diff options
Diffstat (limited to 'tokio/src/executor/current_thread/scheduler.rs')
| -rw-r--r-- | tokio/src/executor/current_thread/scheduler.rs | 808 |
1 files changed, 808 insertions, 0 deletions
diff --git a/tokio/src/executor/current_thread/scheduler.rs b/tokio/src/executor/current_thread/scheduler.rs new file mode 100644 index 00000000..ba14ee88 --- /dev/null +++ b/tokio/src/executor/current_thread/scheduler.rs @@ -0,0 +1,808 @@ +use crate::executor::current_thread::{Borrow, BorrowSpawner}; +use crate::executor::park::Unpark; + +use std::cell::UnsafeCell; +use std::fmt::{self, Debug}; +use std::future::Future; +use std::mem; +use std::pin::Pin; +use std::ptr; +use std::sync::atomic::Ordering::{AcqRel, Acquire, Relaxed, Release, SeqCst}; +use std::sync::atomic::{AtomicBool, AtomicPtr, AtomicUsize}; +use std::sync::{Arc, Weak}; +use std::task::{Context, Poll, RawWaker, RawWakerVTable, Waker}; +use std::thread; +use std::usize; + +/// A generic task-aware scheduler. +/// +/// This is used both by `FuturesUnordered` and the current-thread executor. +pub(crate) struct Scheduler<U> { + inner: Arc<Inner<U>>, + nodes: List<U>, +} + +// A linked-list of nodes +struct List<U> { + len: usize, + head: *const Node<U>, + tail: *const Node<U>, +} + +// Scheduler is implemented using two linked lists. The first linked list tracks +// all items managed by a `Scheduler`. This list is stored on the `Scheduler` +// struct and is **not** thread safe. The second linked list is an +// implementation of the intrusive MPSC queue algorithm described by +// 1024cores.net and is stored on `Inner`. This linked list can push items to +// the back concurrently but only one consumer may pop from the front. To +// enforce this requirement, all popping will be performed via fns on +// `Scheduler` that take `&mut self`. +// +// When a item is submitted to the set a node is allocated and inserted in +// both linked lists. This means that all insertion operations **must** be +// originated from `Scheduler` with `&mut self` The next call to `tick` will +// (eventually) see this node and call `poll` on the item. +// +// Nodes are wrapped in `Arc` cells which manage the lifetime of the node. +// However, `Arc` handles are sometimes cast to `*const Node` pointers. +// Specifically, when a node is stored in at least one of the two lists +// described above, this represents a logical `Arc` handle. This is how +// `Scheduler` maintains its reference to all nodes it manages. Each +// `NotifyHandle` instance is an `Arc<Node>` as well. +// +// When `Scheduler` drops, it clears the linked list of all nodes that it +// manages. When doing so, it must attempt to decrement the reference count (by +// dropping an Arc handle). However, it can **only** decrement the reference +// count if the node is not currently stored in the mpsc channel. If the node +// **is** "queued" in the mpsc channel, then the arc reference count cannot be +// decremented. Once the node is popped from the mpsc channel, then the final +// arc reference count can be decremented, thus freeing the node. + +struct Inner<U> { + // Thread unpark handle + unpark: U, + + // Tick number + tick_num: AtomicUsize, + + // Head/tail of the readiness queue + head_readiness: AtomicPtr<Node<U>>, + tail_readiness: UnsafeCell<*const Node<U>>, + + // Used as part of the mpsc queue algorithm + stub: Arc<Node<U>>, +} + +unsafe impl<U: Sync + Send> Send for Inner<U> {} +unsafe impl<U: Sync + Send> Sync for Inner<U> {} + +struct Node<U> { + // The item + item: UnsafeCell<Option<Task>>, + + // The tick at which this node was notified + notified_at: AtomicUsize, + + // Next pointer for linked list tracking all active nodes + next_all: UnsafeCell<*const Node<U>>, + + // Previous node in linked list tracking all active nodes + prev_all: UnsafeCell<*const Node<U>>, + + // Next pointer in readiness queue + next_readiness: AtomicPtr<Node<U>>, + + // Whether or not this node is currently in the mpsc queue. + queued: AtomicBool, + + // Queue that we'll be enqueued to when notified + queue: Weak<Inner<U>>, +} + +/// Returned by `Inner::dequeue`, representing either a dequeue success (with +/// the dequeued node), an empty list, or an inconsistent state. +/// +/// The inconsistent state is described in more detail at [1024cores], but +/// roughly indicates that a node will be ready to dequeue sometime shortly in +/// the future and the caller should try again soon. +/// +/// [1024cores]: http://www.1024cores.net/home/lock-free-algorithms/queues/intrusive-mpsc-node-based-queue +enum Dequeue<U> { + Data(*const Node<U>), + Empty, + Yield, + Inconsistent, +} + +/// Wraps a spawned boxed future +struct Task(Pin<Box<dyn Future<Output = ()>>>); + +/// A task that is scheduled. `turn` must be called +pub(crate) struct Scheduled<'a, U> { + task: &'a mut Task, + node: &'a Arc<Node<U>>, + done: &'a mut bool, +} + +pub(super) struct TickArgs<'a> { + pub(super) id: u64, + pub(super) num_futures: &'a AtomicUsize, + #[cfg(feature = "blocking")] + pub(super) blocking: &'a crate::executor::blocking::PoolWaiter, +} + +impl<U> Scheduler<U> +where + U: Unpark, +{ + /// Constructs a new, empty `Scheduler` + /// + /// The returned `Scheduler` does not contain any items and, in this + /// state, `Scheduler::poll` will return `Ok(Async::Ready(None))`. + pub(crate) fn new(unpark: U) -> Self { + let stub = Arc::new(Node { + item: UnsafeCell::new(None), + notified_at: AtomicUsize::new(0), + next_all: UnsafeCell::new(ptr::null()), + prev_all: UnsafeCell::new(ptr::null()), + next_readiness: AtomicPtr::new(ptr::null_mut()), + queued: AtomicBool::new(true), + queue: Weak::new(), + }); + let stub_ptr = &*stub as *const Node<U>; + let inner = Arc::new(Inner { + unpark, + tick_num: AtomicUsize::new(0), + head_readiness: AtomicPtr::new(stub_ptr as *mut _), + tail_readiness: UnsafeCell::new(stub_ptr), + stub, + }); + + Scheduler { + inner, + nodes: List::new(), + } + } + + pub(crate) fn waker(&self) -> Waker { + waker_inner(self.inner.clone()) + } + + pub(crate) fn schedule(&mut self, item: Pin<Box<dyn Future<Output = ()>>>) { + // Get the current scheduler tick + let tick_num = self.inner.tick_num.load(SeqCst); + + let node = Arc::new(Node { + item: UnsafeCell::new(Some(Task::new(item))), + notified_at: AtomicUsize::new(tick_num), + next_all: UnsafeCell::new(ptr::null_mut()), + prev_all: UnsafeCell::new(ptr::null_mut()), + next_readiness: AtomicPtr::new(ptr::null_mut()), + queued: AtomicBool::new(true), + queue: Arc::downgrade(&self.inner), + }); + + // Right now our node has a strong reference count of 1. We transfer + // ownership of this reference count to our internal linked list + // and we'll reclaim ownership through the `unlink` function below. + let ptr = self.nodes.push_back(node); + + // We'll need to get the item "into the system" to start tracking it, + // e.g. getting its unpark notifications going to us tracking which + // items are ready. To do that we unconditionally enqueue it for + // polling here. + self.inner.enqueue(ptr); + } + + /// Returns `true` if there are currently any pending futures + pub(crate) fn has_pending_futures(&mut self) -> bool { + // See function definition for why the unsafe is needed and + // correctly used here + unsafe { self.inner.has_pending_futures() } + } + + /// Advance the scheduler state, returning `true` if any futures were + /// processed. + /// + /// This function should be called whenever the caller is notified via a + /// wakeup. + pub(super) fn tick(&mut self, args: TickArgs<'_>) -> bool { + let mut ret = false; + let tick = self.inner.tick_num.fetch_add(1, SeqCst).wrapping_add(1); + + loop { + let node = match unsafe { self.inner.dequeue(Some(tick)) } { + Dequeue::Empty => { + return ret; + } + Dequeue::Yield => { + self.inner.unpark.unpark(); + return ret; + } + Dequeue::Inconsistent => { + thread::yield_now(); + continue; + } + Dequeue::Data(node) => node, + }; + + ret = true; + + debug_assert!(node != self.inner.stub()); + + unsafe { + if (*(*node).item.get()).is_none() { + // The node has already been released. However, while it was + // being released, another thread notified it, which + // resulted in it getting pushed into the mpsc channel. + // + // In this case, we just decrement the ref count. + let node = ptr2arc(node); + assert!((*node.next_all.get()).is_null()); + assert!((*node.prev_all.get()).is_null()); + continue; + }; + + // We're going to need to be very careful if the `poll` + // function below panics. We need to (a) not leak memory and + // (b) ensure that we still don't have any use-after-frees. To + // manage this we do a few things: + // + // * This "bomb" here will call `release_node` if dropped + // abnormally. That way we'll be sure the memory management + // of the `node` is managed correctly. + // + // * We unlink the node from our internal queue to preemptively + // assume is is complete (will return Ready or panic), in + // which case we'll want to discard it regardless. + // + struct Bomb<'a, U: Unpark> { + borrow: &'a mut Borrow<'a, U>, + node: Option<Arc<Node<U>>>, + } + + impl<U: Unpark> Drop for Bomb<'_, U> { + fn drop(&mut self) { + if let Some(node) = self.node.take() { + self.borrow.enter(|| release_node(node)) + } + } + } + + let node = self.nodes.remove(node); + + let mut borrow = Borrow { + spawner: BorrowSpawner { + id: args.id, + scheduler: self, + num_futures: args.num_futures, + }, + #[cfg(feature = "blocking")] + blocking: args.blocking, + }; + + let mut bomb = Bomb { + node: Some(node), + borrow: &mut borrow, + }; + + let mut done = false; + + // Now that the bomb holds the node, create a new scope. This + // scope ensures that the borrow will go out of scope before we + // mutate the node pointer in `bomb` again + { + let node = bomb.node.as_ref().unwrap(); + + // Get a reference to the inner future. We already ensured + // that the item `is_some`. + let item = (*node.item.get()).as_mut().unwrap(); + + // Unset queued flag... this must be done before + // polling. This ensures that the item gets + // rescheduled if it is notified **during** a call + // to `poll`. + let prev = (*node).queued.swap(false, SeqCst); + assert!(prev); + + // Poll the underlying item with the appropriate `notify` + // implementation. This is where a large bit of the unsafety + // starts to stem from internally. The `notify` instance itself + // is basically just our `Arc<Node>` and tracks the mpsc + // queue of ready items. + // + // Critically though `Node` won't actually access `Task`, the + // item, while it's floating around inside of `Task` + // instances. These structs will basically just use `T` to size + // the internal allocation, appropriately accessing fields and + // deallocating the node if need be. + let borrow = &mut *bomb.borrow; + + let mut scheduled = Scheduled { + task: item, + node: bomb.node.as_ref().unwrap(), + done: &mut done, + }; + + if borrow.enter(|| scheduled.tick()) { + // we have a borrow of the Runtime, so we know it's not shut down + borrow.spawner.num_futures.fetch_sub(2, SeqCst); + } + } + + if !done { + // The future is not done, push it back into the "all + // node" list. + let node = bomb.node.take().unwrap(); + bomb.borrow.spawner.scheduler.nodes.push_back(node); + } + } + } + } +} + +impl<U: Unpark> Scheduled<'_, U> { + /// Polls the task, returns `true` if the task has completed. + pub(crate) fn tick(&mut self) -> bool { + let waker = unsafe { + // Safety: we don't hold this waker ref longer than + // this `tick` function + waker_ref(self.node) + }; + let mut cx = Context::from_waker(&waker); + let ret = match self.task.0.as_mut().poll(&mut cx) { + Poll::Ready(()) => true, + Poll::Pending => false, + }; + + *self.done = ret; + ret + } +} + +impl Task { + pub(crate) fn new(future: Pin<Box<dyn Future<Output = ()> + 'static>>) -> Self { + Task(future) + } +} + +impl fmt::Debug for Task { + fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result { + fmt.debug_struct("Task").finish() + } +} + +fn release_node<U>(node: Arc<Node<U>>) { + // The item is done, try to reset the queued flag. This will prevent + // `notify` from doing any work in the item + let prev = node.queued.swap(true, SeqCst); + + // Drop the item, even if it hasn't finished yet. This is safe + // because we're dropping the item on the thread that owns + // `Scheduler`, which correctly tracks T's lifetimes and such. + unsafe { + drop((*node.item.get()).take()); + } + + // If the queued flag was previously set then it means that this node + // is still in our internal mpsc queue. We then transfer ownership + // of our reference count to the mpsc queue, and it'll come along and + // free it later, noticing that the item is `None`. + // + // If, however, the queued flag was *not* set then we're safe to + // release our reference count on the internal node. The queued flag + // was set above so all item `enqueue` operations will not actually + // enqueue the node, so our node will never see the mpsc queue again. + // The node itself will be deallocated once all reference counts have + // been dropped by the various owning tasks elsewhere. + if prev { + mem::forget(node); + } +} + +impl<U> Debug for Scheduler<U> { + fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result { + write!(fmt, "Scheduler {{ ... }}") + } +} + +impl<U> Drop for Scheduler<U> { + fn drop(&mut self) { + // When a `Scheduler` is dropped we want to drop all items associated + // with it. At the same time though there may be tons of `Task` handles + // flying around which contain `Node` references inside them. We'll + // let those naturally get deallocated when the `Task` itself goes out + // of scope or gets notified. + while let Some(node) = self.nodes.pop_front() { + release_node(node); + } + + // Note that at this point we could still have a bunch of nodes in the + // mpsc queue. None of those nodes, however, have items associated + // with them so they're safe to destroy on any thread. At this point + // the `Scheduler` struct, the owner of the one strong reference + // to `Inner` will drop the strong reference. At that point + // whichever thread releases the strong refcount last (be it this + // thread or some other thread as part of an `upgrade`) will clear out + // the mpsc queue and free all remaining nodes. + // + // While that freeing operation isn't guaranteed to happen here, it's + // guaranteed to happen "promptly" as no more "blocking work" will + // happen while there's a strong refcount held. + } +} + +impl<U> Inner<U> { + /// The enqueue function from the 1024cores intrusive MPSC queue algorithm. + fn enqueue(&self, node: *const Node<U>) { + unsafe { + debug_assert!((*node).queued.load(Relaxed)); + + // This action does not require any coordination + (*node).next_readiness.store(ptr::null_mut(), Relaxed); + + // Note that these atomic orderings come from 1024cores + let node = node as *mut _; + let prev = self.head_readiness.swap(node, AcqRel); + (*prev).next_readiness.store(node, Release); + } + } + + /// Returns `true` if there are currently any pending futures + /// + /// See `dequeue` for an explanation why this function is unsafe. + unsafe fn has_pending_futures(&self) -> bool { + let tail = *self.tail_readiness.get(); + let next = (*tail).next_readiness.load(Acquire); + + if tail == self.stub() && next.is_null() { + return false; + } + + true + } + + /// The dequeue function from the 1024cores intrusive MPSC queue algorithm + /// + /// Note that this unsafe as it required mutual exclusion (only one thread + /// can call this) to be guaranteed elsewhere. + unsafe fn dequeue(&self, tick: Option<usize>) -> Dequeue<U> { + let mut tail = *self.tail_readiness.get(); + let mut next = (*tail).next_readiness.load(Acquire); + + if tail == self.stub() { + if next.is_null() { + return Dequeue::Empty; + } + + *self.tail_readiness.get() = next; + tail = next; + next = (*next).next_readiness.load(Acquire); + } + + if let Some(tick) = tick { + let actual = (*tail).notified_at.load(SeqCst); + + // Only dequeue if the node was not scheduled during the current + // tick. + if actual == tick { + // Only doing the check above **should** be enough in + // practice. However, technically there is a potential for + // deadlocking if there are `usize::MAX` ticks while the thread + // scheduling the task is frozen. + // + // If, for some reason, this is not enough, calling `unpark` + // here will resolve the issue. + return Dequeue::Yield; + } + } + + if !next.is_null() { + *self.tail_readiness.get() = next; + debug_assert!(tail != self.stub()); + return Dequeue::Data(tail); + } + + if self.head_readiness.load(Acquire) as *const _ != tail { + return Dequeue::Inconsistent; + } + + self.enqueue(self.stub()); + + next = (*tail).next_readiness.load(Acquire); + + if !next.is_null() { + *self.tail_readiness.get() = next; + return Dequeue::Data(tail); + } + + Dequeue::Inconsistent + } + + fn stub(&self) -> *const Node<U> { + &*self.stub + } +} + +impl<U> Drop for Inner<U> { + fn drop(&mut self) { + // Once we're in the destructor for `Inner` we need to clear out the + // mpsc queue of nodes if there's anything left in there. + // + // Note that each node has a strong reference count associated with it + // which is owned by the mpsc queue. All nodes should have had their + // items dropped already by the `Scheduler` destructor above, + // so we're just pulling out nodes and dropping their refcounts. + unsafe { + loop { + match self.dequeue(None) { + Dequeue::Empty => break, + Dequeue::Yield => unreachable!(), + Dequeue::Inconsistent => abort("inconsistent in drop"), + Dequeue::Data(ptr) => drop(ptr2arc(ptr)), + } + } + } + } +} + +impl<U> List<U> { + fn new() -> Self { + List { + len: 0, + head: ptr::null_mut(), + tail: ptr::null_mut(), + } + } + + /// Appends an element to the back of the list + fn push_back(&mut self, node: Arc<Node<U>>) -> *const Node<U> { + let ptr = arc2ptr(node); + + unsafe { + // Point to the current last node in the list + *(*ptr).prev_all.get() = self.tail; + *(*ptr).next_all.get() = ptr::null_mut(); + + if !self.tail.is_null() { + *(*self.tail).next_all.get() = ptr; + self.tail = ptr; + } else { + // This is the first node + self.tail = ptr; + self.head = ptr; + } + } + + self.len += 1; + + ptr + } + + /// Pop an element from the front of the list + fn pop_front(&mut self) -> Option<Arc<Node<U>>> { + if self.head.is_null() { + // The list is empty + return None; + } + + self.len -= 1; + + unsafe { + // Convert the ptr to Arc<_> + let node = ptr2arc(self.head); + + // Update the head pointer + self.head = *node.next_all.get(); + + // If the pointer is null, then the list is empty + if self.head.is_null() { + self.tail = ptr::null_mut(); + } else { + *(*self.head).prev_all.get() = ptr::null_mut(); + } + + Some(node) + } + } + + /// Remove a specific node + unsafe fn remove(&mut self, node: *const Node<U>) -> Arc<Node<U>> { + let node = ptr2arc(node); + let next = *node.next_all.get(); + let prev = *node.prev_all.get(); + *node.next_all.get() = ptr::null_mut(); + *node.prev_all.get() = ptr::null_mut(); + + if !next.is_null() { + *(*next).prev_all.get() = prev; + } else { + self.tail = prev; + } + + if !prev.is_null() { + *(*prev).next_all.get() = next; + } else { + self.head = next; + } + + self.len -= 1; + + node + } +} + +unsafe fn noop(_: *const ()) {} + +// ===== Raw Waker Inner<U> ====== + +fn waker_inner<U: Unpark>(inner: Arc<Inner<U>>) -> Waker { + let ptr = Arc::into_raw(inner) as *const (); + let vtable = &RawWakerVTable::new( + clone_inner::<U>, + wake_inner::<U>, + wake_by_ref_inner::<U>, + drop_inner::<U>, + ); + + unsafe { Waker::from_raw(RawWaker::new(ptr, vtable)) } +} + +unsafe fn clone_inner<U: Unpark>(data: *const ()) -> RawWaker { + let arc: Arc<Inner<U>> = Arc::from_raw(data as *const Inner<U>); + let clone = arc.clone(); + // forget both Arcs so the refcounts don't get decremented + mem::forget(arc); + mem::forget(clone); + + let vtable = &RawWakerVTable::new( + clone_inner::<U>, + wake_inner::<U>, + wake_by_ref_inner::<U>, + drop_inner::<U>, + ); + RawWaker::new(data, vtable) +} + +unsafe fn wake_inner<U: Unpark>(data: *const ()) { + let arc: Arc<Inner<U>> = Arc::from_raw(data as *const Inner<U>); + arc.unpark.unpark(); +} + +unsafe fn wake_by_ref_inner<U: Unpark>(data: *const ()) { + let arc: Arc<Inner<U>> = Arc::from_raw(data as *const Inner<U>); + arc.unpark.unpark(); + // by_ref means we don't own the Node, so forget the Arc + mem::forget(arc); +} + +unsafe fn drop_inner<U>(data: *const ()) { + drop(Arc::<Inner<U>>::from_raw(data as *const Inner<U>)); +} +// ===== Raw Waker Node<U> ====== + +unsafe fn waker_ref<U: Unpark>(node: &Arc<Node<U>>) -> Waker { + let ptr = &*node as &Node<U> as *const Node<U> as *const (); + let vtable = &RawWakerVTable::new( + clone_node::<U>, + wake_unreachable, + wake_by_ref_node::<U>, + noop, + ); + + Waker::from_raw(RawWaker::new(ptr, vtable)) +} + +unsafe fn wake_unreachable(_data: *const ()) { + unreachable!("waker_ref::wake()"); +} + +unsafe fn clone_node<U: Unpark>(data: *const ()) -> RawWaker { + let arc: Arc<Node<U>> = Arc::from_raw(data as *const Node<U>); + let clone = arc.clone(); + // forget both Arcs so the refcounts don't get decremented + mem::forget(arc); + mem::forget(clone); + + let vtable = &RawWakerVTable::new( + clone_node::<U>, + wake_node::<U>, + wake_by_ref_node::<U>, + drop_node::<U>, + ); + RawWaker::new(data, vtable) +} + +unsafe fn wake_node<U: Unpark>(data: *const ()) { + let arc: Arc<Node<U>> = Arc::from_raw(data as *const Node<U>); + Node::<U>::notify(&arc); +} + +unsafe fn wake_by_ref_node<U: Unpark>(data: *const ()) { + let arc: Arc<Node<U>> = Arc::from_raw(data as *const Node<U>); + Node::<U>::notify(&arc); + // by_ref means we don't own the Node, so forget the Arc + mem::forget(arc); +} + +unsafe fn drop_node<U>(data: *const ()) { + drop(Arc::<Node<U>>::from_raw(data as *const Node<U>)); +} + +impl<U: Unpark> Node<U> { + fn notify(me: &Arc<Node<U>>) { + let inner = match me.queue.upgrade() { + Some(inner) => inner, + None => return, + }; + + // It's our job to notify the node that it's ready to get polled, + // meaning that we need to enqueue it into the readiness queue. To + // do this we flag that we're ready to be queued, and if successful + // we then do the literal queueing operation, ensuring that we're + // only queued once. + // + // Once the node is inserted we be sure to notify the parent task, + // as it'll want to come along and pick up our node now. + // + // Note that we don't change the reference count of the node here, + // we're just enqueueing the raw pointer. The `Scheduler` + // implementation guarantees that if we set the `queued` flag true that + // there's a reference count held by the main `Scheduler` queue + // still. + let prev = me.queued.swap(true, SeqCst); + if !prev { + // Get the current scheduler tick + let tick_num = inner.tick_num.load(SeqCst); + me.notified_at.store(tick_num, SeqCst); + + inner.enqueue(&**me); + inner.unpark.unpark(); + } + } +} + +impl<U> Drop for Node<U> { + fn drop(&mut self) { + // Currently a `Node` is sent across all threads for any lifetime, + // regardless of `T`. This means that for memory safety we can't + // actually touch `T` at any time except when we have a reference to the + // `Scheduler` itself. + // + // Consequently it *should* be the case that we always drop items from + // the `Scheduler` instance, but this is a bomb in place to catch + // any bugs in that logic. + unsafe { + if (*self.item.get()).is_some() { + abort("item still here when dropping"); + } + } + } +} + +fn arc2ptr<T>(ptr: Arc<T>) -> *const T { + let addr = &*ptr as *const T; + mem::forget(ptr); + addr +} + +unsafe fn ptr2arc<T>(ptr: *const T) -> Arc<T> { + let anchor = mem::transmute::<usize, Arc<T>>(0x10); + let addr = &*anchor as *const T; + mem::forget(anchor); + let offset = addr as isize - 0x10; + mem::transmute::<isize, Arc<T>>(ptr as isize - offset) +} + +fn abort(s: &str) -> ! { + struct DoublePanic; + + impl Drop for DoublePanic { + fn drop(&mut self) { + panic!("panicking twice to abort the program"); + } + } + + let _bomb = DoublePanic; + panic!("{}", s); +} |