Auto merge of #28585 - ranma42:simpler-panic, r=alexcrichton

This is part of some cleanup I did while investigating #28129.
This also ensures that `on_panic` is run even if the user has registered too many callbacks.

src/libstd/sys/common/unwind/mod.rs

@@ -92,23 +92,6 @@ pub mod imp;
 #[path = "gcc.rs"] #[doc(hidden)]
 pub mod imp;
 
-pub type Callback = fn(msg: &(Any + Send), file: &'static str, line: u32);
-
-// Variables used for invoking callbacks when a thread starts to unwind.
-//
-// For more information, see below.
-const MAX_CALLBACKS: usize = 16;
-static CALLBACKS: [atomic::AtomicUsize; MAX_CALLBACKS] =
-        [atomic::AtomicUsize::new(0), atomic::AtomicUsize::new(0),
-         atomic::AtomicUsize::new(0), atomic::AtomicUsize::new(0),
-         atomic::AtomicUsize::new(0), atomic::AtomicUsize::new(0),
-         atomic::AtomicUsize::new(0), atomic::AtomicUsize::new(0),
-         atomic::AtomicUsize::new(0), atomic::AtomicUsize::new(0),
-         atomic::AtomicUsize::new(0), atomic::AtomicUsize::new(0),
-         atomic::AtomicUsize::new(0), atomic::AtomicUsize::new(0),
-         atomic::AtomicUsize::new(0), atomic::AtomicUsize::new(0)];
-static CALLBACK_CNT: atomic::AtomicUsize = atomic::AtomicUsize::new(0);
-
 thread_local! { static PANICKING: Cell<bool> = Cell::new(false) }
 
 /// Invoke a closure, capturing the cause of panic if one occurs.
@@ -246,45 +229,11 @@ pub fn begin_unwind<M: Any + Send>(msg: M, file_line: &(&'static str, u32)) -> !
 #[inline(never)] #[cold] // this is the slow path, please never inline this
 fn begin_unwind_inner(msg: Box<Any + Send>,
                       file_line: &(&'static str, u32)) -> ! {
-    // Make sure the default failure handler is registered before we look at the
-    // callbacks. We also use a raw sys-based mutex here instead of a
-    // `std::sync` one as accessing TLS can cause weird recursive problems (and
-    // we don't need poison checking).
-    unsafe {
-        static LOCK: Mutex = Mutex::new();
-        static mut INIT: bool = false;
-        LOCK.lock();
-        if !INIT {
-            register(panicking::on_panic);
-            INIT = true;
-        }
-        LOCK.unlock();
-    }
+    let (file, line) = *file_line;
 
-    // First, invoke call the user-defined callbacks triggered on thread panic.
-    //
-    // By the time that we see a callback has been registered (by reading
-    // MAX_CALLBACKS), the actual callback itself may have not been stored yet,
-    // so we just chalk it up to a race condition and move on to the next
-    // callback. Additionally, CALLBACK_CNT may briefly be higher than
-    // MAX_CALLBACKS, so we're sure to clamp it as necessary.
-    let callbacks = {
-        let amt = CALLBACK_CNT.load(Ordering::SeqCst);
-        &CALLBACKS[..cmp::min(amt, MAX_CALLBACKS)]
-    };
-    for cb in callbacks {
-        match cb.load(Ordering::SeqCst) {
-            0 => {}
-            n => {
-                let f: Callback = unsafe { mem::transmute(n) };
-                let (file, line) = *file_line;
-                f(&*msg, file, line);
-            }
-        }
-    };
+    // First, invoke the default panic handler.
+    panicking::on_panic(&*msg, file, line);
 
-    // Now that we've run all the necessary unwind callbacks, we actually
-    // perform the unwinding.
     if panicking() {
         // If a thread panics while it's already unwinding then we
         // have limited options. Currently our preference is to
@@ -295,34 +244,7 @@ fn begin_unwind_inner(msg: Box<Any + Send>,
         unsafe { intrinsics::abort() }
     }
     PANICKING.with(|s| s.set(true));
-    rust_panic(msg);
-}
 
-/// Register a callback to be invoked when a thread unwinds.
-///
-/// This is an unsafe and experimental API which allows for an arbitrary
-/// callback to be invoked when a thread panics. This callback is invoked on both
-/// the initial unwinding and a double unwinding if one occurs. Additionally,
-/// the local `Thread` will be in place for the duration of the callback, and
-/// the callback must ensure that it remains in place once the callback returns.
-///
-/// Only a limited number of callbacks can be registered, and this function
-/// returns whether the callback was successfully registered or not. It is not
-/// currently possible to unregister a callback once it has been registered.
-pub unsafe fn register(f: Callback) -> bool {
-    match CALLBACK_CNT.fetch_add(1, Ordering::SeqCst) {
-        // The invocation code has knowledge of this window where the count has
-        // been incremented, but the callback has not been stored. We're
-        // guaranteed that the slot we're storing into is 0.
-        n if n < MAX_CALLBACKS => {
-            let prev = CALLBACKS[n].swap(mem::transmute(f), Ordering::SeqCst);
-            rtassert!(prev == 0);
-            true
-        }
-        // If we accidentally bumped the count too high, pull it back.
-        _ => {
-            CALLBACK_CNT.store(MAX_CALLBACKS, Ordering::SeqCst);
-            false
-        }
-    }
+    // Finally, perform the unwinding.
+    rust_panic(msg);
 }