Fixed two races.

The first is that the memory_region destructor would complain there is
still an outstanding allocation. This is because circular_buffer from
rust_chan wasn't refing its task, so the task was being destructed too
soon.

The second was where the program could deadlock while joining a
task. The target task would die in the time between checking whether
the task should block and then actually blocking. The fix is to use
the target task's lock.

src/rt/circular_buffer.cpp

@@ -20,6 +20,7 @@ circular_buffer::circular_buffer(rust_task *task, size_t unit_sz) :
          _buffer_sz, _unread, this);
 
     A(sched, _buffer, "Failed to allocate buffer.");
+    task->ref();
 }
 
 circular_buffer::~circular_buffer() {
@@ -28,6 +29,7 @@ circular_buffer::~circular_buffer() {
     W(sched, _unread == 0,
       "freeing circular_buffer with %d unread bytes", _unread);
     task->free(_buffer);
+    --task->ref_count;
 }
 
 size_t

src/rt/rust_builtin.cpp

@@ -393,11 +393,16 @@ task_yield(rust_task *task) {
 extern "C" CDECL void
 task_join(rust_task *task, rust_task *join_task) {
     // If the other task is already dying, we don't have to wait for it.
+    join_task->lock.lock();
     if (join_task->dead() == false) {
         join_task->tasks_waiting_to_join.push(task);
         task->block(join_task, "joining local task");
+        join_task->lock.unlock();
         task->yield(2);
     }
+    else {
+        join_task->lock.unlock();
+    }
 }
 
 /* Debug builtins for std.dbg. */

src/rt/rust_task.cpp

@@ -118,12 +118,15 @@ void task_start_wrapper(spawn_args *a)
     a->f(&rval, task, a->a3, a->a4);
 
     LOG(task, task, "task exited with value %d", rval);
+    
 
     LOG(task, task, "task ref_count: %d", task->ref_count);
     A(task->sched, task->ref_count >= 0,
       "Task ref_count should not be negative on exit!");
     task->die();
+    task->lock.lock();
     task->notify_tasks_waiting_to_join();
+    task->lock.unlock();
 
     task->yield(1);
 }