feat: wait on dedicated tasks after main is finished (#7958)

This PR ensures that after `main` is finished we still wait on dedicated
tasks instead of exiting forcefully. If users wish to violently kill
their dedicated tasks at the end of main instead they can run
`IO.Process.exit` at the end of `main` instead.

src/runtime/object.cpp

@@ -666,6 +666,7 @@ class task_manager {
     unsigned                                      m_max_prio{0};
     condition_variable                            m_queue_cv;
     condition_variable                            m_task_finished_cv;
+    condition_variable                            m_dedicated_finished_cv;
     bool                                          m_shutting_down{false};
 
     lean_task_object * dequeue() {
@@ -762,6 +763,7 @@ class task_manager {
             unique_lock<mutex> lock(m_mutex);
             run_task(lock, t);
             m_num_dedicated_workers--;
+            m_dedicated_finished_cv.notify_all();
         });
         // `lthread` will be implicitly freed, which frees up its control resources but does not terminate the thread
     }
@@ -863,6 +865,9 @@ public:
         // wait for all workers to finish
         for (auto & t : m_std_workers)
             t->join();
+
+        unique_lock<mutex> lock(m_mutex);
+        m_dedicated_finished_cv.wait(lock, [&]() { return m_num_dedicated_workers == 0; });
         // never seems to terminate under Emscripten
 #endif
     }

tests/compiler/wait_dedicated.lean

@@ -0,0 +1,10 @@
+/- Ensure we wait on dedicated tasks even after main is finished -/
+
+def stuff : IO Unit := do
+  IO.sleep 100
+  IO.println "Hi there"
+
+
+def main : IO Unit := do
+  discard <| IO.asTask (prio := .dedicated) stuff
+

tests/compiler/wait_dedicated.lean.expected.out

@@ -0,0 +1 @@
+Hi there