Conservatively serialize nearly all upcalls. Successfuly ran make check with RUST_THREADS=8, so we're probably fairly safe now. In the future we can relax the synchronization to get better performance.

src/rt/rust_builtin.cpp

@@ -391,12 +391,17 @@ task_yield(rust_task *task) {
 
 extern "C" CDECL void
 task_join(rust_task *task, rust_task *join_task) {
+    task->dom->scheduler_lock.lock();
     // If the other task is already dying, we don't have to wait for it.
     if (join_task->dead() == false) {
         join_task->tasks_waiting_to_join.push(task);
         task->block(join_task, "joining local task");
+        task->dom->scheduler_lock.unlock();
         task->yield(2);
     }
+    else {
+        task->dom->scheduler_lock.unlock();
+    }
 }
 
 /* Debug builtins for std.dbg. */