diff options
Diffstat (limited to 'library/std')
| -rw-r--r-- | library/std/Cargo.toml | 1 | ||||
| -rw-r--r-- | library/std/src/lib.rs | 2 | ||||
| -rw-r--r-- | library/std/src/macros.rs | 74 | ||||
| -rw-r--r-- | library/std/src/panic.rs | 2 | ||||
| -rw-r--r-- | library/std/src/panicking.rs | 14 | ||||
| -rw-r--r-- | library/std/src/path.rs | 5 | ||||
| -rw-r--r-- | library/std/src/sync/lazy_lock.rs | 60 | ||||
| -rw-r--r-- | library/std/src/sync/once_lock.rs | 2 | ||||
| -rw-r--r-- | library/std/src/sync/poison.rs | 24 | ||||
| -rw-r--r-- | library/std/src/sync/poison/mutex.rs | 70 | ||||
| -rw-r--r-- | library/std/src/sync/poison/once.rs | 6 | ||||
| -rw-r--r-- | library/std/src/sync/poison/rwlock.rs | 10 | ||||
| -rw-r--r-- | library/std/src/sys/pal/uefi/helpers.rs | 22 | ||||
| -rw-r--r-- | library/std/src/thread/mod.rs | 3 |
14 files changed, 246 insertions, 49 deletions
diff --git a/library/std/Cargo.toml b/library/std/Cargo.toml index 29ab9be0e69..7bc52976500 100644 --- a/library/std/Cargo.toml +++ b/library/std/Cargo.toml @@ -18,7 +18,6 @@ cfg-if = { version = "1.0", features = ['rustc-dep-of-std'] } panic_unwind = { path = "../panic_unwind", optional = true } panic_abort = { path = "../panic_abort" } core = { path = "../core", public = true } -compiler_builtins = { path = "../compiler-builtins/compiler-builtins" } unwind = { path = "../unwind" } hashbrown = { version = "0.15", default-features = false, features = [ 'rustc-dep-of-std', diff --git a/library/std/src/lib.rs b/library/std/src/lib.rs index 77301d7228e..fd06a3b540c 100644 --- a/library/std/src/lib.rs +++ b/library/std/src/lib.rs @@ -294,6 +294,8 @@ #![feature(f128)] #![feature(ffi_const)] #![feature(formatting_options)] +#![feature(hash_map_internals)] +#![feature(hash_map_macro)] #![feature(if_let_guard)] #![feature(intra_doc_pointers)] #![feature(iter_advance_by)] diff --git a/library/std/src/macros.rs b/library/std/src/macros.rs index 25e2b7ea137..254570ae9c8 100644 --- a/library/std/src/macros.rs +++ b/library/std/src/macros.rs @@ -379,3 +379,77 @@ macro_rules! dbg { ($($crate::dbg!($val)),+,) }; } + +#[doc(hidden)] +#[macro_export] +#[allow_internal_unstable(hash_map_internals)] +#[unstable(feature = "hash_map_internals", issue = "none")] +macro_rules! repetition_utils { + (@count $($tokens:tt),*) => {{ + [$($crate::repetition_utils!(@replace $tokens => ())),*].len() + }}; + + (@replace $x:tt => $y:tt) => { $y } +} + +/// Creates a [`HashMap`] containing the arguments. +/// +/// `hash_map!` allows specifying the entries that make +/// up the [`HashMap`] where the key and value are separated by a `=>`. +/// +/// The entries are separated by commas with a trailing comma being allowed. +/// +/// It is semantically equivalent to using repeated [`HashMap::insert`] +/// on a newly created hashmap. +/// +/// `hash_map!` will attempt to avoid repeated reallocations by +/// using [`HashMap::with_capacity`]. +/// +/// # Examples +/// +/// ```rust +/// #![feature(hash_map_macro)] +/// +/// let map = hash_map! { +/// "key" => "value", +/// "key1" => "value1" +/// }; +/// +/// assert_eq!(map.get("key"), Some(&"value")); +/// assert_eq!(map.get("key1"), Some(&"value1")); +/// assert!(map.get("brrrrrrooooommm").is_none()); +/// ``` +/// +/// And with a trailing comma +/// +///```rust +/// #![feature(hash_map_macro)] +/// +/// let map = hash_map! { +/// "key" => "value", // notice the , +/// }; +/// +/// assert_eq!(map.get("key"), Some(&"value")); +/// ``` +/// +/// The key and value are moved into the HashMap. +/// +/// [`HashMap`]: crate::collections::HashMap +/// [`HashMap::insert`]: crate::collections::HashMap::insert +/// [`HashMap::with_capacity`]: crate::collections::HashMap::with_capacity +#[macro_export] +#[allow_internal_unstable(hash_map_internals)] +#[unstable(feature = "hash_map_macro", issue = "144032")] +macro_rules! hash_map { + () => {{ + $crate::collections::HashMap::new() + }}; + + ( $( $key:expr => $value:expr ),* $(,)? ) => {{ + let mut map = $crate::collections::HashMap::with_capacity( + const { $crate::repetition_utils!(@count $($key),*) } + ); + $( map.insert($key, $value); )* + map + }} +} diff --git a/library/std/src/panic.rs b/library/std/src/panic.rs index 234fb284a59..913ef72f674 100644 --- a/library/std/src/panic.rs +++ b/library/std/src/panic.rs @@ -388,7 +388,7 @@ pub fn catch_unwind<F: FnOnce() -> R + UnwindSafe, R>(f: F) -> Result<R> { /// ``` #[stable(feature = "resume_unwind", since = "1.9.0")] pub fn resume_unwind(payload: Box<dyn Any + Send>) -> ! { - panicking::rust_panic_without_hook(payload) + panicking::resume_unwind(payload) } /// Makes all future panics abort directly without running the panic hook or unwinding. diff --git a/library/std/src/panicking.rs b/library/std/src/panicking.rs index 7873049d20b..224cd39855a 100644 --- a/library/std/src/panicking.rs +++ b/library/std/src/panicking.rs @@ -696,14 +696,14 @@ pub fn begin_panic_handler(info: &core::panic::PanicInfo<'_>) -> ! { let msg = info.message(); crate::sys::backtrace::__rust_end_short_backtrace(move || { if let Some(s) = msg.as_str() { - rust_panic_with_hook( + panic_with_hook( &mut StaticStrPayload(s), loc, info.can_unwind(), info.force_no_backtrace(), ); } else { - rust_panic_with_hook( + panic_with_hook( &mut FormatStringPayload { inner: &msg, string: None }, loc, info.can_unwind(), @@ -767,7 +767,7 @@ pub const fn begin_panic<M: Any + Send>(msg: M) -> ! { let loc = Location::caller(); crate::sys::backtrace::__rust_end_short_backtrace(move || { - rust_panic_with_hook( + panic_with_hook( &mut Payload { inner: Some(msg) }, loc, /* can_unwind */ true, @@ -792,7 +792,7 @@ fn payload_as_str(payload: &dyn Any) -> &str { /// panics, panic hooks, and finally dispatching to the panic runtime to either /// abort or unwind. #[optimize(size)] -fn rust_panic_with_hook( +fn panic_with_hook( payload: &mut dyn PanicPayload, location: &Location<'_>, can_unwind: bool, @@ -861,7 +861,7 @@ fn rust_panic_with_hook( /// This is the entry point for `resume_unwind`. /// It just forwards the payload to the panic runtime. #[cfg_attr(feature = "panic_immediate_abort", inline)] -pub fn rust_panic_without_hook(payload: Box<dyn Any + Send>) -> ! { +pub fn resume_unwind(payload: Box<dyn Any + Send>) -> ! { panic_count::increase(false); struct RewrapBox(Box<dyn Any + Send>); @@ -885,8 +885,8 @@ pub fn rust_panic_without_hook(payload: Box<dyn Any + Send>) -> ! { rust_panic(&mut RewrapBox(payload)) } -/// An unmangled function (through `rustc_std_internal_symbol`) on which to slap -/// yer breakpoints. +/// A function with a fixed suffix (through `rustc_std_internal_symbol`) +/// on which to slap yer breakpoints. #[inline(never)] #[cfg_attr(not(test), rustc_std_internal_symbol)] #[cfg(not(feature = "panic_immediate_abort"))] diff --git a/library/std/src/path.rs b/library/std/src/path.rs index d9c34d4fa04..055e7f81480 100644 --- a/library/std/src/path.rs +++ b/library/std/src/path.rs @@ -3259,8 +3259,8 @@ impl Path { /// /// # Examples /// - #[cfg_attr(unix, doc = "```no_run")] - #[cfg_attr(not(unix), doc = "```ignore")] + /// ```rust,no_run + /// # #[cfg(unix)] { /// use std::path::Path; /// use std::os::unix::fs::symlink; /// @@ -3268,6 +3268,7 @@ impl Path { /// symlink("/origin_does_not_exist/", link_path).unwrap(); /// assert_eq!(link_path.is_symlink(), true); /// assert_eq!(link_path.exists(), false); + /// # } /// ``` /// /// # See Also diff --git a/library/std/src/sync/lazy_lock.rs b/library/std/src/sync/lazy_lock.rs index eba849d16da..a40e29a772a 100644 --- a/library/std/src/sync/lazy_lock.rs +++ b/library/std/src/sync/lazy_lock.rs @@ -25,6 +25,22 @@ union Data<T, F> { /// /// [`LazyCell`]: crate::cell::LazyCell /// +/// # Poisoning +/// +/// If the initialization closure passed to [`LazyLock::new`] panics, the lock will be poisoned. +/// Once the lock is poisoned, any threads that attempt to access this lock (via a dereference +/// or via an explicit call to [`force()`]) will panic. +/// +/// This concept is similar to that of poisoning in the [`std::sync::poison`] module. A key +/// difference, however, is that poisoning in `LazyLock` is _unrecoverable_. All future accesses of +/// the lock from other threads will panic, whereas a type in [`std::sync::poison`] like +/// [`std::sync::poison::Mutex`] allows recovery via [`PoisonError::into_inner()`]. +/// +/// [`force()`]: LazyLock::force +/// [`std::sync::poison`]: crate::sync::poison +/// [`std::sync::poison::Mutex`]: crate::sync::poison::Mutex +/// [`PoisonError::into_inner()`]: crate::sync::poison::PoisonError::into_inner +/// /// # Examples /// /// Initialize static variables with `LazyLock`. @@ -102,6 +118,10 @@ impl<T, F: FnOnce() -> T> LazyLock<T, F> { /// /// Returns `Ok(value)` if `Lazy` is initialized and `Err(f)` otherwise. /// + /// # Panics + /// + /// Panics if the lock is poisoned. + /// /// # Examples /// /// ``` @@ -136,6 +156,15 @@ impl<T, F: FnOnce() -> T> LazyLock<T, F> { /// Forces the evaluation of this lazy value and returns a mutable reference to /// the result. /// + /// # Panics + /// + /// If the initialization closure panics (the one that is passed to the [`new()`] method), the + /// panic is propagated to the caller, and the lock becomes poisoned. This will cause all future + /// accesses of the lock (via [`force()`] or a dereference) to panic. + /// + /// [`new()`]: LazyLock::new + /// [`force()`]: LazyLock::force + /// /// # Examples /// /// ``` @@ -193,6 +222,15 @@ impl<T, F: FnOnce() -> T> LazyLock<T, F> { /// This method will block the calling thread if another initialization /// routine is currently running. /// + /// # Panics + /// + /// If the initialization closure panics (the one that is passed to the [`new()`] method), the + /// panic is propagated to the caller, and the lock becomes poisoned. This will cause all future + /// accesses of the lock (via [`force()`] or a dereference) to panic. + /// + /// [`new()`]: LazyLock::new + /// [`force()`]: LazyLock::force + /// /// # Examples /// /// ``` @@ -227,7 +265,8 @@ impl<T, F: FnOnce() -> T> LazyLock<T, F> { } impl<T, F> LazyLock<T, F> { - /// Returns a mutable reference to the value if initialized, or `None` if not. + /// Returns a mutable reference to the value if initialized. Otherwise (if uninitialized or + /// poisoned), returns `None`. /// /// # Examples /// @@ -256,7 +295,8 @@ impl<T, F> LazyLock<T, F> { } } - /// Returns a reference to the value if initialized, or `None` if not. + /// Returns a reference to the value if initialized. Otherwise (if uninitialized or poisoned), + /// returns `None`. /// /// # Examples /// @@ -307,6 +347,14 @@ impl<T, F: FnOnce() -> T> Deref for LazyLock<T, F> { /// This method will block the calling thread if another initialization /// routine is currently running. /// + /// # Panics + /// + /// If the initialization closure panics (the one that is passed to the [`new()`] method), the + /// panic is propagated to the caller, and the lock becomes poisoned. This will cause all future + /// accesses of the lock (via [`force()`] or a dereference) to panic. + /// + /// [`new()`]: LazyLock::new + /// [`force()`]: LazyLock::force #[inline] fn deref(&self) -> &T { LazyLock::force(self) @@ -315,6 +363,14 @@ impl<T, F: FnOnce() -> T> Deref for LazyLock<T, F> { #[stable(feature = "lazy_deref_mut", since = "1.89.0")] impl<T, F: FnOnce() -> T> DerefMut for LazyLock<T, F> { + /// # Panics + /// + /// If the initialization closure panics (the one that is passed to the [`new()`] method), the + /// panic is propagated to the caller, and the lock becomes poisoned. This will cause all future + /// accesses of the lock (via [`force()`] or a dereference) to panic. + /// + /// [`new()`]: LazyLock::new + /// [`force()`]: LazyLock::force #[inline] fn deref_mut(&mut self) -> &mut T { LazyLock::force_mut(self) diff --git a/library/std/src/sync/once_lock.rs b/library/std/src/sync/once_lock.rs index a5c3a6c46a4..b224044cbe0 100644 --- a/library/std/src/sync/once_lock.rs +++ b/library/std/src/sync/once_lock.rs @@ -16,6 +16,8 @@ use crate::sync::Once; /// A `OnceLock` can be thought of as a safe abstraction over uninitialized data that becomes /// initialized once written. /// +/// Unlike [`Mutex`](crate::sync::Mutex), `OnceLock` is never poisoned on panic. +/// /// [`OnceCell`]: crate::cell::OnceCell /// [`LazyLock<T, F>`]: crate::sync::LazyLock /// [`LazyLock::new(|| ...)`]: crate::sync::LazyLock::new diff --git a/library/std/src/sync/poison.rs b/library/std/src/sync/poison.rs index b901a5701a4..31889dcc10f 100644 --- a/library/std/src/sync/poison.rs +++ b/library/std/src/sync/poison.rs @@ -2,15 +2,16 @@ //! //! # Poisoning //! -//! All synchronization objects in this module implement a strategy called "poisoning" -//! where if a thread panics while holding the exclusive access granted by the primitive, -//! the state of the primitive is set to "poisoned". -//! This information is then propagated to all other threads +//! All synchronization objects in this module implement a strategy called +//! "poisoning" where a primitive becomes poisoned if it recognizes that some +//! thread has panicked while holding the exclusive access granted by the +//! primitive. This information is then propagated to all other threads //! to signify that the data protected by this primitive is likely tainted //! (some invariant is not being upheld). //! -//! The specifics of how this "poisoned" state affects other threads -//! depend on the primitive. See [#Overview] below. +//! The specifics of how this "poisoned" state affects other threads and whether +//! the panics are recognized reliably or on a best-effort basis depend on the +//! primitive. See [Overview](#overview) below. //! //! For the alternative implementations that do not employ poisoning, //! see [`std::sync::nonpoison`]. @@ -36,14 +37,15 @@ //! - [`Mutex`]: Mutual Exclusion mechanism, which ensures that at //! most one thread at a time is able to access some data. //! -//! [`Mutex::lock()`] returns a [`LockResult`], -//! providing a way to deal with the poisoned state. -//! See [`Mutex`'s documentation](Mutex#poisoning) for more. +//! Panicking while holding the lock typically poisons the mutex, but it is +//! not guaranteed to detect this condition in all circumstances. +//! [`Mutex::lock()`] returns a [`LockResult`], providing a way to deal with +//! the poisoned state. See [`Mutex`'s documentation](Mutex#poisoning) for more. //! //! - [`Once`]: A thread-safe way to run a piece of code only once. //! Mostly useful for implementing one-time global initialization. //! -//! [`Once`] is poisoned if the piece of code passed to +//! [`Once`] is reliably poisoned if the piece of code passed to //! [`Once::call_once()`] or [`Once::call_once_force()`] panics. //! When in poisoned state, subsequent calls to [`Once::call_once()`] will panic too. //! [`Once::call_once_force()`] can be used to clear the poisoned state. @@ -53,7 +55,7 @@ //! writer at a time. In some cases, this can be more efficient than //! a mutex. //! -//! This implementation, like [`Mutex`], will become poisoned on a panic. +//! This implementation, like [`Mutex`], usually becomes poisoned on a panic. //! Note, however, that an `RwLock` may only be poisoned if a panic occurs //! while it is locked exclusively (write mode). If a panic occurs in any reader, //! then the lock will not be poisoned. diff --git a/library/std/src/sync/poison/mutex.rs b/library/std/src/sync/poison/mutex.rs index 64744f18c74..6205c4fa4ca 100644 --- a/library/std/src/sync/poison/mutex.rs +++ b/library/std/src/sync/poison/mutex.rs @@ -18,20 +18,69 @@ use crate::sys::sync as sys; /// # Poisoning /// /// The mutexes in this module implement a strategy called "poisoning" where a -/// mutex is considered poisoned whenever a thread panics while holding the -/// mutex. Once a mutex is poisoned, all other threads are unable to access the -/// data by default as it is likely tainted (some invariant is not being -/// upheld). +/// mutex becomes poisoned if it recognizes that the thread holding it has +/// panicked. /// -/// For a mutex, this means that the [`lock`] and [`try_lock`] methods return a +/// Once a mutex is poisoned, all other threads are unable to access the data by +/// default as it is likely tainted (some invariant is not being upheld). For a +/// mutex, this means that the [`lock`] and [`try_lock`] methods return a /// [`Result`] which indicates whether a mutex has been poisoned or not. Most /// usage of a mutex will simply [`unwrap()`] these results, propagating panics /// among threads to ensure that a possibly invalid invariant is not witnessed. /// -/// A poisoned mutex, however, does not prevent all access to the underlying -/// data. The [`PoisonError`] type has an [`into_inner`] method which will return -/// the guard that would have otherwise been returned on a successful lock. This -/// allows access to the data, despite the lock being poisoned. +/// Poisoning is only advisory: the [`PoisonError`] type has an [`into_inner`] +/// method which will return the guard that would have otherwise been returned +/// on a successful lock. This allows access to the data, despite the lock being +/// poisoned. +/// +/// In addition, the panic detection is not ideal, so even unpoisoned mutexes +/// need to be handled with care, since certain panics may have been skipped. +/// Here is a non-exhaustive list of situations where this might occur: +/// +/// - If a mutex is locked while a panic is underway, e.g. within a [`Drop`] +/// implementation or a [panic hook], panicking for the second time while the +/// lock is held will leave the mutex unpoisoned. Note that while double panic +/// usually aborts the program, [`catch_unwind`] can prevent this. +/// +/// - Locking and unlocking the mutex across different panic contexts, e.g. by +/// storing the guard to a [`Cell`] within [`Drop::drop`] and accessing it +/// outside, or vice versa, can affect poisoning status in an unexpected way. +/// +/// - Foreign exceptions do not currently trigger poisoning even in absence of +/// other panics. +/// +/// While this rarely happens in realistic code, `unsafe` code cannot rely on +/// poisoning for soundness, since the behavior of poisoning can depend on +/// outside context. Here's an example of **incorrect** use of poisoning: +/// +/// ```rust +/// use std::sync::Mutex; +/// +/// struct MutexBox<T> { +/// data: Mutex<*mut T>, +/// } +/// +/// impl<T> MutexBox<T> { +/// pub fn new(value: T) -> Self { +/// Self { +/// data: Mutex::new(Box::into_raw(Box::new(value))), +/// } +/// } +/// +/// pub fn replace_with(&self, f: impl FnOnce(T) -> T) { +/// let ptr = self.data.lock().expect("poisoned"); +/// // While `f` is running, the data is moved out of `*ptr`. If `f` +/// // panics, `*ptr` keeps pointing at a dropped value. The intention +/// // is that this will poison the mutex, so the following calls to +/// // `replace_with` will panic without reading `*ptr`. But since +/// // poisoning is not guaranteed to occur if this is run from a panic +/// // hook, this can lead to use-after-free. +/// unsafe { +/// (*ptr).write(f((*ptr).read())); +/// } +/// } +/// } +/// ``` /// /// [`new`]: Self::new /// [`lock`]: Self::lock @@ -39,6 +88,9 @@ use crate::sys::sync as sys; /// [`unwrap()`]: Result::unwrap /// [`PoisonError`]: super::PoisonError /// [`into_inner`]: super::PoisonError::into_inner +/// [panic hook]: crate::panic::set_hook +/// [`catch_unwind`]: crate::panic::catch_unwind +/// [`Cell`]: crate::cell::Cell /// /// # Examples /// diff --git a/library/std/src/sync/poison/once.rs b/library/std/src/sync/poison/once.rs index 103e5195407..faf2913c547 100644 --- a/library/std/src/sync/poison/once.rs +++ b/library/std/src/sync/poison/once.rs @@ -136,7 +136,8 @@ impl Once { /// it will *poison* this [`Once`] instance, causing all future invocations of /// `call_once` to also panic. /// - /// This is similar to [poisoning with mutexes][poison]. + /// This is similar to [poisoning with mutexes][poison], but this mechanism + /// is guaranteed to never skip panics within `f`. /// /// [poison]: struct.Mutex.html#poisoning #[inline] @@ -293,6 +294,9 @@ impl Once { /// Blocks the current thread until initialization has completed, ignoring /// poisoning. + /// + /// If this [`Once`] has been poisoned, this function blocks until it + /// becomes completed, unlike [`Once::wait()`], which panics in this case. #[stable(feature = "once_wait", since = "1.86.0")] pub fn wait_force(&self) { if !self.inner.is_completed() { diff --git a/library/std/src/sync/poison/rwlock.rs b/library/std/src/sync/poison/rwlock.rs index 934a173425a..2c92602bc87 100644 --- a/library/std/src/sync/poison/rwlock.rs +++ b/library/std/src/sync/poison/rwlock.rs @@ -46,10 +46,12 @@ use crate::sys::sync as sys; /// /// # Poisoning /// -/// An `RwLock`, like [`Mutex`], will become poisoned on a panic. Note, however, -/// that an `RwLock` may only be poisoned if a panic occurs while it is locked -/// exclusively (write mode). If a panic occurs in any reader, then the lock -/// will not be poisoned. +/// An `RwLock`, like [`Mutex`], will [usually] become poisoned on a panic. Note, +/// however, that an `RwLock` may only be poisoned if a panic occurs while it is +/// locked exclusively (write mode). If a panic occurs in any reader, then the +/// lock will not be poisoned. +/// +/// [usually]: super::Mutex#poisoning /// /// # Examples /// diff --git a/library/std/src/sys/pal/uefi/helpers.rs b/library/std/src/sys/pal/uefi/helpers.rs index 271dc4d11de..b50574de937 100644 --- a/library/std/src/sys/pal/uefi/helpers.rs +++ b/library/std/src/sys/pal/uefi/helpers.rs @@ -444,17 +444,17 @@ impl<'a> DevicePathNode<'a> { impl<'a> PartialEq for DevicePathNode<'a> { fn eq(&self, other: &Self) -> bool { - let self_len = self.length(); - let other_len = other.length(); - - self_len == other_len - && unsafe { - compiler_builtins::mem::memcmp( - self.protocol.as_ptr().cast(), - other.protocol.as_ptr().cast(), - usize::from(self_len), - ) == 0 - } + // Compare as a single buffer rather than by field since it optimizes better. + // + // SAFETY: `Protocol` is followed by a buffer of `length - sizeof::<Protocol>()`. `Protocol` + // has no padding so it is sound to interpret as a slice. + unsafe { + let s1 = + slice::from_raw_parts(self.protocol.as_ptr().cast::<u8>(), self.length().into()); + let s2 = + slice::from_raw_parts(other.protocol.as_ptr().cast::<u8>(), other.length().into()); + s1 == s2 + } } } diff --git a/library/std/src/thread/mod.rs b/library/std/src/thread/mod.rs index dff981c900c..8d3ab82ace1 100644 --- a/library/std/src/thread/mod.rs +++ b/library/std/src/thread/mod.rs @@ -2018,6 +2018,9 @@ fn _assert_sync_and_send() { /// which may take time on systems with large numbers of mountpoints. /// (This does not apply to cgroup v2, or to processes not in a /// cgroup.) +/// - It does not attempt to take `ulimit` into account. If there is a limit set on the number of +/// threads, `available_parallelism` cannot know how much of that limit a Rust program should +/// take, or know in a reliable and race-free way how much of that limit is already taken. /// /// On all targets: /// - It may overcount the amount of parallelism available when running in a VM |