Guarantee that native tasks wait for children

There was a scheduling race where a child may not increment the global task
count before the parent exits, and the parent would then think that there are no
more tasks left.

Author: alexcrichton

src/libnative/task.rs

@@ -72,6 +72,11 @@ pub fn spawn_opts(opts: TaskOpts, f: proc()) {
     let task = task;
     let ops = ops();
 
+    // Note that this increment must happen *before* the spawn in order to
+    // guarantee that if this task exits it will always end up waiting for the
+    // spawned task to exit.
+    bookeeping::increment();
+
     // Spawning a new OS thread guarantees that __morestack will never get
     // triggered, but we must manually set up the actual stack bounds once this
     // function starts executing. This raises the lower limit by a bit because
@@ -88,7 +93,6 @@ pub fn spawn_opts(opts: TaskOpts, f: proc()) {
         let mut ops = ops;
         ops.stack_bounds = Some((my_stack - stack + 1024, my_stack));
 
-        bookeeping::increment();
         let mut f = Some(f);
         let mut task = task;
         task.put_runtime(ops as ~rt::Runtime);

src/test/run-pass/native-always-waits.rs

@@ -0,0 +1,28 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// xfail-fast
+
+extern mod native;
+
+static mut set: bool = false;
+
+#[start]
+fn start(argc: int, argv: **u8) -> int {
+    // make sure that native::start always waits for all children to finish
+    do native::start(argc, argv) {
+        do spawn {
+            unsafe { set = true; }
+        }
+    };
+
+    // if we didn't set the global, then return a nonzero code
+    if unsafe {set} {0} else {1}
+}