core: tests: Add some basic tests for ThreadPoolBuilder::use_current_…

…thread.

Ideas for testing the "call cleanup function from a job" case would be
great.

rayon-core/Cargo.toml

@@ -55,3 +55,7 @@ path = "tests/simple_panic.rs"
 [[test]]
 name = "scoped_threadpool"
 path = "tests/scoped_threadpool.rs"
+
+[[test]]
+name = "use_current_thread"
+path = "tests/use_current_thread.rs"

rayon-core/tests/use_current_thread.rs

@@ -0,0 +1,47 @@
+use rayon_core::ThreadPoolBuilder;
+use std::sync::{Arc, Condvar, Mutex};
+use std::thread::{self, JoinHandle};
+
+#[test]
+fn use_current_thread_basic() {
+    static JOIN_HANDLES: Mutex<Vec<JoinHandle<()>>> = Mutex::new(Vec::new());
+    let pool = ThreadPoolBuilder::new()
+        .num_threads(2)
+        .use_current_thread()
+        .spawn_handler(|builder| {
+            let handle = thread::Builder::new().spawn(|| builder.run())?;
+            JOIN_HANDLES.lock().unwrap().push(handle);
+            Ok(())
+        })
+        .build()
+        .unwrap();
+    assert_eq!(rayon_core::current_thread_index(), Some(0));
+    assert_eq!(
+        JOIN_HANDLES.lock().unwrap().len(),
+        1,
+        "Should only spawn one extra thread"
+    );
+
+    let pair = Arc::new((Mutex::new(false), Condvar::new()));
+    let pair2 = Arc::clone(&pair);
+    pool.spawn(move || {
+        assert_ne!(rayon_core::current_thread_index(), Some(0));
+        // This should execute even if the current thread is blocked, since we have two threads in
+        // the pool.
+        let &(ref started, ref condvar) = &*pair2;
+        *started.lock().unwrap() = true;
+        condvar.notify_one();
+    });
+
+    let _guard = pair
+        .1
+        .wait_while(pair.0.lock().unwrap(), |ran| !*ran)
+        .unwrap();
+    std::mem::drop(pool); // Drop the pool.
+
+    // Wait until all threads have actually exited. This is not really needed, other than to
+    // reduce noise of leak-checking tools.
+    for handle in std::mem::take(&mut *JOIN_HANDLES.lock().unwrap()) {
+        let _ = handle.join();
+    }
+}