spawn-blocking: Add example from Chapter 20, Asynchronous Programming.

spawn-blocking/.gitignore

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+/target/
+Cargo.lock

spawn-blocking/Cargo.toml

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+[package]
+name = "spawn-blocking"
+version = "0.1.0"
+authors = ["You <you@example.com>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dev-dependencies]
+argonautica = "0.2"
+async-std = "1.7"
+

spawn-blocking/src/lib.rs

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+#[cfg(test)]
+mod tests;
+
+use std::sync::{Arc, Mutex};
+use std::task::Waker;
+
+pub struct SpawnBlocking<T>(Arc<Mutex<Shared<T>>>);
+
+struct Shared<T> {
+    value: Option<T>,
+    waker: Option<Waker>,
+}
+
+pub fn spawn_blocking<T, F>(closure: F) -> SpawnBlocking<T>
+where F: FnOnce() -> T,
+      F: Send + 'static,
+      T: Send + 'static,
+{
+    let inner = Arc::new(Mutex::new(Shared {
+        value: None,
+        waker: None,
+    }));
+
+    std::thread::spawn({
+        let inner = inner.clone();
+        move || {
+            let value = closure();
+
+            let maybe_waker = {
+                let mut guard = inner.lock().unwrap();
+                guard.value = Some(value);
+                guard.waker.take()
+            };
+
+            if let Some(waker) = maybe_waker {
+                waker.wake();
+            }
+        }
+    });
+
+    SpawnBlocking(inner)
+}
+
+use std::future::Future;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+impl<T: Send> Future for SpawnBlocking<T> {
+    type Output = T;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<T> {
+        let mut guard = self.0.lock().unwrap();
+        if let Some(value) = guard.value.take() {
+            return Poll::Ready(value);
+        }
+
+        guard.waker = Some(cx.waker().clone());
+        Poll::Pending
+    }
+}

spawn-blocking/src/tests.rs

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+use super::*;
+
+async fn verify_password(password: &str, hash: &str, key: &str)
+                        -> Result<bool, argonautica::Error>
+{
+    let password = password.to_string();
+    let hash = hash.to_string();
+    let key = key.to_string();
+
+    spawn_blocking(move || {
+        argonautica::Verifier::default()
+            .with_hash(hash)
+            .with_password(password)
+            .with_secret_key(key)
+            .verify()
+    }).await
+}
+
+static PASSWORD: &str = "P@ssw0rd";
+static HASH: &str = "$argon2id$v=19$m=4096,t=192,p=4$\
+                     o2y5PU86Vt+sr93N7YUGgC7AMpTKpTQCk4tNGUPZMY4$\
+                     yzP/ukZRPIbZg6PvgnUUobUMbApfF9RH6NagL9L4Xr4\
+                     ";
+static SECRET_KEY: &str = "secret key that you should really store in a .env file \
+                           instead of in code, but this is just an example\
+                           ";
+
+#[test]
+fn argonautica() {
+    async_std::task::block_on(async {
+        assert!(verify_password(PASSWORD, HASH, SECRET_KEY).await.unwrap());
+    });
+}
+
+#[test]
+fn many_serial() {
+    async_std::task::block_on(async {
+        for i in 0..1000 {
+            assert_eq!(spawn_blocking(move || i).await, i);
+        }
+    });
+}
+
+#[test]
+fn many_parallel() {
+    async_std::task::block_on(async {
+        let futures: Vec<_> = (0..100)
+            .map(|i| (i, spawn_blocking(move || i)))
+            .collect();
+
+        for (i, f) in futures {
+            assert_eq!(f.await, i);
+        }
+    });
+}