Initial vendor packages

Signed-off-by: Valentin Popov <valentin@popov.link>

1 files changed, 61 insertions, 0 deletions

diff --git a/vendor/spin/src/relax.rs b/vendor/spin/src/relax.rs
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+//! Strategies that determine the behaviour of locks when encountering contention.
+
+/// A trait implemented by spinning relax strategies.
+pub trait RelaxStrategy {
+    /// Perform the relaxing operation during a period of contention.
+    fn relax();
+}
+
+/// A strategy that rapidly spins while informing the CPU that it should power down non-essential components via
+/// [`core::hint::spin_loop`].
+///
+/// Note that spinning is a 'dumb' strategy and most schedulers cannot correctly differentiate it from useful work,
+/// thereby misallocating even more CPU time to the spinning process. This is known as
+/// ['priority inversion'](https://matklad.github.io/2020/01/02/spinlocks-considered-harmful.html).
+///
+/// If you see signs that priority inversion is occurring, consider switching to [`Yield`] or, even better, not using a
+/// spinlock at all and opting for a proper scheduler-aware lock. Remember also that different targets, operating
+/// systems, schedulers, and even the same scheduler with different workloads will exhibit different behaviour. Just
+/// because priority inversion isn't occurring in your tests does not mean that it will not occur. Use a scheduler-
+/// aware lock if at all possible.
+pub struct Spin;
+
+impl RelaxStrategy for Spin {
+    #[inline(always)]
+    fn relax() {
+        // Use the deprecated spin_loop_hint() to ensure that we don't get
+        // a higher MSRV than we need to.
+        #[allow(deprecated)]
+        core::sync::atomic::spin_loop_hint();
+    }
+}
+
+/// A strategy that yields the current time slice to the scheduler in favour of other threads or processes.
+///
+/// This is generally used as a strategy for minimising power consumption and priority inversion on targets that have a
+/// standard library available. Note that such targets have scheduler-integrated concurrency primitives available, and
+/// you should generally use these instead, except in rare circumstances.
+#[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+pub struct Yield;
+
+#[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+impl RelaxStrategy for Yield {
+    #[inline(always)]
+    fn relax() {
+        std::thread::yield_now();
+    }
+}
+
+/// A strategy that rapidly spins, without telling the CPU to do any powering down.
+///
+/// You almost certainly do not want to use this. Use [`Spin`] instead. It exists for completeness and for targets
+/// that, for some reason, miscompile or do not support spin hint intrinsics despite attempting to generate code for
+/// them (i.e: this is a workaround for possible compiler bugs).
+pub struct Loop;
+
+impl RelaxStrategy for Loop {
+    #[inline(always)]
+    fn relax() {}
+}