use callbacks in parallel test

tests/parallel/setup.rs

@@ -56,6 +56,9 @@ pub(crate) struct KnobsStruct {
     /// threads to ensure we reach various weird states.
     pub(crate) signal: Arc<Signal>,
 
+    /// When this database is about to block, send a signal.
+    pub(crate) signal_on_will_block: Cell<usize>,
+
     /// Invocations of `sum` will signal this stage on entry.
     pub(crate) sum_signal_on_entry: Cell<usize>,
 
@@ -114,6 +117,17 @@ impl Database for ParDatabaseImpl {
     fn salsa_runtime(&self) -> &salsa::Runtime<ParDatabaseImpl> {
         &self.runtime
     }
+
+    fn salsa_event(&self, event_fn: impl Fn() -> salsa::Event<Self>) {
+        let event = event_fn();
+        match event.kind {
+            salsa::EventKind::WillBlockOn { .. } => {
+                self.signal(self.knobs().signal_on_will_block.get());
+            }
+
+            _ => {}
+        }
+    }
 }
 
 impl ParallelDatabase for ParDatabaseImpl {

tests/parallel/true_parallel.rs

@@ -66,13 +66,15 @@ fn true_parallel_same_keys() {
         }
     });
 
-    // Thread 2 will sync barrier *just* before calling `sum`. Doesn't
+    // Thread 2 will wait until Thread 1 has entered sum and then --
-    // guarantee the race we want but makes it highly likely.
+    // once it has set tself to block -- signal Thread 1 to
+    // continue. This way, we test out the mechanism of one thread
+    // blocking on another.
     let thread2 = std::thread::spawn({
         let db = db.fork();
         move || {
             db.knobs().signal.wait_for(1);
-            db.knobs().signal.signal(2);
+            db.knobs().signal_on_will_block.set(2);
             db.sum("abc")
         }
     });