Ticker thread might block final draw on progress bar

[chris-laplante]

The order of events would be:

1. You install a Ticker and it starts chugging along.
2. main returns or panics, the ProgressBar(s) go out of scope and should be dropped.
3. Simultaneously, let's say the Ticker is executing this line:

   indicatif/src/state.rs

   Line 310 in 515a2a9

   state.draw(false, Instant::now()).ok();

4. Since main returned, the thread is killed while Ticker is executing that line.
5. The Arc is still held, so finish_using_style is never called and the spinner is not cleared.

From #407 (comment):

Please give this fork a try: https://github.com/chris-laplante/indicatif/tree/cpl/fun_with_barriers and try running: cargo run --example=long-spinner

By default I have Barrier #1 and #2 uncommented. If you comment out barrier #2 and uncomment #3, you see correct behavior (since executing proceeds after the Arc is dropped). You can also comment out all the Barriers and you should get correct behavior (unless of course, the race happens organically 😉).