"failed at 'sending on a closed channel'" when using Future

[schmee]

While playing around with opening a bunch of files and collecting the results I ran into task '<unnamed>' failed at 'sending on a closed channel', C:\bot\slave\nightly-win\build\src\libstd\comm\mod.rs:509 errors. The errors are not deterministic, so sometimes the code works and sometimes not. The error happens with and without -O. Here is a reduced test case:

extern crate sync;
use std::sync::Future;

fn main() {
    let mut futures = range(0, 134).map(|i| {
        Future::spawn(proc() { vec![] } )
    }).collect::<Vec<Future<Vec<String>>>>();
}

Here's another one courtesy of @huonw (this one errors more rarely, at least for me):

extern crate sync;
use std::sync::Future;

fn main() {
    let r = {
        Vec::from_fn(1000, |_| Future::spawn(proc() {}));
    };
}

$ rustc --version
rustc 0.11.0-pre-nightly (18c451f 2014-06-14 06:36:49 +0000)
host: i686-pc-mingw32

[huonw]

cc @alexcrichton

[alexcrichton]

Closing as working as intended. Each future will send the result of its computation along a channel, but what's happening here is that you're spawning a future and the closing the receiving half of the channel by dropping the future. When the task is done it then sends the result on an empty channel, failing the task.

[huonw]

The user doesn't care that a Future is implemented with channels internally, so I think the failure message should either be specific to Future, or just not be printed at all.