fix: revert the waitAny refactoring (#11000)

This PR fixes a memleak caused by the Lean based `IO.waitAny`
implementation by reverting it.

This the faulty Lean implementation:
```lean
def IO.waitAny (tasks : @& List (Task α)) (h : tasks.length > 0 := by exact Nat.zero_lt_succ _) :
    BaseIO α := do
  have : Nonempty α := ⟨tasks[0].get⟩
  let promise : IO.Promise α ← IO.Promise.new
  tasks.forM <| fun t => BaseIO.chainTask (sync := true) t promise.resolve
  return promise.result!.get
```
In a situation where we call this function repeatedly in a loop with a
pair of tasks `[t1, t2]`
where `t2` is a long lived task that we pass every time and `t1` is
fresh a short lived task, `t2` will
accumlate more and more children from `BaseIO.chainTask` that fill
memory over time. The old C++
implementation did not have this issue so we are reverting.

src/Init/System/IO.lean

@@ -557,6 +557,13 @@ Waits for the task to finish, then returns its result.
 @[extern "lean_io_wait"] opaque wait (t : Task α) : BaseIO α :=
   return t.get
 
+/--
+Waits until any of the tasks in the list has finished, then return its result.
+-/
+@[extern "lean_io_wait_any"] opaque waitAny (tasks : @& List (Task α))
+    (h : tasks.length > 0 := by exact Nat.zero_lt_succ _) : BaseIO α :=
+  return tasks[0].get
+
 /--
 Returns the number of _heartbeats_ that have occurred during the current thread's execution. The
 heartbeat count is the number of “small” memory allocations performed in a thread.

src/Init/Task.lean

@@ -12,17 +12,6 @@ public import Init.System.Promise
 
 public section
 
-/--
-Waits until any of the tasks in the list has finished, then return its result.
--/
-@[noinline]
-def IO.waitAny (tasks : @& List (Task α)) (h : tasks.length > 0 := by exact Nat.zero_lt_succ _) :
-    BaseIO α := do
-  have : Nonempty α := ⟨tasks[0].get⟩
-  let promise : IO.Promise α ← IO.Promise.new
-  tasks.forM <| fun t => BaseIO.chainTask (sync := true) t promise.resolve
-  return promise.result!.get
-
 namespace Task
 
 /--

src/include/lean/lean.h

@@ -1225,6 +1225,8 @@ LEAN_EXPORT bool lean_io_check_canceled_core(void);
 LEAN_EXPORT void lean_io_cancel_core(b_lean_obj_arg t);
 /* primitive for implementing `IO.getTaskState : Task a -> IO TaskState` */
 LEAN_EXPORT uint8_t lean_io_get_task_state_core(b_lean_obj_arg t);
+/* primitive for implementing `IO.waitAny : List (Task a) -> IO (Task a)` */
+LEAN_EXPORT b_lean_obj_res lean_io_wait_any_core(b_lean_obj_arg task_list);
 
 /* External objects */
 

src/runtime/io.cpp

@@ -1554,6 +1554,13 @@ extern "C" LEAN_EXPORT obj_res lean_io_wait(obj_arg t) {
     return lean_task_get_own(t);
 }
 
+extern "C" LEAN_EXPORT obj_res lean_io_wait_any(b_obj_arg task_list) {
+    object * t = lean_io_wait_any_core(task_list);
+    object * v = lean_task_get(t);
+    lean_inc(v);
+    return v;
+}
+
 extern "C" LEAN_EXPORT obj_res lean_io_exit(uint8_t code) {
     exit(code);
 }

src/runtime/object.cpp

@@ -847,6 +847,17 @@ class task_manager {
         }
     }
 
+    object * wait_any_check(object * task_list) {
+        object * it = task_list;
+        while (!is_scalar(it)) {
+            object * head = lean_ctor_get(it, 0);
+            if (lean_to_task(head)->m_value)
+                return head;
+            it = cnstr_get(it, 1);
+        }
+        return nullptr;
+    }
+
 public:
     task_manager(unsigned max_std_workers):
         m_max_std_workers(max_std_workers) {
@@ -929,6 +940,17 @@ public:
         }
     }
 
+    object * wait_any(object * task_list) {
+        if (object * t = wait_any_check(task_list))
+            return t;
+        unique_lock<mutex> lock(m_mutex);
+        while (true) {
+            if (object * t = wait_any_check(task_list))
+                return t;
+            m_task_finished_cv.wait(lock);
+        }
+    }
+
     void deactivate_task(lean_task_object * t) {
         unique_lock<mutex> lock(m_mutex);
         if (object * v = t->m_value) {
@@ -1166,6 +1188,10 @@ extern "C" LEAN_EXPORT uint8_t lean_io_get_task_state_core(b_obj_arg t) {
     return g_task_manager->get_task_state(o);
 }
 
+extern "C" LEAN_EXPORT b_obj_res lean_io_wait_any_core(b_obj_arg task_list) {
+    return g_task_manager->wait_any(task_list);
+}
+
 obj_res lean_promise_new() {
     lean_always_assert(g_task_manager);
 

src/runtime/object.h

@@ -287,6 +287,7 @@ inline b_obj_res task_get(b_obj_arg t) { return lean_task_get(t); }
 inline bool io_check_canceled_core() { return lean_io_check_canceled_core(); }
 inline void io_cancel_core(b_obj_arg t) { return lean_io_cancel_core(t); }
 inline bool io_get_task_state_core(b_obj_arg t) { return lean_io_get_task_state_core(t); }
+inline b_obj_res io_wait_any_core(b_obj_arg task_list) { return lean_io_wait_any_core(task_list); }
 
 // =======================================
 // External