Poisoned Mutex at runtime with nightly

[daschl]

Hi folks,

I'm not sure if I messed up here, but since this looks suspicious at runtime and works sometime I thought I gotta raise it here.

The code is as follows:

use std::sync::{Arc, Mutex};

fn main() {
    let num_threads = 4;

    let vec_arc = Arc::new(Mutex::new(Vec::with_capacity(num_threads)));

    for i in range(0u, num_threads) {
        let mutex = vec_arc.clone();

        spawn(proc() {
            let mut vec = mutex.lock();
            vec.insert(i, i);
            println!("vec is now {}", *vec);
        })
    }
}

When I run it on the command line, most of the time it works correctly:

vec is now [0]
vec is now [0, 1]
vec is now [0, 1, 2]
vec is now [0, 1, 2, 3]

But sometimes this comes up:

task '<unnamed>' failed at 'assertion failed: index <= len', /Users/rustbuild/src/rust-buildbot/slave/nightly-mac/build/src/libcollections/vec.rs:1000
task '<unnamed>' failed at 'Poisoned Mutex - another task failed inside!', /Users/rustbuild/src/rust-buildbot/slave/nightly-mac/build/src/libsync/lock.rs:46
task '<unnamed>' failed at 'Poisoned Mutex - another task failed inside!', /Users/rustbuild/src/rust-buildbot/slave/nightly-mac/build/src/libsync/lock.rs:46
task '<unnamed>' failed at 'Poisoned Mutex - another task failed inside!', /Users/rustbuild/src/rust-buildbot/slave/nightly-mac/build/src/libsync/lock.rs:46

Is this expected to happen? I'm on rustc 0.12.0-pre-nightly (f15d6d2 2014-07-20 22:46:29 +0000)

[chris-morgan]

This is a bug in your own code; Vec.insert(&mut self, index: uint, element: T) is defined as something that will fail “if index is not between 0 and the vector's length (both bounds inclusive).” This means that your code will only run successfully if the four tasks are run in sequence; if they are not, the insert call fails and the mutex is poisoned—once the task has failed, the mutex’s integrity cannot be guaranteed.

Everything is behaving as specified.

[daschl]

@chris-morgan thanks much :)

[chris-morgan]

It works if the four tasks are run in sequence, because that boils down to being equivalent to this code:

fn main() {
    let mut vec = Vec::with_capacity(4);
    vec.insert(0, 0u);
    vec.insert(1, 1);
    vec.insert(2, 2);
    vec.insert(3, 3);
    println!("{}", vec);
}

[0, 1, 2, 3]

(Remember that Vec::with_capacity is just setting the capacity of the vector, not inserting any items—it’s an efficiency measure only.)