show that executing queries on distinct keys can proceed in parallel

tests/parallel/main.rs

@@ -3,3 +3,4 @@ mod setup;
 mod cancellation;
 mod independent;
 mod race;
+mod true_parallel;

tests/parallel/true_parallel.rs

@@ -0,0 +1,41 @@
+use crate::setup::{Input, Knobs, ParDatabase, ParDatabaseImpl, WithValue};
+use salsa::{Database, ParallelDatabase};
+
+/// Test where two threads are executing sum. We show that they can
+/// both be executing sum in parallel by having thread1 wait for
+/// thread2 to send a signal before it leaves (similarly, thread2
+/// waits for thread1 to send a signal before it enters).
+#[test]
+fn true_parallel_different_keys() {
+    let db = ParDatabaseImpl::default();
+
+    db.query(Input).set('a', 100);
+    db.query(Input).set('b', 010);
+    db.query(Input).set('c', 001);
+
+    // Thread 1 will signal stage 1 when it enters and wait for stage 2.
+    let thread1 = std::thread::spawn({
+        let db = db.fork();
+        move || {
+            let v = db.knobs().sum_signal_on_entry.with_value(1, || {
+                db.knobs().sum_await_on_exit.with_value(2, || db.sum("a"))
+            });
+            v
+        }
+    });
+
+    // Thread 2 will await stage 1 when it enters and signal stage 2
+    // when it leaves.
+    let thread2 = std::thread::spawn({
+        let db = db.fork();
+        move || {
+            let v = db.knobs().sum_await_on_entry.with_value(1, || {
+                db.knobs().sum_signal_on_exit.with_value(2, || db.sum("b"))
+            });
+            v
+        }
+    });
+
+    assert_eq!(thread1.join().unwrap(), 100);
+    assert_eq!(thread2.join().unwrap(), 010);
+}