Add a fairness example

Author: Stjepan Glavina

examples/fairness.rs

@@ -0,0 +1,37 @@
+//! Demonstrates fairness properties of the lock.
+//!
+//! A number of threads run a loop in which they hold the lock for a little bit and re-acquire it
+//! immediately after. In the end we print the number of times each thread acquired the lock.
+
+use std::thread;
+use std::time::{Duration, Instant};
+
+use async_lock::Lock;
+use smol::Timer;
+
+fn main() {
+    let num_threads = 30;
+    let mut threads = Vec::new();
+    let hits = Lock::new(vec![0; num_threads]);
+
+    for i in 0..num_threads {
+        let hits = hits.clone();
+        threads.push(thread::spawn(move || {
+            smol::run(async {
+                let start = Instant::now();
+
+                while start.elapsed() < Duration::from_secs(1) {
+                    let mut hits = hits.lock().await;
+                    hits[i] += 1;
+                    Timer::after(Duration::from_micros(5000)).await;
+                }
+            })
+        }));
+    }
+
+    for t in threads {
+        t.join().unwrap();
+    }
+
+    dbg!(hits);
+}