Adding upstream version 124.0.1.upstream/124.0.1

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

1 files changed, 465 insertions, 0 deletions

diff --git a/third_party/rust/regex-automata/src/util/lazy.rs b/third_party/rust/regex-automata/src/util/lazy.rs
new file mode 100644
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+/*!
+A lazily initialized value for safe sharing between threads.
+
+The principal type in this module is `Lazy`, which makes it easy to construct
+values that are shared safely across multiple threads simultaneously.
+*/
+
+use core::fmt;
+
+/// A lazily initialized value that implements `Deref` for `T`.
+///
+/// A `Lazy` takes an initialization function and permits callers from any
+/// thread to access the result of that initialization function in a safe
+/// manner. In effect, this permits one-time initialization of global resources
+/// in a (possibly) multi-threaded program.
+///
+/// This type and its functionality are available even when neither the `alloc`
+/// nor the `std` features are enabled. In exchange, a `Lazy` does **not**
+/// guarantee that the given `create` function is called at most once. It
+/// might be called multiple times. Moreover, a call to `Lazy::get` (either
+/// explicitly or implicitly via `Lazy`'s `Deref` impl) may block until a `T`
+/// is available.
+///
+/// This is very similar to `lazy_static` or `once_cell`, except it doesn't
+/// guarantee that the initialization function will be run once and it works
+/// in no-alloc no-std environments. With that said, if you need stronger
+/// guarantees or a more flexible API, then it is recommended to use either
+/// `lazy_static` or `once_cell`.
+///
+/// # Warning: may use a spin lock
+///
+/// When this crate is compiled _without_ the `alloc` feature, then this type
+/// may used a spin lock internally. This can have subtle effects that may
+/// be undesirable. See [Spinlocks Considered Harmful][spinharm] for a more
+/// thorough treatment of this topic.
+///
+/// [spinharm]: https://matklad.github.io/2020/01/02/spinlocks-considered-harmful.html
+///
+/// # Example
+///
+/// This type is useful for creating regexes once, and then using them from
+/// multiple threads simultaneously without worrying about synchronization.
+///
+/// ```
+/// use regex_automata::{dfa::regex::Regex, util::lazy::Lazy, Match};
+///
+/// static RE: Lazy<Regex> = Lazy::new(|| Regex::new("foo[0-9]+bar").unwrap());
+///
+/// let expected = Some(Match::must(0, 3..14));
+/// assert_eq!(expected, RE.find(b"zzzfoo12345barzzz"));
+/// ```
+pub struct Lazy<T, F = fn() -> T>(lazy::Lazy<T, F>);
+
+impl<T, F> Lazy<T, F> {
+    /// Create a new `Lazy` value that is initialized via the given function.
+    ///
+    /// The `T` type is automatically inferred from the return type of the
+    /// `create` function given.
+    pub const fn new(create: F) -> Lazy<T, F> {
+        Lazy(lazy::Lazy::new(create))
+    }
+}
+
+impl<T, F: Fn() -> T> Lazy<T, F> {
+    /// Return a reference to the lazily initialized value.
+    ///
+    /// This routine may block if another thread is initializing a `T`.
+    ///
+    /// Note that given a `x` which has type `Lazy`, this must be called via
+    /// `Lazy::get(x)` and not `x.get()`. This routine is defined this way
+    /// because `Lazy` impls `Deref` with a target of `T`.
+    ///
+    /// # Panics
+    ///
+    /// This panics if the `create` function inside this lazy value panics.
+    /// If the panic occurred in another thread, then this routine _may_ also
+    /// panic (but is not guaranteed to do so).
+    pub fn get(this: &Lazy<T, F>) -> &T {
+        this.0.get()
+    }
+}
+
+impl<T, F: Fn() -> T> core::ops::Deref for Lazy<T, F> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        Lazy::get(self)
+    }
+}
+
+impl<T: fmt::Debug, F: Fn() -> T> fmt::Debug for Lazy<T, F> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self.0.fmt(f)
+    }
+}
+
+#[cfg(feature = "alloc")]
+mod lazy {
+    use core::{
+        fmt,
+        marker::PhantomData,
+        sync::atomic::{AtomicPtr, Ordering},
+    };
+
+    use alloc::boxed::Box;
+
+    /// A non-std lazy initialized value.
+    ///
+    /// This might run the initialization function more than once, but will
+    /// never block.
+    ///
+    /// I wish I could get these semantics into the non-alloc non-std Lazy
+    /// type below, but I'm not sure how to do it. If you can do an alloc,
+    /// then the implementation becomes very simple if you don't care about
+    /// redundant work precisely because a pointer can be atomically swapped.
+    ///
+    /// Perhaps making this approach work in the non-alloc non-std case
+    /// requires asking the caller for a pointer? It would make the API less
+    /// convenient I think.
+    pub(super) struct Lazy<T, F> {
+        data: AtomicPtr<T>,
+        create: F,
+        // This indicates to the compiler that this type can drop T. It's not
+        // totally clear how the absence of this marker could lead to trouble,
+        // but putting here doesn't have any downsides so we hedge until somone
+        // can from the Unsafe Working Group can tell us definitively that we
+        // don't need it.
+        //
+        // See: https://github.com/BurntSushi/regex-automata/issues/30
+        owned: PhantomData<Box<T>>,
+    }
+
+    // SAFETY: So long as T and &T (and F and &F) can themselves be safely
+    // shared among threads, so to can a Lazy<T, _>. Namely, the Lazy API only
+    // permits accessing a &T and initialization is free of data races. So if T
+    // is thread safe, then so to is Lazy<T, _>.
+    //
+    // We specifically require that T: Send in order for Lazy<T> to be Sync.
+    // Without that requirement, it's possible to send a T from one thread to
+    // another via Lazy's destructor.
+    //
+    // It's not clear whether we need F: Send+Sync for Lazy to be Sync. But
+    // we're conservative for now and keep both.
+    unsafe impl<T: Send + Sync, F: Send + Sync> Sync for Lazy<T, F> {}
+
+    impl<T, F> Lazy<T, F> {
+        /// Create a new alloc but non-std lazy value that is racily
+        /// initialized. That is, the 'create' function may be called more than
+        /// once.
+        pub(super) const fn new(create: F) -> Lazy<T, F> {
+            Lazy {
+                data: AtomicPtr::new(core::ptr::null_mut()),
+                create,
+                owned: PhantomData,
+            }
+        }
+    }
+
+    impl<T, F: Fn() -> T> Lazy<T, F> {
+        /// Get the underlying lazy value. If it hasn't been initialized
+        /// yet, then always attempt to initialize it (even if some other
+        /// thread is initializing it) and atomically attach it to this lazy
+        /// value before returning it.
+        pub(super) fn get(&self) -> &T {
+            if let Some(data) = self.poll() {
+                return data;
+            }
+            let data = (self.create)();
+            let mut ptr = Box::into_raw(Box::new(data));
+            // We attempt to stuff our initialized value into our atomic
+            // pointer. Upon success, we don't need to do anything. But if
+            // someone else beat us to the punch, then we need to make sure
+            // our newly created value is dropped.
+            let result = self.data.compare_exchange(
+                core::ptr::null_mut(),
+                ptr,
+                Ordering::AcqRel,
+                Ordering::Acquire,
+            );
+            if let Err(old) = result {
+                // SAFETY: We created 'ptr' via Box::into_raw above, so turning
+                // it back into a Box via from_raw is safe.
+                drop(unsafe { Box::from_raw(ptr) });
+                ptr = old;
+            }
+            // SAFETY: We just set the pointer above to a non-null value, even
+            // in the error case, and set it to a fully initialized value
+            // returned by 'create'.
+            unsafe { &*ptr }
+        }
+
+        /// If this lazy value has been initialized successfully, then return
+        /// that value. Otherwise return None immediately. This never attempts
+        /// to run initialization itself.
+        fn poll(&self) -> Option<&T> {
+            let ptr = self.data.load(Ordering::Acquire);
+            if ptr.is_null() {
+                return None;
+            }
+            // SAFETY: We just checked that the pointer is not null. Since it's
+            // not null, it must have been fully initialized by 'get' at some
+            // point.
+            Some(unsafe { &*ptr })
+        }
+    }
+
+    impl<T: fmt::Debug, F: Fn() -> T> fmt::Debug for Lazy<T, F> {
+        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+            f.debug_struct("Lazy").field("data", &self.poll()).finish()
+        }
+    }
+
+    impl<T, F> Drop for Lazy<T, F> {
+        fn drop(&mut self) {
+            let ptr = *self.data.get_mut();
+            if !ptr.is_null() {
+                // SAFETY: We just checked that 'ptr' is not null. And since
+                // we have exclusive access, there are no races to worry about.
+                drop(unsafe { Box::from_raw(ptr) });
+            }
+        }
+    }
+}
+
+#[cfg(not(feature = "alloc"))]
+mod lazy {
+    use core::{
+        cell::Cell,
+        fmt,
+        mem::MaybeUninit,
+        panic::{RefUnwindSafe, UnwindSafe},
+        sync::atomic::{AtomicU8, Ordering},
+    };
+
+    /// Our 'Lazy' value can be in one of three states:
+    ///
+    /// * INIT is where it starts, and also ends up back here if the
+    /// 'create' routine panics.
+    /// * BUSY is where it sits while initialization is running in exactly
+    /// one thread.
+    /// * DONE is where it sits after 'create' has completed and 'data' has
+    /// been fully initialized.
+    const LAZY_STATE_INIT: u8 = 0;
+    const LAZY_STATE_BUSY: u8 = 1;
+    const LAZY_STATE_DONE: u8 = 2;
+
+    /// A non-alloc non-std lazy initialized value.
+    ///
+    /// This guarantees initialization only happens once, but uses a spinlock
+    /// to block in the case of simultaneous access. Blocking occurs so that
+    /// one thread waits while another thread initializes the value.
+    ///
+    /// I would much rather have the semantics of the 'alloc' Lazy type above.
+    /// Namely, that we might run the initialization function more than once,
+    /// but we never otherwise block. However, I don't know how to do that in
+    /// a non-alloc non-std context.
+    pub(super) struct Lazy<T, F> {
+        state: AtomicU8,
+        create: Cell<Option<F>>,
+        data: Cell<MaybeUninit<T>>,
+    }
+
+    // SAFETY: So long as T and &T (and F and &F) can themselves be safely
+    // shared among threads, so to can a Lazy<T, _>. Namely, the Lazy API only
+    // permits accessing a &T and initialization is free of data races. So if T
+    // is thread safe, then so to is Lazy<T, _>.
+    unsafe impl<T: Send + Sync, F: Send + Sync> Sync for Lazy<T, F> {}
+    // A reference to a Lazy is unwind safe because we specifically take
+    // precautions to poison all accesses to a Lazy if the caller-provided
+    // 'create' function panics.
+    impl<T: UnwindSafe, F: UnwindSafe + RefUnwindSafe> RefUnwindSafe
+        for Lazy<T, F>
+    {
+    }
+
+    impl<T, F> Lazy<T, F> {
+        /// Create a new non-alloc non-std lazy value that is initialized
+        /// exactly once on first use using the given function.
+        pub(super) const fn new(create: F) -> Lazy<T, F> {
+            Lazy {
+                state: AtomicU8::new(LAZY_STATE_INIT),
+                create: Cell::new(Some(create)),
+                data: Cell::new(MaybeUninit::uninit()),
+            }
+        }
+    }
+
+    impl<T, F: FnOnce() -> T> Lazy<T, F> {
+        /// Get the underlying lazy value. If it isn't been initialized
+        /// yet, then either initialize it or block until some other thread
+        /// initializes it. If the 'create' function given to Lazy::new panics
+        /// (even in another thread), then this panics too.
+        pub(super) fn get(&self) -> &T {
+            // This is effectively a spinlock. We loop until we enter a DONE
+            // state, and if possible, initialize it ourselves. The only way
+            // we exit the loop is if 'create' panics, we initialize 'data' or
+            // some other thread initializes 'data'.
+            //
+            // Yes, I have read spinlocks considered harmful[1]. And that
+            // article is why this spinlock is only active when 'alloc' isn't
+            // enabled. I did this because I don't think there is really
+            // another choice without 'alloc', other than not providing this at
+            // all. But I think that's a big bummer.
+            //
+            // [1]: https://matklad.github.io/2020/01/02/spinlocks-considered-harmful.html
+            while self.state.load(Ordering::Acquire) != LAZY_STATE_DONE {
+                // Check if we're the first ones to get here. If so, we'll be
+                // the ones who initialize.
+                let result = self.state.compare_exchange(
+                    LAZY_STATE_INIT,
+                    LAZY_STATE_BUSY,
+                    Ordering::AcqRel,
+                    Ordering::Acquire,
+                );
+                // This means we saw the INIT state and nobody else can. So we
+                // must take responsibility for initializing. And by virtue of
+                // observing INIT, we have also told anyone else trying to
+                // get here that we are BUSY. If someone else sees BUSY, then
+                // they will spin until we finish initialization.
+                if let Ok(_) = result {
+                    // Since we are guaranteed to be the only ones here, we
+                    // know that 'create' is there... Unless someone else got
+                    // here before us and 'create' panicked. In which case,
+                    // 'self.create' is now 'None' and we forward the panic
+                    // to the caller. (i.e., We implement poisoning.)
+                    //
+                    // SAFETY: Our use of 'self.state' guarantees that we are
+                    // the only thread executing this line, and thus there are
+                    // no races.
+                    let create = unsafe {
+                        (*self.create.as_ptr()).take().expect(
+                            "Lazy's create function panicked, \
+                             preventing initialization,
+                             poisoning current thread",
+                        )
+                    };
+                    let guard = Guard { state: &self.state };
+                    // SAFETY: Our use of 'self.state' guarantees that we are
+                    // the only thread executing this line, and thus there are
+                    // no races.
+                    unsafe {
+                        (*self.data.as_ptr()).as_mut_ptr().write(create());
+                    }
+                    // All is well. 'self.create' ran successfully, so we
+                    // forget the guard.
+                    core::mem::forget(guard);
+                    // Everything is initialized, so we can declare success.
+                    self.state.store(LAZY_STATE_DONE, Ordering::Release);
+                    break;
+                }
+                core::hint::spin_loop();
+            }
+            // We only get here if data is fully initialized, and thus poll
+            // will always return something.
+            self.poll().unwrap()
+        }
+
+        /// If this lazy value has been initialized successfully, then return
+        /// that value. Otherwise return None immediately. This never blocks.
+        fn poll(&self) -> Option<&T> {
+            if self.state.load(Ordering::Acquire) == LAZY_STATE_DONE {
+                // SAFETY: The DONE state only occurs when data has been fully
+                // initialized.
+                Some(unsafe { &*(*self.data.as_ptr()).as_ptr() })
+            } else {
+                None
+            }
+        }
+    }
+
+    impl<T: fmt::Debug, F: FnMut() -> T> fmt::Debug for Lazy<T, F> {
+        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+            f.debug_struct("Lazy")
+                .field("state", &self.state.load(Ordering::Acquire))
+                .field("create", &"<closure>")
+                .field("data", &self.poll())
+                .finish()
+        }
+    }
+
+    impl<T, F> Drop for Lazy<T, F> {
+        fn drop(&mut self) {
+            if *self.state.get_mut() == LAZY_STATE_DONE {
+                // SAFETY: state is DONE if and only if data has been fully
+                // initialized. At which point, it is safe to drop.
+                unsafe {
+                    // MSRV(1.60): Use assume_init_drop. The below is how
+                    // assume_init_drop is implemented.
+                    core::ptr::drop_in_place(
+                        (*self.data.as_ptr()).as_mut_ptr(),
+                    )
+                }
+            }
+        }
+    }
+
+    /// A guard that will reset a Lazy's state back to INIT when dropped. The
+    /// idea here is to 'forget' this guard on success. On failure (when a
+    /// panic occurs), the Drop impl runs and causes all in-progress and future
+    /// 'get' calls to panic. Without this guard, all in-progress and future
+    /// 'get' calls would spin forever. Crashing is much better than getting
+    /// stuck in an infinite loop.
+    struct Guard<'a> {
+        state: &'a AtomicU8,
+    }
+
+    impl<'a> Drop for Guard<'a> {
+        fn drop(&mut self) {
+            // We force ourselves back into an INIT state. This will in turn
+            // cause any future 'get' calls to attempt calling 'self.create'
+            // again which will in turn panic because 'self.create' will now
+            // be 'None'.
+            self.state.store(LAZY_STATE_INIT, Ordering::Release);
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn assert_send<T: Send>() {}
+    fn assert_sync<T: Sync>() {}
+    fn assert_unwind<T: core::panic::UnwindSafe>() {}
+    fn assert_refunwind<T: core::panic::RefUnwindSafe>() {}
+
+    #[test]
+    fn oibits() {
+        assert_send::<Lazy<u64>>();
+        assert_sync::<Lazy<u64>>();
+        assert_unwind::<Lazy<u64>>();
+        assert_refunwind::<Lazy<u64>>();
+    }
+
+    // This is a regression test because we used to rely on the inferred Sync
+    // impl for the Lazy type defined above (for 'alloc' mode). In the
+    // inferred impl, it only requires that T: Sync for Lazy<T>: Sync. But
+    // if we have that, we can actually make use of the fact that Lazy<T> drops
+    // T to create a value on one thread and drop it on another. This *should*
+    // require T: Send, but our missing bounds before let it sneak by.
+    //
+    // Basically, this test should not compile, so we... comment it out. We
+    // don't have a great way of testing compile-fail tests right now.
+    //
+    // See: https://github.com/BurntSushi/regex-automata/issues/30
+    /*
+    #[test]
+    fn sync_not_send() {
+        #[allow(dead_code)]
+        fn inner<T: Sync + Default>() {
+            let lazy = Lazy::new(move || T::default());
+            std::thread::scope(|scope| {
+                scope.spawn(|| {
+                    Lazy::get(&lazy); // We create T in this thread
+                });
+            });
+            // And drop in this thread.
+            drop(lazy);
+            // So we have send a !Send type over threads. (with some more
+            // legwork, its possible to even sneak the value out of drop
+            // through thread local)
+        }
+    }
+    */
+}