add a test for the "AB peeping C" scenario

tests/cycle.rs

@@ -130,7 +130,7 @@ fn max_panic<'db>(db: &'db dyn Db, inputs: Inputs) -> u8 {
 // - `Xi` for `max_iterate`
 // - `Np` for `min_panic`
 // - `Xp` for `max_panic`
-//
+//\
 // and `ii` is the inputs for that query, represented as a comma-separated list, with each
 // component representing an input:
 // - `a`, `b`, `c`... where the input is another node,

tests/parallel/cycle_ab_peeping_c.rs

@@ -0,0 +1,97 @@
+//! Test a specific cycle scenario:
+//!
+//! Thread T1 calls A which calls B which calls A.
+//!
+//! Thread T2 calls C which calls B.
+//!
+//! The trick is that the call from Thread T2 comes before B has reached a fixed point.
+//! We want to be sure that C sees the final value (and blocks until it is complete).
+
+use salsa::CycleRecoveryAction;
+
+use crate::setup::{Knobs, KnobsDatabase};
+
+#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Clone, Copy)]
+struct CycleValue(u32);
+
+const MIN: CycleValue = CycleValue(0);
+const MID: CycleValue = CycleValue(11);
+const MAX: CycleValue = CycleValue(22);
+
+#[salsa::tracked(cycle_fn=query_a_cycle_fn, cycle_initial=query_a_initial)]
+fn query_a(db: &dyn KnobsDatabase) -> CycleValue {
+    eprintln!("query_a()");
+    let b_value = query_b(db);
+
+    eprintln!("query_a: {:?}", b_value);
+
+    // When we reach the mid point, signal stage 1 (unblocking T2)
+    // and then wait for T2 to signal stage 2.
+    if b_value == MID {
+        eprintln!("query_a: signal");
+        db.signal(1);
+        db.wait_for(2);
+    }
+
+    b_value
+}
+
+fn query_a_cycle_fn(
+    _db: &dyn KnobsDatabase,
+    value: &CycleValue,
+    count: u32,
+) -> CycleRecoveryAction<CycleValue> {
+    eprintln!("query_a_cycle_fn({:?}, {:?})", value, count);
+    CycleRecoveryAction::Iterate
+}
+
+fn query_a_initial(_db: &dyn KnobsDatabase) -> CycleValue {
+    MIN
+}
+
+#[salsa::tracked]
+fn query_b(db: &dyn KnobsDatabase) -> CycleValue {
+    eprintln!("query_b()");
+
+    let a_value = query_a(db);
+
+    eprintln!("query_b: {:?}", a_value);
+
+    CycleValue(a_value.0 + 1).min(MAX)
+}
+
+#[salsa::tracked]
+fn query_c(db: &dyn KnobsDatabase) -> CycleValue {
+    eprintln!("query_c()");
+
+    // Wait until T1 has reached MID then execute `query_b`.
+    // This shoul block and (due to the configuration on our database) signal stage 2.
+    db.wait_for(1);
+
+    eprintln!("query_c: signaled");
+
+    query_b(db)
+}
+
+#[test]
+fn the_test() {
+    eprintln!("hi");
+    std::thread::scope(|scope| {
+        let db_t1 = Knobs::default();
+
+        let db_t2 = db_t1.clone();
+        db_t2.signal_on_will_block.store(2);
+
+        // Thread 1:
+        scope.spawn(move || {
+            let r = query_a(&db_t1);
+            assert_eq!(r, MAX);
+        });
+
+        // Thread 2:
+        scope.spawn(move || {
+            let r = query_c(&db_t2);
+            assert_eq!(r, MAX);
+        });
+    });
+}

tests/parallel/main.rs

@@ -1,5 +1,6 @@
 mod setup;
 
+mod cycle_ab_peeping_c;
 mod parallel_cancellation;
 mod parallel_map;
 mod signal;