std: Fix a TLS destructor bug on OSX

src/libstd/thread/local.rs

@@ -275,6 +275,7 @@ mod imp {
 
     use cell::{Cell, UnsafeCell};
     use intrinsics;
+    use ptr;
 
     pub struct Key<T> {
         inner: UnsafeCell<Option<T>>,
@@ -327,7 +328,6 @@ mod imp {
     #[cfg(target_os = "linux")]
     unsafe fn register_dtor(t: *mut u8, dtor: unsafe extern fn(*mut u8)) {
         use mem;
-        use ptr;
         use libc;
         use sys_common::thread_local as os;
 
@@ -394,7 +394,24 @@ mod imp {
         // destructor as running for this thread so calls to `get` will return
         // `None`.
         (*ptr).dtor_running.set(true);
-        intrinsics::drop_in_place((*ptr).inner.get());
+
+        // The OSX implementation of TLS apparently had an odd aspect to it
+        // where the pointer we have may be overwritten while this destructor
+        // is running. Specifically if a TLS destructor re-accesses TLS it may
+        // trigger a re-initialization of all TLS variables, paving over at
+        // least some destroyed ones with initial values.
+        //
+        // This means that if we drop a TLS value in place on OSX that we could
+        // revert the value to its original state halfway through the
+        // destructor, which would be bad!
+        //
+        // Hence, we use `ptr::read` on OSX (to move to a "safe" location)
+        // instead of drop_in_place.
+        if cfg!(target_os = "macos") {
+            ptr::read((*ptr).inner.get());
+        } else {
+            intrinsics::drop_in_place((*ptr).inner.get());
+        }
     }
 }
 

src/test/run-pass/down-with-thread-dtors.rs

@@ -0,0 +1,38 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+thread_local!(static FOO: Foo = Foo);
+thread_local!(static BAR: Bar = Bar(1));
+thread_local!(static BAZ: Baz = Baz);
+
+struct Foo;
+struct Bar(i32);
+struct Baz;
+
+impl Drop for Foo {
+    fn drop(&mut self) {
+        BAR.with(|_| {});
+    }
+}
+
+impl Drop for Bar {
+    fn drop(&mut self) {
+        assert_eq!(self.0, 1);
+        self.0 = 2;
+        BAZ.with(|_| {});
+        assert_eq!(self.0, 2);
+    }
+}
+
+fn main() {
+    std::thread::spawn(|| {
+        FOO.with(|_| {});
+    }).join().unwrap();
+}