make sum invoke is_current_revision_canceled deterministically

tests/parallel/race.rs

@@ -25,9 +25,14 @@ fn in_par_get_set_race() {
     });
 
     // If the 1st thread runs first, you get 111, otherwise you get
-    // 1011.
+    // 1011; if they run concurrently and the 1st thread observes the
+    // cancelation, you get back usize::max.
     let value1 = thread1.join().unwrap();
-    assert!(value1 == 111 || value1 == 1011, "illegal result {}", value1);
+    assert!(
+        value1 == 111 || value1 == 1011 || value1 == std::usize::MAX,
+        "illegal result {}",
+        value1
+    );
 
     assert_eq!(thread2.join().unwrap(), 1000);
 }

tests/parallel/setup.rs

@@ -109,6 +109,17 @@ fn sum(db: &impl ParDatabase, key: &'static str) -> usize {
             std::thread::yield_now();
         }
         log::debug!("cancellation observed");
+    }
+
+    // Check for cancelation and return MAX if so. Note that we check
+    // for cancelation *deterministically* -- but if
+    // `sum_wait_for_cancellation` is set, we will block
+    // beforehand. Deterministic execution is a requirement for valid
+    // salsa user code. It's also important to some tests that `sum`
+    // *attempts* to invoke `is_current_revision_canceled` even if we
+    // know it will not be canceled, because that helps us keep the
+    // accounting up to date.
+    if db.salsa_runtime().is_current_revision_canceled() {
         return std::usize::MAX; // when we are cancelled, we return usize::MAX.
     }