Added a task wakeup callback. Closes #599.

The callback happens when a task moves from the "blocked" state to the
"running" state. The callback is also inherited by child tasks. There
is currently only a native API.

This code hasn't been heavily exercised yet.

src/rt/rust_scheduler.cpp

@@ -291,8 +291,10 @@ rust_scheduler::create_task(rust_task *spawner, const char *name) {
         new (this->kernel) rust_task (this, &newborn_tasks, spawner, name);
     DLOG(this, task, "created task: " PTR ", spawner: %s, name: %s",
                         task, spawner ? spawner->name : "null", name);
-    if(spawner)
+    if(spawner) {
         task->pin(spawner->pinned_on);
+        task->on_wakeup(spawner->_on_wakeup);
+    }
     newborn_tasks.append(task);
     return task;
 }

src/rt/rust_task.cpp

@@ -73,7 +73,8 @@ rust_task::rust_task(rust_scheduler *sched, rust_task_list *state,
     running_on(-1),
     pinned_on(-1),
     local_region(&sched->srv->local_region),
-    synchronized_region(&sched->srv->synchronized_region)
+    synchronized_region(&sched->srv->synchronized_region),
+    _on_wakeup(NULL)
 {
     LOGPTR(sched, "new task", (uintptr_t)this);
     DLOG(sched, task, "sizeof(task) = %d (0x%x)", sizeof *this, sizeof *this);
@@ -431,6 +432,10 @@ rust_task::wakeup(rust_cond *from) {
     I(sched, cond == from);
     cond = NULL;
     cond_name = "none";
+
+    if(_on_wakeup) {
+        _on_wakeup->on_wakeup();
+    }
 }
 
 void
@@ -541,6 +546,10 @@ void rust_task::unpin() {
     pinned_on = -1;
 }
 
+void rust_task::on_wakeup(rust_task::wakeup_callback *callback) {
+    _on_wakeup = callback;
+}
+
 //
 // Local Variables:
 // mode: C++

src/rt/rust_task.h

@@ -83,6 +83,13 @@ rust_task : public maybe_proxy<rust_task>,
     memory_region local_region;
     memory_region synchronized_region;
 
+    class wakeup_callback {
+    public:
+        virtual void on_wakeup() = 0;
+    };
+
+    wakeup_callback *_on_wakeup;
+
     // Only a pointer to 'name' is kept, so it must live as long as this task.
     rust_task(rust_scheduler *sched,
               rust_task_list *state,
@@ -156,6 +163,8 @@ rust_task : public maybe_proxy<rust_task>,
     void pin();
     void pin(int id);
     void unpin();
+
+    void on_wakeup(wakeup_callback *callback);
 };
 
 //