Child tasks take a ref to their parents

This is so that when a child dies after the parent, it still holds a valid
pointer and can call supervisor->kill() safely.

src/rt/rust_task.cpp

@@ -86,6 +86,9 @@ rust_task::rust_task(rust_scheduler *sched, rust_task_list *state,
 
     stk = new_stk(sched, this, 0);
     user.rust_sp = stk->limit;
+    if (supervisor) {
+        supervisor->ref();
+    }
 }
 
 rust_task::~rust_task()
@@ -107,6 +110,10 @@ rust_task::~rust_task()
         }
     }
 
+    if (supervisor) {
+        supervisor->deref();
+    }
+
     kernel->release_task_id(user.id);
 
     /* FIXME: tighten this up, there are some more
@@ -294,6 +301,9 @@ rust_task::unsupervise()
              "task %s @0x%" PRIxPTR
              " disconnecting from supervisor %s @0x%" PRIxPTR,
              name, this, supervisor->name, supervisor);
+    if (supervisor) {
+        supervisor->deref();
+    }
     supervisor = NULL;
     propagate_failure = false;
 }