtime(),
+        }
+    }
+
+    fn get_cfg(&self) -> driver::Cfg {
+        driver::Cfg {
+            enable_pause_time: match self.kind {
+                Kind::CurrentThread => true,
+                #[cfg(all(feature = "rt-multi-thread", not(tokio_wasi)))]
+                Kind::MultiThread => false,
+            },
+            enable_io: self.enable_io,
+            enable_time: self.enable_time,
+            start_paused: self.start_paused,
+            nevents: self.nevents,
+        }
+    }
+
+    /// Sets a custom timeout for a thread in the blocking pool.
+    ///
+    /// By default, the timeout for a thread is set to 10 seconds. This can
+    /// be overridden using .thread_keep_alive().
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # use tokio::runtime;
+    /// # use std::time::Duration;
+    /// # pub fn main() {
+    /// let rt = runtime::Builder::new_multi_thread()
+    ///     .thread_keep_alive(Duration::from_millis(100))
+    ///     .build();
+    /// # }
+    /// ```
+    pub fn thread_keep_alive(&mut self, duration: Duration) -> &mut Self {
+        self.keep_alive = Some(duration);
+        self
+    }
+
+    /// Sets the number of scheduler ticks after which the scheduler will poll the global
+    /// task queue.
+    ///
+    /// A scheduler "tick" roughly corresponds to one `poll` invocation on a task.
+    ///
+    /// By default the global queue interval is:
+    ///
+    /// * `31` for the current-thread scheduler.
+    /// * `61` for the multithreaded scheduler.
+    ///
+    /// Schedulers have a local queue of already-claimed tasks, and a global queue of incoming
+    /// tasks. Setting the interval to a smaller value increases the fairness of the scheduler,
+    /// at the cost of more synchronization overhead. That can be beneficial for prioritizing
+    /// getting started on new work, especially if tasks frequently yield rather than complete
+    /// or await on further I/O. Conversely, a higher value prioritizes existing work, and
+    /// is a good choice when most tasks quickly complete polling.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use tokio::runtime;
+    /// # pub fn main() {
+    /// let rt = runtime::Builder::new_multi_thread()
+    ///     .global_queue_interval(31)
+    ///     .build();
+    /// # }
+    /// ```
+    pub fn global_queue_interval(&mut self, val: u32) -> &mut Self {
+        self.global_queue_interval = Some(val);
+        self
+    }
+
+    /// Sets the number of scheduler ticks after which the scheduler will poll for
+    /// external events (timers, I/O, and so on).
+    ///
+    /// A scheduler "tick" roughly corresponds to one `poll` invocation on a task.
+    ///
+    /// By default, the event interval is `61` for all scheduler types.
+    ///
+    /// Setting the event interval determines the effective "priority" of delivering
+    /// these external events (which may wake up additional tasks), compared to
+    /// executing tasks that are currently ready to run. A smaller value is useful
+    /// when tasks frequently spend a long time in polling, or frequently yield,
+    /// which can result in overly long delays picking up I/O events. Conversely,
+    /// picking up new events requires extra synchronization and syscall overhead,
+    /// so if tasks generally complete their polling quickly, a higher event interval
+    /// will minimize that overhead while still keeping the scheduler responsive to
+    /// events.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use tokio::runtime;
+    /// # pub fn main() {
+    /// let rt = runtime::Builder::new_multi_thread()
+    ///     .event_interval(31)
+    ///     .build();
+    /// # }
+    /// ```
+    pub fn event_interval(&mut self, val: u32) -> &mut Self {
+        self.event_interval = val;
+        self
+    }
+
+    cfg_unstable! {
+        /// Configure how the runtime responds to an unhandled panic on a
+        /// spawned task.
+        ///
+        /// By default, an unhandled panic (i.e. a panic not caught by
+        /// [`std::panic::catch_unwind`]) has no impact on the runtime's
+        /// execution. The panic is error value is forwarded to the task's
+        /// [`JoinHandle`] and all other spawned tasks continue running.
+        ///
+        /// The `unhandled_panic` option enables configuring this behavior.
+        ///
+        /// * `UnhandledPanic::Ignore` is the default behavior. Panics on
+        ///   spawned tasks have no impact on the runtime's execution.
+        /// * `UnhandledPanic::ShutdownRuntime` will force the runtime to
+        ///   shutdown immediately when a spawned task panics even if that
+        ///   task's `JoinHandle` has not been dropped. All other spawned tasks
+        ///   will immediately terminate and further calls to
+        ///   [`Runtime::block_on`] will panic.
+        ///
+        /// # Unstable
+        ///
+        /// This option is currently unstable and its implementation is
+        /// incomplete. The API may change or be removed in the future. See
+        /// tokio-rs/tokio#4516 for more details.
+        ///
+        /// # Examples
+        ///
+        /// The following demonstrates a runtime configured to shutdown on
+        /// panic. The first spawned task panics and results in the runtime
+        /// shutting down. The second spawned task never has a chance to
+        /// execute. The call to `block_on` will panic due to the runtime being
+        /// forcibly shutdown.
+        ///
+        /// ```should_panic
+        /// use tokio::runtime::{self, UnhandledPanic};
+        ///
+        /// # pub fn main() {
+        /// let rt = runtime::Builder::new_current_thread()
+        ///     .unhandled_panic(UnhandledPanic::ShutdownRuntime)
+        ///     .build()
+        ///     .unwrap();
+        ///
+        /// rt.spawn(async { panic!("boom"); });
+        /// rt.spawn(async {
+        ///     // This task never completes.
+        /// });
+        ///
+        /// rt.block_on(async {
+        ///     // Do some work
+        /// # loop { tokio::task::yield_now().await; }
+        /// })
+        /// # }
+        /// ```
+        ///
+        /// [`JoinHandle`]: struct@crate::task::JoinHandle
+        pub fn unhandled_panic(&mut self, behavior: UnhandledPanic) -> &mut Self {
+            self.unhandled_panic = behavior;
+            self
+        }
+
+        /// Disables the LIFO task scheduler heuristic.
+        ///
+        /// The multi-threaded scheduler includes a heuristic for optimizing
+        /// message-passing patterns. This heuristic results in the **last**
+        /// scheduled task being polled first.
+        ///
+        /// To implement this heuristic, each worker thread has a slot which
+        /// holds the task that should be polled next. However, this slot cannot
+        /// be stolen by other worker threads, which can result in lower total
+        /// throughput when tasks tend to have longer poll times.
+        ///
+        /// This configuration option will disable this heuristic resulting in
+        /// all scheduled tasks being pushed into the worker-local queue, which
+        /// is stealable.
+        ///
+        /// Consider trying this option when the task "scheduled" time is high
+        /// but the runtime is underutilized. Use tokio-rs/tokio-metrics to
+        /// collect this data.
+        ///
+        /// # Unstable
+        ///
+        /// This configuration option is considered a workaround for the LIFO
+        /// slot not being stealable. When the slot becomes stealable, we will
+        /// revisit whether or not this option is necessary. See
+        /// tokio-rs/tokio#4941.
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// use tokio::runtime;
+        ///
+        /// let rt = runtime::Builder::new_multi_thread()
+        ///     .disable_lifo_slot()
+        ///     .build()
+        ///     .unwrap();
+        /// ```
+        pub fn disable_lifo_slot(&mut self) -> &mut Self {
+            self.disable_lifo_slot = true;
+            self
+        }
+
+        /// Specifies the random number generation seed to use within all
+        /// threads associated with the runtime being built.
+        ///
+        /// This option is intended to make certain parts of the runtime
+        /// deterministic (e.g. the [`tokio::select!`] macro). In the case of
+        /// [`tokio::select!`] it will ensure that the order that branches are
+        /// polled is deterministic.
+        ///
+        /// In addition to the code specifying `rng_seed` and interacting with
+        /// the runtime, the internals of Tokio and the Rust compiler may affect
+        /// the sequences of random numbers. In order to ensure repeatable
+        /// results, the version of Tokio, the versions of all other
+        /// dependencies that interact with Tokio, and the Rust compiler version
+        /// should also all remain constant.
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// # use tokio::runtime::{self, RngSeed};
+        /// # pub fn main() {
+        /// let seed = RngSeed::from_bytes(b"place your seed here");
+        /// let rt = runtime::Builder::new_current_thread()
+        ///     .rng_seed(seed)
+        ///     .build();
+        /// # }
+        /// ```
+        ///
+        /// [`tokio::select!`]: crate::select
+        pub fn rng_seed(&mut self, seed: RngSeed) -> &mut Self {
+            self.seed_generator = RngSeedGenerator::new(seed);
+            self
+        }
+    }
+
+    cfg_metrics! {
+        /// Enables tracking the distribution of task poll times.
+        ///
+        /// Task poll times are not instrumented by default as doing so requires
+        /// calling [`Instant::now()`] twice per task poll, which could add
+        /// measurable overhead. Use the [`Handle::metrics()`] to access the
+        /// metrics data.
+        ///
+        /// The histogram uses fixed bucket sizes. In other words, the histogram
+        /// buckets are not dynamic based on input values. Use the
+        /// `metrics_poll_count_histogram_` builder methods to configure the
+        /// histogram details.
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// use tokio::runtime;
+        ///
+        /// let rt = runtime::Builder::new_multi_thread()
+        ///     .enable_metrics_poll_count_histogram()
+        ///     .build()
+        ///     .unwrap();
+        /// # // Test default values here
+        /// # fn us(n: u64) -> std::time::Duration { std::time::Duration::from_micros(n) }
+        /// # let m = rt.handle().metrics();
+        /// # assert_eq!(m.poll_count_histogram_num_buckets(), 10);
+        /// # assert_eq!(m.poll_count_histogram_bucket_range(0), us(0)..us(100));
+        /// # assert_eq!(m.poll_count_histogram_bucket_range(1), us(100)..us(200));
+        /// ```
+        ///
+        /// [`Handle::metrics()`]: crate::runtime::Handle::metrics
+        /// [`Instant::now()`]: std::time::Instant::now
+        pub fn enable_metrics_poll_count_histogram(&mut self) -> &mut Self {
+            self.metrics_poll_count_histogram_enable = true;
+            self
+        }
+
+        /// Sets the histogram scale for tracking the distribution of task poll
+        /// times.
+        ///
+        /// Tracking the distribution of task poll times can be done using a
+        /// linear or log scale. When using linear scale, each histogram bucket
+        /// will represent the same range of poll times. When using log scale,
+        /// each histogram bucket will cover a range twice as big as the
+        /// previous bucket.
+        ///
+        /// **Default:** linear scale.
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// use tokio::runtime::{self, HistogramScale};
+        ///
+        /// let rt = runtime::Builder::new_multi_thread()
+        ///     .enable_metrics_poll_count_histogram()
+        ///     .metrics_poll_count_histogram_scale(HistogramScale::Log)
+        ///     .build()
+        ///     .unwrap();
+        /// ```
+        pub fn metrics_poll_count_histogram_scale(&mut self, histogram_scale: crate::runtime::HistogramScale) -> &mut Self {
+            self.metrics_poll_count_histogram.scale = histogram_scale;
+            self
+        }
+
+        /// Sets the histogram resolution for tracking the distribution of task
+        /// poll times.
+        ///
+        /// The resolution is the histogram's first bucket's range. When using a
+        /// linear histogram scale, each bucket will cover the same range. When
+        /// using a log scale, each bucket will cover a range twice as big as
+        /// the previous bucket. In the log case, the resolution represents the
+        /// smallest bucket range.
+        ///
+        /// Note that, when using log scale, the resolution is rounded up to the
+        /// nearest power of 2 in nanoseconds.
+        ///
+        /// **Default:** 100 microseconds.
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// use tokio::runtime;
+        /// use std::time::Duration;
+        ///
+        /// let rt = runtime::Builder::new_multi_thread()
+        ///     .enable_metrics_poll_count_histogram()
+        ///     .metrics_poll_count_histogram_resolution(Duration::from_micros(100))
+        ///     .build()
+        ///     .unwrap();
+        /// ```
+        pub fn metrics_poll_count_histogram_resolution(&mut self, resolution: Duration) -> &mut Self {
+            assert!(resolution > Duration::from_secs(0));
+            // Sanity check the argument and also make the cast below safe.
+            assert!(resolution <= Duration::from_secs(1));
+
+            let resolution = resolution.as_nanos() as u64;
+            self.metrics_poll_count_histogram.resolution = resolution;
+            self
+        }
+
+        /// Sets the number of buckets for the histogram tracking the
+        /// distribution of task poll times.
+        ///
+        /// The last bucket tracks all greater values that fall out of other
+        /// ranges. So, configuring the histogram using a linear scale,
+        /// resolution of 50ms, and 10 buckets, the 10th bucket will track task
+        /// polls that take more than 450ms to complete.
+        ///
+        /// **Default:** 10
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// use tokio::runtime;
+        ///
+        /// let rt = runtime::Builder::new_multi_thread()
+        ///     .enable_metrics_poll_count_histogram()
+        ///     .metrics_poll_count_histogram_buckets(15)
+        ///     .build()
+        ///     .unwrap();
+        /// ```
+        pub fn metrics_poll_count_histogram_buckets(&mut self, buckets: usize) -> &mut Self {
+            self.metrics_poll_count_histogram.num_buckets = buckets;
+            self
+        }
+    }
+
+    fn build_current_thread_runtime(&mut self) -> io::Result<Runtime> {
+        use crate::runtime::scheduler::{self, CurrentThread};
+        use crate::runtime::{runtime::Scheduler, Config};
+
+        let (driver, driver_handle) = driver::Driver::new(self.get_cfg())?;
+
+        // Blocking pool
+        let blocking_pool = blocking::create_blocking_pool(self, self.max_blocking_threads);
+        let blocking_spawner = blocking_pool.spawner().clone();
+
+        // Generate a rng seed for this runtime.
+        let seed_generator_1 = self.seed_generator.next_generator();
+        let seed_generator_2 = self.seed_generator.next_generator();
+
+        // And now put a single-threaded scheduler on top of the timer. When
+        // there are no futures ready to do something, it'll let the timer or
+        // the reactor to generate some new stimuli for the futures to continue
+        // in their life.
+        let (scheduler, handle) = CurrentThread::new(
+            driver,
+            driver_handle,
+            blocking_spawner,
+            seed_generator_2,
+            Config {
+                before_park: self.before_park.clone(),
+                after_unpark: self.after_unpark.clone(),
+                global_queue_interval: self.global_queue_interval,
+                event_interval: self.event_interval,
+                #[cfg(tokio_unstable)]
+                unhandled_panic: self.unhandled_panic.clone(),
+                disable_lifo_slot: self.disable_lifo_slot,
+                seed_generator: seed_generator_1,
+                metrics_poll_count_histogram: self.metrics_poll_count_histogram_builder(),
+            },
+        );
+
+        let handle = Handle {
+            inner: scheduler::Handle::CurrentThread(handle),
+        };
+
+        Ok(Runtime::from_parts(
+            Scheduler::CurrentThread(scheduler),
+            handle,
+            blocking_pool,
+        ))
+    }
+
+    fn metrics_poll_count_histogram_builder(&self) -> Option<HistogramBuilder> {
+        if self.metrics_poll_count_histogram_enable {
+            Some(self.metrics_poll_count_histogram.clone())
+        } else {
+            None
+        }
+    }
+}
+
+cfg_io_driver! {
+    impl Builder {
+        /// Enables the I/O driver.
+        ///
+        /// Doing this enables using net, process, signal, and some I/O types on
+        /// the runtime.
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// use tokio::runtime;
+        ///
+        /// let rt = runtime::Builder::new_multi_thread()
+        ///     .enable_io()
+        ///     .build()
+        ///     .unwrap();
+        /// ```
+        pub fn enable_io(&mut self) -> &mut Self {
+            self.enable_io = true;
+            self
+        }
+
+        /// Enables the I/O driver and configures the max number of events to be
+        /// processed per tick.
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// use tokio::runtime;
+        ///
+        /// let rt = runtime::Builder::new_current_thread()
+        ///     .enable_io()
+        ///     .max_io_events_per_tick(1024)
+        ///     .build()
+        ///     .unwrap();
+        /// ```
+        pub fn max_io_events_per_tick(&mut self, capacity: usize) -> &mut Self {
+            self.nevents = capacity;
+            self
+        }
+    }
+}
+
+cfg_time! {
+    impl Builder {
+        /// Enables the time driver.
+        ///
+        /// Doing this enables using `tokio::time` on the runtime.
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// use tokio::runtime;
+        ///
+        /// let rt = runtime::Builder::new_multi_thread()
+        ///     .enable_time()
+        ///     .build()
+        ///     .unwrap();
+        /// ```
+        pub fn enable_time(&mut self) -> &mut Self {
+            self.enable_time = true;
+            self
+        }
+    }
+}
+
+cfg_test_util! {
+    impl Builder {
+        /// Controls if the runtime's clock starts paused or advancing.
+        ///
+        /// Pausing time requires the current-thread runtime; construction of
+        /// the runtime will panic otherwise.
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// use tokio::runtime;
+        ///
+        /// let rt = runtime::Builder::new_current_thread()
+        ///     .enable_time()
+        ///     .start_paused(true)
+        ///     .build()
+        ///     .unwrap();
+        /// ```
+        pub fn start_paused(&mut self, start_paused: bool) -> &mut Self {
+            self.start_paused = start_paused;
+            self
+        }
+    }
+}
+
+cfg_rt_multi_thread! {
+    impl Builder {
+        fn build_threaded_runtime(&mut self) -> io::Result<Runtime> {
+            use crate::loom::sys::num_cpus;
+            use crate::runtime::{Config, runtime::Scheduler};
+            use crate::runtime::scheduler::{self, MultiThread};
+
+            let core_threads = self.worker_threads.unwrap_or_else(num_cpus);
+
+            let (driver, driver_handle) = driver::Driver::new(self.get_cfg())?;
+
+            // Create the blocking pool
+            let blocking_pool =
+                blocking::create_blocking_pool(self, self.max_blocking_threads + core_threads);
+            let blocking_spawner = blocking_pool.spawner().clone();
+
+            // Generate a rng seed for this runtime.
+            let seed_generator_1 = self.seed_generator.next_generator();
+            let seed_generator_2 = self.seed_generator.next_generator();
+
+            let (scheduler, handle, launch) = MultiThread::new(
+                core_threads,
+                driver,
+                driver_handle,
+                blocking_spawner,
+                seed_generator_2,
+                Config {
+                    before_park: self.before_park.clone(),
+                    after_unpark: self.after_unpark.clone(),
+                    global_queue_interval: self.global_queue_interval,
+                    event_interval: self.event_interval,
+                    #[cfg(tokio_unstable)]
+                    unhandled_panic: self.unhandled_panic.clone(),
+                    disable_lifo_slot: self.disable_lifo_slot,
+                    seed_generator: seed_generator_1,
+                    metrics_poll_count_histogram: self.metrics_poll_count_histogram_builder(),
+                },
+            );
+
+            let handle = Handle { inner: scheduler::Handle::MultiThread(handle) };
+
+            // Spawn the thread pool workers
+            let _enter = handle.enter();
+            launch.launch();
+
+            Ok(Runtime::from_parts(Scheduler::MultiThread(scheduler), handle, blocking_pool))
+        }
+    }
+}
+
+impl fmt::Debug for Builder {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("Builder")
+            .field("worker_threads", &self.worker_threads)
+            .field("max_blocking_threads", &self.max_blocking_threads)
+            .field(
+                "thread_name",
+                &"<dyn Fn() -> String + Send + Sync + 'static>",
+            )
+            .field("thread_stack_size", &self.thread_stack_size)
+            .field("after_start", &self.after_start.as_ref().map(|_| "..."))
+            .field("before_stop", &self.before_stop.as_ref().map(|_| "..."))
+            .field("before_park", &self.before_park.as_ref().map(|_| "..."))
+            .field("after_unpark", &self.after_unpark.as_ref().map(|_| "..."))
+            .finish()
+    }
+}