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Revert "Remove the Arc rt::init allocation for thread info"
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This reverts commit 1ee38271cae49178080ea4df8f5aae5ad3a81cd6.
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@joboet
joboet authored and gitbot committed Feb 20, 2025
1 parent d18398a commit 1b33103
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Showing 2 changed files with 54 additions and 118 deletions.
2 changes: 1 addition & 1 deletion std/src/rt.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -110,7 +110,7 @@ unsafe fn init(argc: isize, argv: *const *const u8, sigpipe: u8) {
// handle does not match the current ID, we should attempt to use the
// current thread ID here instead of unconditionally creating a new
// one. Also see #130210.
let thread = unsafe { Thread::new_main(thread::current_id()) };
let thread = Thread::new_main(thread::current_id());
if let Err(_thread) = thread::set_current(thread) {
// `thread::current` will create a new handle if none has been set yet.
// Thus, if someone uses it before main, this call will fail. That's a
Expand Down
170 changes: 53 additions & 117 deletions std/src/thread/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -158,12 +158,9 @@
#[cfg(all(test, not(any(target_os = "emscripten", target_os = "wasi"))))]
mod tests;

use core::cell::SyncUnsafeCell;
use core::ffi::CStr;
use core::mem::MaybeUninit;

use crate::any::Any;
use crate::cell::UnsafeCell;
use crate::ffi::CStr;
use crate::marker::PhantomData;
use crate::mem::{self, ManuallyDrop, forget};
use crate::num::NonZero;
Expand Down Expand Up @@ -1259,114 +1256,65 @@ impl ThreadId {
// Thread
////////////////////////////////////////////////////////////////////////////////

/// The internal representation of a `Thread`'s name.
enum ThreadName {
Main,
Other(ThreadNameString),
Unnamed,
}

// This module ensures private fields are kept private, which is necessary to enforce the safety requirements.
mod thread_name_string {
use core::str;

use super::ThreadName;
use crate::ffi::{CStr, CString};

/// Like a `String` it's guaranteed UTF-8 and like a `CString` it's null terminated.
pub(crate) struct ThreadNameString {
inner: CString,
}

impl ThreadNameString {
pub fn as_str(&self) -> &str {
// SAFETY: `self.inner` is only initialised via `String`, which upholds the validity invariant of `str`.
unsafe { str::from_utf8_unchecked(self.inner.to_bytes()) }
}
}

impl core::ops::Deref for ThreadNameString {
type Target = CStr;
fn deref(&self) -> &CStr {
&self.inner
}
}

impl From<String> for ThreadNameString {
fn from(s: String) -> Self {
Self {
inner: CString::new(s).expect("thread name may not contain interior null bytes"),
}
}
}
impl ThreadName {
pub fn as_cstr(&self) -> Option<&CStr> {
match self {
ThreadName::Main => Some(c"main"),
ThreadName::Other(other) => Some(other),
ThreadName::Unnamed => None,
}
}

pub fn as_str(&self) -> Option<&str> {
// SAFETY: `as_cstr` can only return `Some` for a fixed CStr or a `ThreadNameString`,
// which is guaranteed to be UTF-8.
self.as_cstr().map(|s| unsafe { str::from_utf8_unchecked(s.to_bytes()) })
}
}
}
pub(crate) use thread_name_string::ThreadNameString;

static MAIN_THREAD_INFO: SyncUnsafeCell<(MaybeUninit<ThreadId>, MaybeUninit<Parker>)> =
SyncUnsafeCell::new((MaybeUninit::uninit(), MaybeUninit::uninit()));

/// The internal representation of a `Thread` that is not the main thread.
struct OtherInner {
name: Option<ThreadNameString>,
/// The internal representation of a `Thread` handle
struct Inner {
name: ThreadName, // Guaranteed to be UTF-8
id: ThreadId,
parker: Parker,
}

/// The internal representation of a `Thread` handle.
#[derive(Clone)]
enum Inner {
/// Represents the main thread. May only be constructed by Thread::new_main.
Main(&'static (ThreadId, Parker)),
/// Represents any other thread.
Other(Pin<Arc<OtherInner>>),
}

impl Inner {
fn id(&self) -> ThreadId {
match self {
Self::Main((thread_id, _)) => *thread_id,
Self::Other(other) => other.id,
}
}

fn cname(&self) -> Option<&CStr> {
match self {
Self::Main(_) => Some(c"main"),
Self::Other(other) => other.name.as_deref(),
}
}

fn name(&self) -> Option<&str> {
match self {
Self::Main(_) => Some("main"),
Self::Other(other) => other.name.as_ref().map(ThreadNameString::as_str),
}
}

fn into_raw(self) -> *const () {
match self {
// Just return the pointer to `MAIN_THREAD_INFO`.
Self::Main(ptr) => crate::ptr::from_ref(ptr).cast(),
Self::Other(arc) => {
// Safety: We only expose an opaque pointer, which maintains the `Pin` invariant.
let inner = unsafe { Pin::into_inner_unchecked(arc) };
Arc::into_raw(inner) as *const ()
}
}
}

/// # Safety
///
/// See [`Thread::from_raw`].
unsafe fn from_raw(ptr: *const ()) -> Self {
// If the pointer is to `MAIN_THREAD_INFO`, we know it is the `Main` variant.
if crate::ptr::eq(ptr.cast(), &MAIN_THREAD_INFO) {
Self::Main(unsafe { &*ptr.cast() })
} else {
// Safety: Upheld by caller
Self::Other(unsafe { Pin::new_unchecked(Arc::from_raw(ptr as *const OtherInner)) })
}
}

fn parker(&self) -> Pin<&Parker> {
match self {
Self::Main((_, parker_ref)) => Pin::static_ref(parker_ref),
Self::Other(inner) => unsafe {
Pin::map_unchecked(inner.as_ref(), |inner| &inner.parker)
},
}
fn parker(self: Pin<&Self>) -> Pin<&Parker> {
unsafe { Pin::map_unchecked(self, |inner| &inner.parker) }
}
}

Expand All @@ -1390,55 +1338,41 @@ impl Inner {
/// docs of [`Builder`] and [`spawn`] for more details.
///
/// [`thread::current`]: current::current
pub struct Thread(Inner);
pub struct Thread {
inner: Pin<Arc<Inner>>,
}

impl Thread {
/// Used only internally to construct a thread object without spawning.
pub(crate) fn new(id: ThreadId, name: String) -> Thread {
Self::new_inner(id, Some(ThreadNameString::from(name)))
Self::new_inner(id, ThreadName::Other(name.into()))
}

pub(crate) fn new_unnamed(id: ThreadId) -> Thread {
Self::new_inner(id, None)
Self::new_inner(id, ThreadName::Unnamed)
}

/// Used in runtime to construct main thread
///
/// # Safety
///
/// This must only ever be called once, and must be called on the main thread.
pub(crate) unsafe fn new_main(thread_id: ThreadId) -> Thread {
// Safety: As this is only called once and on the main thread, nothing else is accessing MAIN_THREAD_INFO
// as the only other read occurs in `main_thread_info` *after* the main thread has been constructed,
// and this function is the only one that constructs the main thread.
//
// Pre-main thread spawning cannot hit this either, as the caller promises that this is only called on the main thread.
let main_thread_info = unsafe { &mut *MAIN_THREAD_INFO.get() };

unsafe { Parker::new_in_place((&raw mut main_thread_info.1).cast()) };
main_thread_info.0.write(thread_id);

// Store a `'static` ref to the initialised ThreadId and Parker,
// to avoid having to repeatedly prove initialisation.
Self(Inner::Main(unsafe { &*MAIN_THREAD_INFO.get().cast() }))
/// Constructs the thread handle for the main thread.
pub(crate) fn new_main(id: ThreadId) -> Thread {
Self::new_inner(id, ThreadName::Main)
}

fn new_inner(id: ThreadId, name: Option<ThreadNameString>) -> Thread {
fn new_inner(id: ThreadId, name: ThreadName) -> Thread {
// We have to use `unsafe` here to construct the `Parker` in-place,
// which is required for the UNIX implementation.
//
// SAFETY: We pin the Arc immediately after creation, so its address never
// changes.
let inner = unsafe {
let mut arc = Arc::<OtherInner>::new_uninit();
let mut arc = Arc::<Inner>::new_uninit();
let ptr = Arc::get_mut_unchecked(&mut arc).as_mut_ptr();
(&raw mut (*ptr).name).write(name);
(&raw mut (*ptr).id).write(id);
Parker::new_in_place(&raw mut (*ptr).parker);
Pin::new_unchecked(arc.assume_init())
};

Self(Inner::Other(inner))
Thread { inner }
}

/// Like the public [`park`], but callable on any handle. This is used to
Expand All @@ -1447,7 +1381,7 @@ impl Thread {
/// # Safety
/// May only be called from the thread to which this handle belongs.
pub(crate) unsafe fn park(&self) {
unsafe { self.0.parker().park() }
unsafe { self.inner.as_ref().parker().park() }
}

/// Like the public [`park_timeout`], but callable on any handle. This is
Expand All @@ -1456,7 +1390,7 @@ impl Thread {
/// # Safety
/// May only be called from the thread to which this handle belongs.
pub(crate) unsafe fn park_timeout(&self, dur: Duration) {
unsafe { self.0.parker().park_timeout(dur) }
unsafe { self.inner.as_ref().parker().park_timeout(dur) }
}

/// Atomically makes the handle's token available if it is not already.
Expand Down Expand Up @@ -1492,7 +1426,7 @@ impl Thread {
#[stable(feature = "rust1", since = "1.0.0")]
#[inline]
pub fn unpark(&self) {
self.0.parker().unpark();
self.inner.as_ref().parker().unpark();
}

/// Gets the thread's unique identifier.
Expand All @@ -1512,7 +1446,7 @@ impl Thread {
#[stable(feature = "thread_id", since = "1.19.0")]
#[must_use]
pub fn id(&self) -> ThreadId {
self.0.id()
self.inner.id
}

/// Gets the thread's name.
Expand Down Expand Up @@ -1555,11 +1489,7 @@ impl Thread {
#[stable(feature = "rust1", since = "1.0.0")]
#[must_use]
pub fn name(&self) -> Option<&str> {
self.0.name()
}

fn cname(&self) -> Option<&CStr> {
self.0.cname()
self.inner.name.as_str()
}

/// Consumes the `Thread`, returning a raw pointer.
Expand All @@ -1583,7 +1513,9 @@ impl Thread {
/// ```
#[unstable(feature = "thread_raw", issue = "97523")]
pub fn into_raw(self) -> *const () {
self.0.into_raw()
// Safety: We only expose an opaque pointer, which maintains the `Pin` invariant.
let inner = unsafe { Pin::into_inner_unchecked(self.inner) };
Arc::into_raw(inner) as *const ()
}

/// Constructs a `Thread` from a raw pointer.
Expand All @@ -1605,7 +1537,11 @@ impl Thread {
#[unstable(feature = "thread_raw", issue = "97523")]
pub unsafe fn from_raw(ptr: *const ()) -> Thread {
// Safety: Upheld by caller.
unsafe { Thread(Inner::from_raw(ptr)) }
unsafe { Thread { inner: Pin::new_unchecked(Arc::from_raw(ptr as *const Inner)) } }
}

fn cname(&self) -> Option<&CStr> {
self.inner.name.as_cstr()
}
}

Expand Down

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