Auto merge of #97924 - cuviper:unguarded-poison, r=Mark-Simulacrum

Avoid `thread::panicking()` in non-poisoning methods of `Mutex` and `RwLock`

`Mutex::lock()` and `RwLock::write()` are poison-guarded against panics,
in that they set the poison flag if a panic occurs while they're locked.
But if we're already in a panic (`thread::panicking()`), they leave the
poison flag alone.

That check is a bit of a waste for methods that never set the poison
flag though, namely `get_mut()`, `into_inner()`, and `RwLock::read()`.
These use-cases are now split to avoid that unnecessary call.

library/std/src/sync/mutex.rs

@@ -423,7 +423,7 @@ impl<T: ?Sized> Mutex<T> {
         T: Sized,
     {
         let data = self.data.into_inner();
-        poison::map_result(self.poison.borrow(), |_| data)
+        poison::map_result(self.poison.borrow(), |()| data)
     }
 
     /// Returns a mutable reference to the underlying data.
@@ -448,7 +448,7 @@ impl<T: ?Sized> Mutex<T> {
     #[stable(feature = "mutex_get_mut", since = "1.6.0")]
     pub fn get_mut(&mut self) -> LockResult<&mut T> {
         let data = self.data.get_mut();
-        poison::map_result(self.poison.borrow(), |_| data)
+        poison::map_result(self.poison.borrow(), |()| data)
     }
 }
 
@@ -497,7 +497,7 @@ impl<T: ?Sized + fmt::Debug> fmt::Debug for Mutex<T> {
 
 impl<'mutex, T: ?Sized> MutexGuard<'mutex, T> {
     unsafe fn new(lock: &'mutex Mutex<T>) -> LockResult<MutexGuard<'mutex, T>> {
-        poison::map_result(lock.poison.borrow(), |guard| MutexGuard { lock, poison: guard })
+        poison::map_result(lock.poison.guard(), |guard| MutexGuard { lock, poison: guard })
     }
 }
 

library/std/src/sync/poison.rs

@@ -23,8 +23,15 @@ impl Flag {
         Flag { failed: AtomicBool::new(false) }
     }
 
+    /// Check the flag for an unguarded borrow, where we only care about existing poison.
     #[inline]
-    pub fn borrow(&self) -> LockResult<Guard> {
+    pub fn borrow(&self) -> LockResult<()> {
+        if self.get() { Err(PoisonError::new(())) } else { Ok(()) }
+    }
+
+    /// Check the flag for a guarded borrow, where we may also set poison when `done`.
+    #[inline]
+    pub fn guard(&self) -> LockResult<Guard> {
         let ret = Guard { panicking: thread::panicking() };
         if self.get() { Err(PoisonError::new(ret)) } else { Ok(ret) }
     }

library/std/src/sync/rwlock.rs

@@ -434,7 +434,7 @@ impl<T: ?Sized> RwLock<T> {
         T: Sized,
     {
         let data = self.data.into_inner();
-        poison::map_result(self.poison.borrow(), |_| data)
+        poison::map_result(self.poison.borrow(), |()| data)
     }
 
     /// Returns a mutable reference to the underlying data.
@@ -461,7 +461,7 @@ impl<T: ?Sized> RwLock<T> {
     #[stable(feature = "rwlock_get_mut", since = "1.6.0")]
     pub fn get_mut(&mut self) -> LockResult<&mut T> {
         let data = self.data.get_mut();
-        poison::map_result(self.poison.borrow(), |_| data)
+        poison::map_result(self.poison.borrow(), |()| data)
     }
 }
 
@@ -510,13 +510,13 @@ impl<T> From<T> for RwLock<T> {
 
 impl<'rwlock, T: ?Sized> RwLockReadGuard<'rwlock, T> {
     unsafe fn new(lock: &'rwlock RwLock<T>) -> LockResult<RwLockReadGuard<'rwlock, T>> {
-        poison::map_result(lock.poison.borrow(), |_| RwLockReadGuard { lock })
+        poison::map_result(lock.poison.borrow(), |()| RwLockReadGuard { lock })
     }
 }
 
 impl<'rwlock, T: ?Sized> RwLockWriteGuard<'rwlock, T> {
     unsafe fn new(lock: &'rwlock RwLock<T>) -> LockResult<RwLockWriteGuard<'rwlock, T>> {
-        poison::map_result(lock.poison.borrow(), |guard| RwLockWriteGuard { lock, poison: guard })
+        poison::map_result(lock.poison.guard(), |guard| RwLockWriteGuard { lock, poison: guard })
     }
 }