rust-analyzer/crates/ra-salsa/tests/parallel/signal.rs

use parking_lot::{Condvar, Mutex};

#[derive(Default)]
pub(crate) struct Signal {
    value: Mutex<usize>,
    cond_var: Condvar,
}

impl Signal {
    pub(crate) fn signal(&self, stage: usize) {
        tracing::debug!("signal({})", stage);

        // This check avoids acquiring the lock for things that will
        // clearly be a no-op. Not *necessary* but helps to ensure we
        // are more likely to encounter weird race conditions;
        // otherwise calls to `sum` will tend to be unnecessarily
        // synchronous.
        if stage > 0 {
            let mut v = self.value.lock();
            if stage > *v {
                *v = stage;
                self.cond_var.notify_all();
            }
        }
    }

    /// Waits until the given condition is true; the fn is invoked
    /// with the current stage.
    pub(crate) fn wait_for(&self, stage: usize) {
        tracing::debug!("wait_for({})", stage);

        // As above, avoid lock if clearly a no-op.
        if stage > 0 {
            let mut v = self.value.lock();
            while *v < stage {
                self.cond_var.wait(&mut v);
            }
        }
    }
}