Unwind the stack on task failure

When a task fails, we will throw an exception, then catch it at the bottom of
the stack.

On Windows we don't do this yet because the exception doesn't propagate
correctly.

No cleanups yet.

Issue #236

src/rt/rust_task.cpp

@@ -147,12 +147,6 @@ void task_exit(rust_closure_env *env, int rval, rust_task *task) {
         //env->td->free_glue(NULL, task, NULL, env->td->first_param, env);
         task->free(env);
     }
-    task->die();
-    task->lock.lock();
-    task->notify_tasks_waiting_to_join();
-    task->lock.unlock();
-
-    task->yield(1);
 }
 
 extern "C" CDECL
@@ -161,8 +155,37 @@ void task_start_wrapper(spawn_args *a)
     rust_task *task = a->task;
     int rval = 42;
 
+    bool failed = false;
+    try {
         a->f(&rval, task, a->a3, a->a4);
-    task_exit((rust_closure_env*)a->a3, rval, task);
+    } catch (rust_task *ex) {
+        A(task->sched, ex == task,
+          "Expected this task to be thrown for unwinding");
+        failed = true;
+    }
+
+    rust_closure_env* env = (rust_closure_env*)a->a3;
+    if(env) {
+        // free the environment.
+        I(task->sched, 1 == env->ref_count); // the ref count better be 1
+        //env->td->drop_glue(NULL, task, NULL, env->td->first_param, env);
+        //env->td->free_glue(NULL, task, NULL, env->td->first_param, env);
+        task->free(env);
+    }
+
+    if (failed) {
+#ifndef __WIN32__
+        task->conclude_failure();
+#else
+        A(task->sched, false, "Shouldn't happen");
+#endif
+    } else {
+        task->die();
+        task->lock.lock();
+        task->notify_tasks_waiting_to_join();
+        task->lock.unlock();
+        task->yield(1);
+    }
 }
 
 /* We spawn a rust (fastcc) function through a CDECL function
@@ -276,6 +299,15 @@ rust_task::fail() {
     // See note in ::kill() regarding who should call this.
     DLOG(sched, task, "task %s @0x%" PRIxPTR " failing", name, this);
     backtrace();
+#ifndef __WIN32__
+    throw this;
+#else
+    conclude_failure();
+#endif
+}
+
+void
+rust_task::conclude_failure() {
     die();
     // Unblock the task so it can unwind.
     unblock();

src/rt/rust_task.h

@@ -155,6 +155,7 @@ rust_task : public kernel_owned<rust_task>, rust_cond
 
     // Fail self, assuming caller-on-stack is this task.
     void fail();
+    void conclude_failure();
 
     // Disconnect from our supervisor.
     void unsupervise();