Auto merge of rust-lang#3739 - joboet:macos_tls_dtors, r=RalfJung

Implement support for multiple TLS destructors on macOS

I want to get rid of [this `#[cfg]` block](https://github.com/rust-lang/rust/blob/98dcbae5c9ac615d5acfbf42d42b19a77cb9ec11/library/std/src/thread/mod.rs#L195-L211) in `std`, but it is currently required for miri, as it does not support multiple macOS TLS destructors. This is not true for the platform itself, however, as can be observed in the [implementation](https://github.com/apple-oss-distributions/dyld/blob/d552c40cd1de105f0ec95008e0e0c0972de43456/dyld/DyldRuntimeState.cpp#L2239).

Author: bors

src/tools/miri/src/shims/tls.rs

@@ -36,9 +36,9 @@ pub struct TlsData<'tcx> {
     /// pthreads-style thread-local storage.
     keys: BTreeMap<TlsKey, TlsEntry<'tcx>>,
 
-    /// A single per thread destructor of the thread local storage (that's how
-    /// things work on macOS) with a data argument.
-    macos_thread_dtors: BTreeMap<ThreadId, (ty::Instance<'tcx>, Scalar)>,
+    /// On macOS, each thread holds a list of destructor functions with their
+    /// respective data arguments.
+    macos_thread_dtors: BTreeMap<ThreadId, Vec<(ty::Instance<'tcx>, Scalar)>>,
 }
 
 impl<'tcx> Default for TlsData<'tcx> {
@@ -119,26 +119,15 @@ impl<'tcx> TlsData<'tcx> {
         }
     }
 
-    /// Set the thread wide destructor of the thread local storage for the given
-    /// thread. This function is used to implement `_tlv_atexit` shim on MacOS.
-    ///
-    /// Thread wide dtors are available only on MacOS. There is one destructor
-    /// per thread as can be guessed from the following comment in the
-    /// [`_tlv_atexit`
-    /// implementation](https://github.com/opensource-apple/dyld/blob/195030646877261f0c8c7ad8b001f52d6a26f514/src/threadLocalVariables.c#L389):
-    ///
-    /// NOTE: this does not need locks because it only operates on current thread data
-    pub fn set_macos_thread_dtor(
+    /// Add a thread local storage destructor for the given thread. This function
+    /// is used to implement the `_tlv_atexit` shim on MacOS.
+    pub fn add_macos_thread_dtor(
         &mut self,
         thread: ThreadId,
         dtor: ty::Instance<'tcx>,
         data: Scalar,
     ) -> InterpResult<'tcx> {
-        if self.macos_thread_dtors.insert(thread, (dtor, data)).is_some() {
-            throw_unsup_format!(
-                "setting more than one thread local storage destructor for the same thread is not supported"
-            );
-        }
+        self.macos_thread_dtors.entry(thread).or_default().push((dtor, data));
         Ok(())
     }
 
@@ -202,6 +191,10 @@ impl<'tcx> TlsData<'tcx> {
         for TlsEntry { data, .. } in self.keys.values_mut() {
             data.remove(&thread_id);
         }
+
+        if let Some(dtors) = self.macos_thread_dtors.remove(&thread_id) {
+            assert!(dtors.is_empty(), "the destructors should have already been run");
+        }
     }
 }
 
@@ -212,7 +205,7 @@ impl VisitProvenance for TlsData<'_> {
         for scalar in keys.values().flat_map(|v| v.data.values()) {
             scalar.visit_provenance(visit);
         }
-        for (_, scalar) in macos_thread_dtors.values() {
+        for (_, scalar) in macos_thread_dtors.values().flatten() {
             scalar.visit_provenance(visit);
         }
     }
@@ -225,6 +218,7 @@ pub struct TlsDtorsState<'tcx>(TlsDtorsStatePriv<'tcx>);
 enum TlsDtorsStatePriv<'tcx> {
     #[default]
     Init,
+    MacOsDtors,
     PthreadDtors(RunningDtorState),
     /// For Windows Dtors, we store the list of functions that we still have to call.
     /// These are functions from the magic `.CRT$XLB` linker section.
@@ -243,11 +237,10 @@ impl<'tcx> TlsDtorsState<'tcx> {
                 Init => {
                     match this.tcx.sess.target.os.as_ref() {
                         "macos" => {
-                            // The macOS thread wide destructor runs "before any TLS slots get
-                            // freed", so do that first.
-                            this.schedule_macos_tls_dtor()?;
-                            // When that destructor is done, go on with the pthread dtors.
-                            break 'new_state PthreadDtors(Default::default());
+                            // macOS has a _tlv_atexit function that allows
+                            // registering destructors without associated keys.
+                            // These are run first.
+                            break 'new_state MacOsDtors;
                         }
                         _ if this.target_os_is_unix() => {
                             // All other Unixes directly jump to running the pthread dtors.
@@ -266,6 +259,14 @@ impl<'tcx> TlsDtorsState<'tcx> {
                         }
                     }
                 }
+                MacOsDtors => {
+                    match this.schedule_macos_tls_dtor()? {
+                        Poll::Pending => return Ok(Poll::Pending),
+                        // After all macOS destructors are run, the system switches
+                        // to destroying the pthread destructors.
+                        Poll::Ready(()) => break 'new_state PthreadDtors(Default::default()),
+                    }
+                }
                 PthreadDtors(state) => {
                     match this.schedule_next_pthread_tls_dtor(state)? {
                         Poll::Pending => return Ok(Poll::Pending), // just keep going
@@ -328,12 +329,15 @@ trait EvalContextPrivExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
         Ok(())
     }
 
-    /// Schedule the MacOS thread destructor of the thread local storage to be
-    /// executed.
-    fn schedule_macos_tls_dtor(&mut self) -> InterpResult<'tcx> {
+    /// Schedule the macOS thread local storage destructors to be executed.
+    fn schedule_macos_tls_dtor(&mut self) -> InterpResult<'tcx, Poll<()>> {
         let this = self.eval_context_mut();
         let thread_id = this.active_thread();
-        if let Some((instance, data)) = this.machine.tls.macos_thread_dtors.remove(&thread_id) {
+        // macOS keeps track of TLS destructors in a stack. If a destructor
+        // registers another destructor, it will be run next.
+        // See https://github.com/apple-oss-distributions/dyld/blob/d552c40cd1de105f0ec95008e0e0c0972de43456/dyld/DyldRuntimeState.cpp#L2277
+        let dtor = this.machine.tls.macos_thread_dtors.get_mut(&thread_id).and_then(Vec::pop);
+        if let Some((instance, data)) = dtor {
             trace!("Running macos dtor {:?} on {:?} at {:?}", instance, data, thread_id);
 
             this.call_function(
@@ -343,8 +347,11 @@ trait EvalContextPrivExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
                 None,
                 StackPopCleanup::Root { cleanup: true },
             )?;
+
+            return Ok(Poll::Pending);
         }
-        Ok(())
+
+        Ok(Poll::Ready(()))
     }
 
     /// Schedule a pthread TLS destructor. Returns `true` if found

src/tools/miri/src/shims/unix/macos/foreign_items.rs

@@ -132,7 +132,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
                 let dtor = this.get_ptr_fn(dtor)?.as_instance()?;
                 let data = this.read_scalar(data)?;
                 let active_thread = this.active_thread();
-                this.machine.tls.set_macos_thread_dtor(active_thread, dtor, data)?;
+                this.machine.tls.add_macos_thread_dtor(active_thread, dtor, data)?;
             }
 
             // Querying system information

src/tools/miri/tests/pass/tls/macos_tlv_atexit.rs

@@ -0,0 +1,43 @@
+//@only-target-darwin
+
+use std::thread;
+
+extern "C" {
+    fn _tlv_atexit(dtor: unsafe extern "C" fn(*mut u8), arg: *mut u8);
+}
+
+fn register<F>(f: F)
+where
+    F: FnOnce() + 'static,
+{
+    // This will receive the pointer passed into `_tlv_atexit`, which is the
+    // original `f` but boxed up.
+    unsafe extern "C" fn run<F>(ptr: *mut u8)
+    where
+        F: FnOnce() + 'static,
+    {
+        let f = unsafe { Box::from_raw(ptr as *mut F) };
+        f()
+    }
+
+    unsafe {
+        _tlv_atexit(run::<F>, Box::into_raw(Box::new(f)) as *mut u8);
+    }
+}
+
+fn main() {
+    thread::spawn(|| {
+        register(|| println!("dtor 2"));
+        register(|| println!("dtor 1"));
+        println!("exiting thread");
+    })
+    .join()
+    .unwrap();
+
+    println!("exiting main");
+    register(|| println!("dtor 5"));
+    register(|| {
+        println!("registering dtor in dtor 3");
+        register(|| println!("dtor 4"));
+    });
+}

src/tools/miri/tests/pass/tls/macos_tlv_atexit.stdout

@@ -0,0 +1,7 @@
+exiting thread
+dtor 1
+dtor 2
+exiting main
+registering dtor in dtor 3
+dtor 4
+dtor 5