Auto merge of #127912 - joboet:tls_dtor_thread_current, r=cuviper

std: make `thread::current` available in all `thread_local!` destructors

... and thereby allow the panic runtime to always print the right thread name.

This works by modifying the TLS destructor system to schedule a runtime cleanup function after all other TLS destructors registered by `std` have run. Unfortunately, this doesn't affect foreign TLS destructors, `thread::current` will still panic there.

Additionally, the thread ID returned by `current_id` will now always be available, even inside the global allocator, and will not change during the lifetime of one thread (this was previously the case with key-based TLS).

The mechanisms I added for this (`local_pointer` and `thread_cleanup`) will also allow finally fixing #111272 by moving the signal stack to a similar runtime-cleanup TLS variable.

Author: bors

library/std/src/rt.rs

@@ -21,9 +21,10 @@ pub use crate::panicking::{begin_panic, panic_count};
 pub use core::panicking::{panic_display, panic_fmt};
 
 #[rustfmt::skip]
+use crate::any::Any;
 use crate::sync::Once;
-use crate::sys;
 use crate::thread::{self, Thread};
+use crate::{mem, panic, sys};
 
 // Prints to the "panic output", depending on the platform this may be:
 // - the standard error output
@@ -66,6 +67,11 @@ macro_rules! rtunwrap {
     };
 }
 
+fn handle_rt_panic(e: Box<dyn Any + Send>) {
+    mem::forget(e);
+    rtabort!("initialization or cleanup bug");
+}
+
 // One-time runtime initialization.
 // Runs before `main`.
 // SAFETY: must be called only once during runtime initialization.
@@ -101,6 +107,20 @@ unsafe fn init(argc: isize, argv: *const *const u8, sigpipe: u8) {
     thread::set_current(thread);
 }
 
+/// Clean up the thread-local runtime state. This *should* be run after all other
+/// code managed by the Rust runtime, but will not cause UB if that condition is
+/// not fulfilled. Also note that this function is not guaranteed to be run, but
+/// skipping it will cause leaks and therefore is to be avoided.
+pub(crate) fn thread_cleanup() {
+    // This function is run in situations where unwinding leads to an abort
+    // (think `extern "C"` functions). Abort here instead so that we can
+    // print a nice message.
+    panic::catch_unwind(|| {
+        crate::thread::drop_current();
+    })
+    .unwrap_or_else(handle_rt_panic);
+}
+
 // One-time runtime cleanup.
 // Runs after `main` or at program exit.
 // NOTE: this is not guaranteed to run, for example when the program aborts.
@@ -123,11 +143,6 @@ fn lang_start_internal(
     argv: *const *const u8,
     sigpipe: u8,
 ) -> Result<isize, !> {
-    use crate::{mem, panic};
-    let rt_abort = move |e| {
-        mem::forget(e);
-        rtabort!("initialization or cleanup bug");
-    };
     // Guard against the code called by this function from unwinding outside of the Rust-controlled
     // code, which is UB. This is a requirement imposed by a combination of how the
     // `#[lang="start"]` attribute is implemented as well as by the implementation of the panicking
@@ -139,16 +154,17 @@ fn lang_start_internal(
     // prevent std from accidentally introducing a panic to these functions. Another is from
     // user code from `main` or, more nefariously, as described in e.g. issue #86030.
     // SAFETY: Only called once during runtime initialization.
-    panic::catch_unwind(move || unsafe { init(argc, argv, sigpipe) }).map_err(rt_abort)?;
+    panic::catch_unwind(move || unsafe { init(argc, argv, sigpipe) })
+        .unwrap_or_else(handle_rt_panic);
     let ret_code = panic::catch_unwind(move || panic::catch_unwind(main).unwrap_or(101) as isize)
         .map_err(move |e| {
             mem::forget(e);
             rtabort!("drop of the panic payload panicked");
         });
-    panic::catch_unwind(cleanup).map_err(rt_abort)?;
+    panic::catch_unwind(cleanup).unwrap_or_else(handle_rt_panic);
     // Guard against multiple threads calling `libc::exit` concurrently.
     // See the documentation for `unique_thread_exit` for more information.
-    panic::catch_unwind(|| crate::sys::exit_guard::unique_thread_exit()).map_err(rt_abort)?;
+    panic::catch_unwind(crate::sys::exit_guard::unique_thread_exit).unwrap_or_else(handle_rt_panic);
     ret_code
 }
 

library/std/src/sys/pal/hermit/mod.rs

@@ -92,7 +92,11 @@ pub unsafe extern "C" fn runtime_entry(
     unsafe {
         crate::sys::thread_local::destructors::run();
     }
-    unsafe { hermit_abi::exit(result) }
+    crate::rt::thread_cleanup();
+
+    unsafe {
+        hermit_abi::exit(result);
+    }
 }
 
 #[inline]

library/std/src/sys/pal/hermit/thread.rs

@@ -53,6 +53,7 @@ impl Thread {
 
                 // run all destructors
                 crate::sys::thread_local::destructors::run();
+                crate::rt::thread_cleanup();
             }
         }
     }

library/std/src/sys/sync/rwlock/queue.rs

@@ -113,7 +113,7 @@ use crate::mem;
 use crate::ptr::{self, NonNull, null_mut, without_provenance_mut};
 use crate::sync::atomic::Ordering::{AcqRel, Acquire, Relaxed, Release};
 use crate::sync::atomic::{AtomicBool, AtomicPtr};
-use crate::thread::{self, Thread};
+use crate::thread::{self, Thread, ThreadId};
 
 // Locking uses exponential backoff. `SPIN_COUNT` indicates how many times the
 // locking operation will be retried.
@@ -200,7 +200,9 @@ impl Node {
     fn prepare(&mut self) {
         // Fall back to creating an unnamed `Thread` handle to allow locking in
         // TLS destructors.
-        self.thread.get_or_init(|| thread::try_current().unwrap_or_else(Thread::new_unnamed));
+        self.thread.get_or_init(|| {
+            thread::try_current().unwrap_or_else(|| Thread::new_unnamed(ThreadId::new()))
+        });
         self.completed = AtomicBool::new(false);
     }
 

library/std/src/sys/thread_local/guard/apple.rs

@@ -26,6 +26,7 @@ pub fn enable() {
     unsafe extern "C" fn run_dtors(_: *mut u8) {
         unsafe {
             destructors::run();
+            crate::rt::thread_cleanup();
         }
     }
 }

library/std/src/sys/thread_local/guard/key.rs

@@ -3,10 +3,12 @@
 //! that will run all native TLS destructors in the destructor list.
 
 use crate::ptr;
-use crate::sys::thread_local::destructors;
 use crate::sys::thread_local::key::{LazyKey, set};
 
+#[cfg(target_thread_local)]
 pub fn enable() {
+    use crate::sys::thread_local::destructors;
+
     static DTORS: LazyKey = LazyKey::new(Some(run));
 
     // Setting the key value to something other than NULL will result in the
@@ -18,6 +20,41 @@ pub fn enable() {
     unsafe extern "C" fn run(_: *mut u8) {
         unsafe {
             destructors::run();
+            // On platforms with `__cxa_thread_atexit_impl`, `destructors::run`
+            // does nothing on newer systems as the TLS destructors are
+            // registered with the system. But because all of those platforms
+            // call the destructors of TLS keys after the registered ones, this
+            // function will still be run last (at the time of writing).
+            crate::rt::thread_cleanup();
+        }
+    }
+}
+
+/// On platforms with key-based TLS, the system runs the destructors for us.
+/// We still have to make sure that [`crate::rt::thread_cleanup`] is called,
+/// however. This is done by defering the execution of a TLS destructor to
+/// the next round of destruction inside the TLS destructors.
+#[cfg(not(target_thread_local))]
+pub fn enable() {
+    const DEFER: *mut u8 = ptr::without_provenance_mut(1);
+    const RUN: *mut u8 = ptr::without_provenance_mut(2);
+
+    static CLEANUP: LazyKey = LazyKey::new(Some(run));
+
+    unsafe { set(CLEANUP.force(), DEFER) }
+
+    unsafe extern "C" fn run(state: *mut u8) {
+        if state == DEFER {
+            // Make sure that this function is run again in the next round of
+            // TLS destruction. If there is no futher round, there will be leaks,
+            // but that's okay, `thread_cleanup` is not guaranteed to be called.
+            unsafe { set(CLEANUP.force(), RUN) }
+        } else {
+            debug_assert_eq!(state, RUN);
+            // If the state is still RUN in the next round of TLS destruction,
+            // it means that no other TLS destructors defined by this runtime
+            // have been run, as they would have set the state to DEFER.
+            crate::rt::thread_cleanup();
         }
     }
 }

library/std/src/sys/thread_local/guard/solid.rs

@@ -19,6 +19,9 @@ pub fn enable() {
     }
 
     unsafe extern "C" fn tls_dtor(_unused: *mut u8) {
-        unsafe { destructors::run() };
+        unsafe {
+            destructors::run();
+            crate::rt::thread_cleanup();
+        }
     }
 }

library/std/src/sys/thread_local/guard/windows.rs

@@ -80,13 +80,13 @@ pub static CALLBACK: unsafe extern "system" fn(*mut c_void, u32, *mut c_void) =
 
 unsafe extern "system" fn tls_callback(_h: *mut c_void, dw_reason: u32, _pv: *mut c_void) {
     if dw_reason == c::DLL_THREAD_DETACH || dw_reason == c::DLL_PROCESS_DETACH {
-        #[cfg(target_thread_local)]
         unsafe {
+            #[cfg(target_thread_local)]
             super::super::destructors::run();
-        }
-        #[cfg(not(target_thread_local))]
-        unsafe {
+            #[cfg(not(target_thread_local))]
             super::super::key::run_dtors();
+
+            crate::rt::thread_cleanup();
         }
     }
 }

library/std/src/sys/thread_local/key/xous.rs

@@ -212,4 +212,6 @@ unsafe fn run_dtors() {
             unsafe { cur = (*cur).next };
         }
     }
+
+    crate::rt::thread_cleanup();
 }

library/std/src/sys/thread_local/mod.rs

@@ -31,12 +31,15 @@ cfg_if::cfg_if! {
     ))] {
         mod statik;
         pub use statik::{EagerStorage, LazyStorage, thread_local_inner};
+        pub(crate) use statik::{LocalPointer, local_pointer};
     } else if #[cfg(target_thread_local)] {
         mod native;
         pub use native::{EagerStorage, LazyStorage, thread_local_inner};
+        pub(crate) use native::{LocalPointer, local_pointer};
     } else {
         mod os;
         pub use os::{Storage, thread_local_inner};
+        pub(crate) use os::{LocalPointer, local_pointer};
     }
 }
 
@@ -72,36 +75,47 @@ pub(crate) mod destructors {
 }
 
 /// This module provides a way to schedule the execution of the destructor list
-/// on systems without a per-variable destructor system.
-mod guard {
+/// and the [runtime cleanup](crate::rt::thread_cleanup) function. Calling `enable`
+/// should ensure that these functions are called at the right times.
+pub(crate) mod guard {
     cfg_if::cfg_if! {
         if #[cfg(all(target_thread_local, target_vendor = "apple"))] {
             mod apple;
-            pub(super) use apple::enable;
+            pub(crate) use apple::enable;
         } else if #[cfg(target_os = "windows")] {
             mod windows;
-            pub(super) use windows::enable;
+            pub(crate) use windows::enable;
         } else if #[cfg(any(
-            all(target_family = "wasm", target_feature = "atomics"),
+            target_family = "wasm",
+            target_os = "uefi",
+            target_os = "zkvm",
         ))] {
-            pub(super) fn enable() {
-                // FIXME: Right now there is no concept of "thread exit", but
-                // this is likely going to show up at some point in the form of
-                // an exported symbol that the wasm runtime is going to be
-                // expected to call. For now we just leak everything, but if
-                // such a function starts to exist it will probably need to
-                // iterate the destructor list with this function:
+            pub(crate) fn enable() {
+                // FIXME: Right now there is no concept of "thread exit" on
+                // wasm, but this is likely going to show up at some point in
+                // the form of an exported symbol that the wasm runtime is going
+                // to be expected to call. For now we just leak everything, but
+                // if such a function starts to exist it will probably need to
+                // iterate the destructor list with these functions:
+                #[cfg(all(target_family = "wasm", target_feature = "atomics"))]
                 #[allow(unused)]
                 use super::destructors::run;
+                #[allow(unused)]
+                use crate::rt::thread_cleanup;
             }
-        } else if #[cfg(target_os = "hermit")] {
-            pub(super) fn enable() {}
+        } else if #[cfg(any(
+            target_os = "hermit",
+            target_os = "xous",
+        ))] {
+            // `std` is the only runtime, so it just calls the destructor functions
+            // itself when the time comes.
+            pub(crate) fn enable() {}
         } else if #[cfg(target_os = "solid_asp3")] {
             mod solid;
-            pub(super) use solid::enable;
-        } else if #[cfg(all(target_thread_local, not(target_family = "wasm")))] {
+            pub(crate) use solid::enable;
+        } else {
             mod key;
-            pub(super) use key::enable;
+            pub(crate) use key::enable;
         }
     }
 }

library/std/src/sys/thread_local/native/mod.rs

@@ -29,6 +29,9 @@
 //! eliminates the `Destroyed` state for these values, which can allow more niche
 //! optimizations to occur for the `State` enum. For `Drop` types, `()` is used.
 
+use crate::cell::Cell;
+use crate::ptr;
+
 mod eager;
 mod lazy;
 
@@ -107,3 +110,31 @@ pub macro thread_local_inner {
             $crate::thread::local_impl::thread_local_inner!(@key $t, $($init)*);
     },
 }
+
+#[rustc_macro_transparency = "semitransparent"]
+pub(crate) macro local_pointer {
+    () => {},
+    ($vis:vis static $name:ident; $($rest:tt)*) => {
+        #[thread_local]
+        $vis static $name: $crate::sys::thread_local::LocalPointer = $crate::sys::thread_local::LocalPointer::__new();
+        $crate::sys::thread_local::local_pointer! { $($rest)* }
+    },
+}
+
+pub(crate) struct LocalPointer {
+    p: Cell<*mut ()>,
+}
+
+impl LocalPointer {
+    pub const fn __new() -> LocalPointer {
+        LocalPointer { p: Cell::new(ptr::null_mut()) }
+    }
+
+    pub fn get(&self) -> *mut () {
+        self.p.get()
+    }
+
+    pub fn set(&self, p: *mut ()) {
+        self.p.set(p)
+    }
+}

library/std/src/sys/thread_local/os.rs

@@ -1,8 +1,8 @@
-use super::abort_on_dtor_unwind;
+use super::key::{Key, LazyKey, get, set};
+use super::{abort_on_dtor_unwind, guard};
 use crate::cell::Cell;
 use crate::marker::PhantomData;
 use crate::ptr;
-use crate::sys::thread_local::key::{Key, LazyKey, get, set};
 
 #[doc(hidden)]
 #[allow_internal_unstable(thread_local_internals)]
@@ -138,5 +138,35 @@ unsafe extern "C" fn destroy_value<T: 'static>(ptr: *mut u8) {
         drop(ptr);
         // SAFETY: `key` is the TLS key `ptr` was stored under.
         unsafe { set(key, ptr::null_mut()) };
+        // Make sure that the runtime cleanup will be performed
+        // after the next round of TLS destruction.
+        guard::enable();
     });
 }
+
+#[rustc_macro_transparency = "semitransparent"]
+pub(crate) macro local_pointer {
+    () => {},
+    ($vis:vis static $name:ident; $($rest:tt)*) => {
+        $vis static $name: $crate::sys::thread_local::LocalPointer = $crate::sys::thread_local::LocalPointer::__new();
+        $crate::sys::thread_local::local_pointer! { $($rest)* }
+    },
+}
+
+pub(crate) struct LocalPointer {
+    key: LazyKey,
+}
+
+impl LocalPointer {
+    pub const fn __new() -> LocalPointer {
+        LocalPointer { key: LazyKey::new(None) }
+    }
+
+    pub fn get(&'static self) -> *mut () {
+        unsafe { get(self.key.force()) as *mut () }
+    }
+
+    pub fn set(&'static self, p: *mut ()) {
+        unsafe { set(self.key.force(), p as *mut u8) }
+    }
+}