1 files changed, 75 insertions, 0 deletions
diff --git a/library/std/src/sys_common/at_exit_imp.rs b/library/std/src/sys_common/at_exit_imp.rs
new file mode 100644
index 00000000000..6b799db856e
--- /dev/null
+++ b/library/std/src/sys_common/at_exit_imp.rs
@@ -0,0 +1,75 @@
+//! Implementation of running at_exit routines
+//!
+//! Documentation can be found on the `rt::at_exit` function.
+
+use crate::mem;
+use crate::ptr;
+use crate::sys_common::mutex::Mutex;
+
+type Queue = Vec<Box<dyn FnOnce()>>;
+
+// NB these are specifically not types from `std::sync` as they currently rely
+// on poisoning and this module needs to operate at a lower level than requiring
+// the thread infrastructure to be in place (useful on the borders of
+// initialization/destruction).
+// We never call `LOCK.init()`, so it is UB to attempt to
+// acquire this mutex reentrantly!
+static LOCK: Mutex = Mutex::new();
+static mut QUEUE: *mut Queue = ptr::null_mut();
+
+const DONE: *mut Queue = 1_usize as *mut _;
+
+// The maximum number of times the cleanup routines will be run. While running
+// the at_exit closures new ones may be registered, and this count is the number
+// of times the new closures will be allowed to register successfully. After
+// this number of iterations all new registrations will return `false`.
+const ITERS: usize = 10;
+
+unsafe fn init() -> bool {
+    if QUEUE.is_null() {
+        let state: Box<Queue> = box Vec::new();
+        QUEUE = Box::into_raw(state);
+    } else if QUEUE == DONE {
+        // can't re-init after a cleanup
+        return false;
+    }
+
+    true
+}
+
+pub fn cleanup() {
+    for i in 1..=ITERS {
+        unsafe {
+            let queue = {
+                let _guard = LOCK.lock();
+                mem::replace(&mut QUEUE, if i == ITERS { DONE } else { ptr::null_mut() })
+            };
+
+            // make sure we're not recursively cleaning up
+            assert!(queue != DONE);
+
+            // If we never called init, not need to cleanup!
+            if !queue.is_null() {
+                let queue: Box<Queue> = Box::from_raw(queue);
+                for to_run in *queue {
+                    // We are not holding any lock, so reentrancy is fine.
+                    to_run();
+                }
+            }
+        }
+    }
+}
+
+pub fn push(f: Box<dyn FnOnce()>) -> bool {
+    unsafe {
+        let _guard = LOCK.lock();
+        if init() {
+            // We are just moving `f` around, not calling it.
+            // There is no possibility of reentrancy here.
+            (*QUEUE).push(f);
+            true
+        } else {
+            false
+        }
+    }
+}