Rollup merge of #69955 - alexcrichton:stderr-infallible, r=sfackler

Fix abort-on-eprintln during process shutdown

This commit fixes an issue where if `eprintln!` is used in a TLS
destructor it can accidentally cause the process to abort. TLS
destructors are executed after `main` returns on the main thread, and at
this point we've also deinitialized global `Lazy` values like those
which store the `Stderr` and `Stdout` internals. This means that despite
handling TLS not being accessible in `eprintln!`, we will fail due to
not being able to call `stderr()`. This means that we'll double-panic
quickly because panicking also attempt to write to stderr.

The fix here is to reimplement the global stderr handle to avoid the
need for destruction. This avoids the need for `Lazy` as well as the
hidden panic inside of the `stderr` function.

Overall this should improve the robustness of printing errors and/or
panics in weird situations, since the `stderr` accessor should be
infallible in more situations.

11 files changed, 149 insertions, 100 deletions

diff --git a/src/libstd/io/stdio.rs b/src/libstd/io/stdio.rs
index 0fb0757792e..9a82ae7626d 100644
--- a/src/libstd/io/stdio.rs
+++ b/src/libstd/io/stdio.rs
@@ -6,7 +6,7 @@ use crate::cell::RefCell;
 use crate::fmt;
 use crate::io::lazy::Lazy;
 use crate::io::{self, BufReader, Initializer, IoSlice, IoSliceMut, LineWriter};
-use crate::sync::{Arc, Mutex, MutexGuard};
+use crate::sync::{Arc, Mutex, MutexGuard, Once};
 use crate::sys::stdio;
 use crate::sys_common::remutex::{ReentrantMutex, ReentrantMutexGuard};
 use crate::thread::LocalKey;
@@ -493,7 +493,11 @@ pub fn stdout() -> Stdout {
             Ok(stdout) => Maybe::Real(stdout),
             _ => Maybe::Fake,
         };
-        Arc::new(ReentrantMutex::new(RefCell::new(LineWriter::new(stdout))))
+        unsafe {
+            let ret = Arc::new(ReentrantMutex::new(RefCell::new(LineWriter::new(stdout))));
+            ret.init();
+            return ret;
+        }
     }
 }
 
@@ -520,7 +524,7 @@ impl Stdout {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn lock(&self) -> StdoutLock<'_> {
-        StdoutLock { inner: self.inner.lock().unwrap_or_else(|e| e.into_inner()) }
+        StdoutLock { inner: self.inner.lock() }
     }
 }
 
@@ -581,7 +585,7 @@ impl fmt::Debug for StdoutLock<'_> {
 /// an error.
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Stderr {
-    inner: Arc<ReentrantMutex<RefCell<Maybe<StderrRaw>>>>,
+    inner: &'static ReentrantMutex<RefCell<Maybe<StderrRaw>>>,
 }
 
 /// A locked reference to the `Stderr` handle.
@@ -639,19 +643,28 @@ pub struct StderrLock<'a> {
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn stderr() -> Stderr {
-    static INSTANCE: Lazy<ReentrantMutex<RefCell<Maybe<StderrRaw>>>> = Lazy::new();
-    return Stderr {
-        inner: unsafe { INSTANCE.get(stderr_init).expect("cannot access stderr during shutdown") },
-    };
-
-    fn stderr_init() -> Arc<ReentrantMutex<RefCell<Maybe<StderrRaw>>>> {
-        // This must not reentrantly access `INSTANCE`
-        let stderr = match stderr_raw() {
-            Ok(stderr) => Maybe::Real(stderr),
-            _ => Maybe::Fake,
-        };
-        Arc::new(ReentrantMutex::new(RefCell::new(stderr)))
-    }
+    // Note that unlike `stdout()` we don't use `Lazy` here which registers a
+    // destructor. Stderr is not buffered nor does the `stderr_raw` type consume
+    // any owned resources, so there's no need to run any destructors at some
+    // point in the future.
+    //
+    // This has the added benefit of allowing `stderr` to be usable during
+    // process shutdown as well!
+    static INSTANCE: ReentrantMutex<RefCell<Maybe<StderrRaw>>> =
+        unsafe { ReentrantMutex::new(RefCell::new(Maybe::Fake)) };
+
+    // When accessing stderr we need one-time initialization of the reentrant
+    // mutex, followed by one-time detection of whether we actually have a
+    // stderr handle or not. Afterwards we can just always use the now-filled-in
+    // `INSTANCE` value.
+    static INIT: Once = Once::new();
+    INIT.call_once(|| unsafe {
+        INSTANCE.init();
+        if let Ok(stderr) = stderr_raw() {
+            *INSTANCE.lock().borrow_mut() = Maybe::Real(stderr);
+        }
+    });
+    return Stderr { inner: &INSTANCE };
 }
 
 impl Stderr {
@@ -677,7 +690,7 @@ impl Stderr {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn lock(&self) -> StderrLock<'_> {
-        StderrLock { inner: self.inner.lock().unwrap_or_else(|e| e.into_inner()) }
+        StderrLock { inner: self.inner.lock() }
     }
 }
 
diff --git a/src/libstd/sys/cloudabi/mutex.rs b/src/libstd/sys/cloudabi/mutex.rs
index 4aa25e25052..580ab0e8ad8 100644
--- a/src/libstd/sys/cloudabi/mutex.rs
+++ b/src/libstd/sys/cloudabi/mutex.rs
@@ -53,16 +53,16 @@ pub struct ReentrantMutex {
 }
 
 impl ReentrantMutex {
-    pub unsafe fn uninitialized() -> ReentrantMutex {
+    pub const unsafe fn uninitialized() -> ReentrantMutex {
         ReentrantMutex {
             lock: UnsafeCell::new(MaybeUninit::uninit()),
             recursion: UnsafeCell::new(MaybeUninit::uninit()),
         }
     }
 
-    pub unsafe fn init(&mut self) {
-        self.lock = UnsafeCell::new(MaybeUninit::new(AtomicU32::new(abi::LOCK_UNLOCKED.0)));
-        self.recursion = UnsafeCell::new(MaybeUninit::new(0));
+    pub unsafe fn init(&self) {
+        *self.lock.get() = MaybeUninit::new(AtomicU32::new(abi::LOCK_UNLOCKED.0));
+        *self.recursion.get() = MaybeUninit::new(0);
     }
 
     pub unsafe fn try_lock(&self) -> bool {
diff --git a/src/libstd/sys/hermit/mutex.rs b/src/libstd/sys/hermit/mutex.rs
index b5c75f738d2..3d4813209cb 100644
--- a/src/libstd/sys/hermit/mutex.rs
+++ b/src/libstd/sys/hermit/mutex.rs
@@ -46,13 +46,13 @@ pub struct ReentrantMutex {
 }
 
 impl ReentrantMutex {
-    pub unsafe fn uninitialized() -> ReentrantMutex {
+    pub const unsafe fn uninitialized() -> ReentrantMutex {
         ReentrantMutex { inner: ptr::null() }
     }
 
     #[inline]
-    pub unsafe fn init(&mut self) {
-        let _ = abi::recmutex_init(&mut self.inner as *mut *const c_void);
+    pub unsafe fn init(&self) {
+        let _ = abi::recmutex_init(&self.inner as *const *const c_void as *mut _);
     }
 
     #[inline]
diff --git a/src/libstd/sys/sgx/mutex.rs b/src/libstd/sys/sgx/mutex.rs
index eebbea1b285..4911c2f5387 100644
--- a/src/libstd/sys/sgx/mutex.rs
+++ b/src/libstd/sys/sgx/mutex.rs
@@ -75,7 +75,7 @@ impl ReentrantMutex {
     }
 
     #[inline]
-    pub unsafe fn init(&mut self) {}
+    pub unsafe fn init(&self) {}
 
     #[inline]
     pub unsafe fn lock(&self) {
diff --git a/src/libstd/sys/unix/mutex.rs b/src/libstd/sys/unix/mutex.rs
index b38375a2e03..103d87e3d2f 100644
--- a/src/libstd/sys/unix/mutex.rs
+++ b/src/libstd/sys/unix/mutex.rs
@@ -92,11 +92,11 @@ unsafe impl Send for ReentrantMutex {}
 unsafe impl Sync for ReentrantMutex {}
 
 impl ReentrantMutex {
-    pub unsafe fn uninitialized() -> ReentrantMutex {
+    pub const unsafe fn uninitialized() -> ReentrantMutex {
         ReentrantMutex { inner: UnsafeCell::new(libc::PTHREAD_MUTEX_INITIALIZER) }
     }
 
-    pub unsafe fn init(&mut self) {
+    pub unsafe fn init(&self) {
         let mut attr = MaybeUninit::<libc::pthread_mutexattr_t>::uninit();
         let result = libc::pthread_mutexattr_init(attr.as_mut_ptr());
         debug_assert_eq!(result, 0);
diff --git a/src/libstd/sys/vxworks/mutex.rs b/src/libstd/sys/vxworks/mutex.rs
index b38375a2e03..103d87e3d2f 100644
--- a/src/libstd/sys/vxworks/mutex.rs
+++ b/src/libstd/sys/vxworks/mutex.rs
@@ -92,11 +92,11 @@ unsafe impl Send for ReentrantMutex {}
 unsafe impl Sync for ReentrantMutex {}
 
 impl ReentrantMutex {
-    pub unsafe fn uninitialized() -> ReentrantMutex {
+    pub const unsafe fn uninitialized() -> ReentrantMutex {
         ReentrantMutex { inner: UnsafeCell::new(libc::PTHREAD_MUTEX_INITIALIZER) }
     }
 
-    pub unsafe fn init(&mut self) {
+    pub unsafe fn init(&self) {
         let mut attr = MaybeUninit::<libc::pthread_mutexattr_t>::uninit();
         let result = libc::pthread_mutexattr_init(attr.as_mut_ptr());
         debug_assert_eq!(result, 0);
diff --git a/src/libstd/sys/wasm/mutex.rs b/src/libstd/sys/wasm/mutex.rs
index 07238d08730..7aaf1b3a343 100644
--- a/src/libstd/sys/wasm/mutex.rs
+++ b/src/libstd/sys/wasm/mutex.rs
@@ -47,11 +47,11 @@ impl Mutex {
 pub struct ReentrantMutex {}
 
 impl ReentrantMutex {
-    pub unsafe fn uninitialized() -> ReentrantMutex {
+    pub const unsafe fn uninitialized() -> ReentrantMutex {
         ReentrantMutex {}
     }
 
-    pub unsafe fn init(&mut self) {}
+    pub unsafe fn init(&self) {}
 
     pub unsafe fn lock(&self) {}
 
diff --git a/src/libstd/sys/wasm/mutex_atomics.rs b/src/libstd/sys/wasm/mutex_atomics.rs
index 90c628a19c2..268a53bb564 100644
--- a/src/libstd/sys/wasm/mutex_atomics.rs
+++ b/src/libstd/sys/wasm/mutex_atomics.rs
@@ -80,11 +80,11 @@ unsafe impl Sync for ReentrantMutex {}
 // released when this recursion counter reaches 0.
 
 impl ReentrantMutex {
-    pub unsafe fn uninitialized() -> ReentrantMutex {
+    pub const unsafe fn uninitialized() -> ReentrantMutex {
         ReentrantMutex { owner: AtomicU32::new(0), recursions: UnsafeCell::new(0) }
     }
 
-    pub unsafe fn init(&mut self) {
+    pub unsafe fn init(&self) {
         // nothing to do...
     }
 
diff --git a/src/libstd/sys/windows/mutex.rs b/src/libstd/sys/windows/mutex.rs
index 281eb294c65..63dfc640908 100644
--- a/src/libstd/sys/windows/mutex.rs
+++ b/src/libstd/sys/windows/mutex.rs
@@ -109,7 +109,7 @@ impl Mutex {
             0 => {}
             n => return n as *mut _,
         }
-        let mut re = box ReentrantMutex::uninitialized();
+        let re = box ReentrantMutex::uninitialized();
         re.init();
         let re = Box::into_raw(re);
         match self.lock.compare_and_swap(0, re as usize, Ordering::SeqCst) {
@@ -157,11 +157,11 @@ unsafe impl Send for ReentrantMutex {}
 unsafe impl Sync for ReentrantMutex {}
 
 impl ReentrantMutex {
-    pub fn uninitialized() -> ReentrantMutex {
+    pub const fn uninitialized() -> ReentrantMutex {
         ReentrantMutex { inner: UnsafeCell::new(MaybeUninit::uninit()) }
     }
 
-    pub unsafe fn init(&mut self) {
+    pub unsafe fn init(&self) {
         c::InitializeCriticalSection((&mut *self.inner.get()).as_mut_ptr());
     }
 
diff --git a/src/libstd/sys_common/remutex.rs b/src/libstd/sys_common/remutex.rs
index a1ad44a3666..4f19bbc467f 100644
--- a/src/libstd/sys_common/remutex.rs
+++ b/src/libstd/sys_common/remutex.rs
@@ -3,7 +3,6 @@ use crate::marker;
 use crate::ops::Deref;
 use crate::panic::{RefUnwindSafe, UnwindSafe};
 use crate::sys::mutex as sys;
-use crate::sys_common::poison::{self, LockResult, TryLockError, TryLockResult};
 
 /// A re-entrant mutual exclusion
 ///
@@ -11,8 +10,7 @@ use crate::sys_common::poison::{self, LockResult, TryLockError, TryLockResult};
 /// available. The thread which has already locked the mutex can lock it
 /// multiple times without blocking, preventing a common source of deadlocks.
 pub struct ReentrantMutex<T> {
-    inner: Box<sys::ReentrantMutex>,
-    poison: poison::Flag,
+    inner: sys::ReentrantMutex,
     data: T,
 }
 
@@ -39,23 +37,30 @@ pub struct ReentrantMutexGuard<'a, T: 'a> {
     // funny underscores due to how Deref currently works (it disregards field
     // privacy).
     __lock: &'a ReentrantMutex<T>,
-    __poison: poison::Guard,
 }
 
 impl<T> !marker::Send for ReentrantMutexGuard<'_, T> {}
 
 impl<T> ReentrantMutex<T> {
     /// Creates a new reentrant mutex in an unlocked state.
-    pub fn new(t: T) -> ReentrantMutex<T> {
-        unsafe {
-            let mut mutex = ReentrantMutex {
-                inner: box sys::ReentrantMutex::uninitialized(),
-                poison: poison::Flag::new(),
-                data: t,
-            };
-            mutex.inner.init();
-            mutex
-        }
+    ///
+    /// # Unsafety
+    ///
+    /// This function is unsafe because it is required that `init` is called
+    /// once this mutex is in its final resting place, and only then are the
+    /// lock/unlock methods safe.
+    pub const unsafe fn new(t: T) -> ReentrantMutex<T> {
+        ReentrantMutex { inner: sys::ReentrantMutex::uninitialized(), data: t }
+    }
+
+    /// Initializes this mutex so it's ready for use.
+    ///
+    /// # Unsafety
+    ///
+    /// Unsafe to call more than once, and must be called after this will no
+    /// longer move in memory.
+    pub unsafe fn init(&self) {
+        self.inner.init();
     }
 
     /// Acquires a mutex, blocking the current thread until it is able to do so.
@@ -70,7 +75,7 @@ impl<T> ReentrantMutex<T> {
     /// If another user of this mutex panicked while holding the mutex, then
     /// this call will return failure if the mutex would otherwise be
     /// acquired.
-    pub fn lock(&self) -> LockResult<ReentrantMutexGuard<'_, T>> {
+    pub fn lock(&self) -> ReentrantMutexGuard<'_, T> {
         unsafe { self.inner.lock() }
         ReentrantMutexGuard::new(&self)
     }
@@ -87,12 +92,8 @@ impl<T> ReentrantMutex<T> {
     /// If another user of this mutex panicked while holding the mutex, then
     /// this call will return failure if the mutex would otherwise be
     /// acquired.
-    pub fn try_lock(&self) -> TryLockResult<ReentrantMutexGuard<'_, T>> {
-        if unsafe { self.inner.try_lock() } {
-            Ok(ReentrantMutexGuard::new(&self)?)
-        } else {
-            Err(TryLockError::WouldBlock)
-        }
+    pub fn try_lock(&self) -> Option<ReentrantMutexGuard<'_, T>> {
+        if unsafe { self.inner.try_lock() } { Some(ReentrantMutexGuard::new(&self)) } else { None }
     }
 }
 
@@ -108,11 +109,8 @@ impl<T> Drop for ReentrantMutex<T> {
 impl<T: fmt::Debug + 'static> fmt::Debug for ReentrantMutex<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self.try_lock() {
-            Ok(guard) => f.debug_struct("ReentrantMutex").field("data", &*guard).finish(),
-            Err(TryLockError::Poisoned(err)) => {
-                f.debug_struct("ReentrantMutex").field("data", &**err.get_ref()).finish()
-            }
-            Err(TryLockError::WouldBlock) => {
+            Some(guard) => f.debug_struct("ReentrantMutex").field("data", &*guard).finish(),
+            None => {
                 struct LockedPlaceholder;
                 impl fmt::Debug for LockedPlaceholder {
                     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
@@ -127,11 +125,8 @@ impl<T: fmt::Debug + 'static> fmt::Debug for ReentrantMutex<T> {
 }
 
 impl<'mutex, T> ReentrantMutexGuard<'mutex, T> {
-    fn new(lock: &'mutex ReentrantMutex<T>) -> LockResult<ReentrantMutexGuard<'mutex, T>> {
-        poison::map_result(lock.poison.borrow(), |guard| ReentrantMutexGuard {
-            __lock: lock,
-            __poison: guard,
-        })
+    fn new(lock: &'mutex ReentrantMutex<T>) -> ReentrantMutexGuard<'mutex, T> {
+        ReentrantMutexGuard { __lock: lock }
     }
 }
 
@@ -147,7 +142,6 @@ impl<T> Drop for ReentrantMutexGuard<'_, T> {
     #[inline]
     fn drop(&mut self) {
         unsafe {
-            self.__lock.poison.done(&self.__poison);
             self.__lock.inner.unlock();
         }
     }
@@ -162,13 +156,17 @@ mod tests {
 
     #[test]
     fn smoke() {
-        let m = ReentrantMutex::new(());
+        let m = unsafe {
+            let m = ReentrantMutex::new(());
+            m.init();
+            m
+        };
         {
-            let a = m.lock().unwrap();
+            let a = m.lock();
             {
-                let b = m.lock().unwrap();
+                let b = m.lock();
                 {
-                    let c = m.lock().unwrap();
+                    let c = m.lock();
                     assert_eq!(*c, ());
                 }
                 assert_eq!(*b, ());
@@ -179,15 +177,19 @@ mod tests {
 
     #[test]
     fn is_mutex() {
-        let m = Arc::new(ReentrantMutex::new(RefCell::new(0)));
+        let m = unsafe {
+            let m = Arc::new(ReentrantMutex::new(RefCell::new(0)));
+            m.init();
+            m
+        };
         let m2 = m.clone();
-        let lock = m.lock().unwrap();
+        let lock = m.lock();
         let child = thread::spawn(move || {
-            let lock = m2.lock().unwrap();
+            let lock = m2.lock();
             assert_eq!(*lock.borrow(), 4950);
         });
         for i in 0..100 {
-            let lock = m.lock().unwrap();
+            let lock = m.lock();
             *lock.borrow_mut() += i;
         }
         drop(lock);
@@ -196,17 +198,21 @@ mod tests {
 
     #[test]
     fn trylock_works() {
-        let m = Arc::new(ReentrantMutex::new(()));
+        let m = unsafe {
+            let m = Arc::new(ReentrantMutex::new(()));
+            m.init();
+            m
+        };
         let m2 = m.clone();
-        let _lock = m.try_lock().unwrap();
-        let _lock2 = m.try_lock().unwrap();
+        let _lock = m.try_lock();
+        let _lock2 = m.try_lock();
         thread::spawn(move || {
             let lock = m2.try_lock();
-            assert!(lock.is_err());
+            assert!(lock.is_none());
         })
         .join()
         .unwrap();
-        let _lock3 = m.try_lock().unwrap();
+        let _lock3 = m.try_lock();
     }
 
     pub struct Answer<'a>(pub ReentrantMutexGuard<'a, RefCell<u32>>);
@@ -215,22 +221,4 @@ mod tests {
             *self.0.borrow_mut() = 42;
         }
     }
-
-    #[test]
-    fn poison_works() {
-        let m = Arc::new(ReentrantMutex::new(RefCell::new(0)));
-        let mc = m.clone();
-        let result = thread::spawn(move || {
-            let lock = mc.lock().unwrap();
-            *lock.borrow_mut() = 1;
-            let lock2 = mc.lock().unwrap();
-            *lock.borrow_mut() = 2;
-            let _answer = Answer(lock2);
-            panic!("What the answer to my lifetimes dilemma is?");
-        })
-        .join();
-        assert!(result.is_err());
-        let r = m.lock().err().unwrap().into_inner();
-        assert_eq!(*r.borrow(), 42);
-    }
 }
diff --git a/src/test/ui/eprint-on-tls-drop.rs b/src/test/ui/eprint-on-tls-drop.rs
new file mode 100644
index 00000000000..9c4800c1a3f
--- /dev/null
+++ b/src/test/ui/eprint-on-tls-drop.rs
@@ -0,0 +1,48 @@
+// run-pass
+// ignore-emscripten no processes
+
+use std::cell::RefCell;
+use std::env;
+use std::process::Command;
+
+fn main() {
+    let name = "YOU_ARE_THE_TEST";
+    if env::var(name).is_ok() {
+        std::thread::spawn(|| {
+            TLS.with(|f| f.borrow().ensure());
+        })
+        .join()
+        .unwrap();
+    } else {
+        let me = env::current_exe().unwrap();
+        let output = Command::new(&me).env(name, "1").output().unwrap();
+        println!("{:?}", output);
+        assert!(output.status.success());
+        let stderr = String::from_utf8(output.stderr).unwrap();
+        assert!(stderr.contains("hello new\n"));
+        assert!(stderr.contains("hello drop\n"));
+    }
+}
+
+struct Stuff {
+    _x: usize,
+}
+
+impl Stuff {
+    fn new() -> Self {
+        eprintln!("hello new");
+        Self { _x: 0 }
+    }
+
+    fn ensure(&self) {}
+}
+
+impl Drop for Stuff {
+    fn drop(&mut self) {
+        eprintln!("hello drop");
+    }
+}
+
+thread_local! {
+    static TLS: RefCell<Stuff> = RefCell::new(Stuff::new());
+}