allow reuse of cached provisional memos within the same cycle iteration during maybe_changed_after (#954)

* Allow `validate_same_iteration` slow path for `maybe_changed_after`

* Add logging to cycle_panic test and enable output capture for debugging

* Add RUST_LOG=trace to CI for detailed tracing output

* Possible fix

* Discard changes to .github/workflows/test.yml

* Delete logs/.eb4cbc62113c2d27a93f1a2e33d842313ed0aa05-audit.json

* Delete logs/mcp-puppeteer-2025-08-02.log

* docs

* Add regression test

* Discard changes to tests/parallel/cycle_panic.rs

* Discard changes to Cargo.toml

* Fix comment

* Discard changes to Cargo.toml

* Update src/function/maybe_changed_after.rs

Co-authored-by: Carl Meyer <[email protected]>

---------

Co-authored-by: Carl Meyer <[email protected]>

Author: MichaReiser

src/function.rs

@@ -311,7 +311,7 @@ where
     fn wait_for<'me>(&'me self, zalsa: &'me Zalsa, key_index: Id) -> WaitForResult<'me> {
         match self.sync_table.try_claim(zalsa, key_index) {
             ClaimResult::Running(blocked_on) => WaitForResult::Running(blocked_on),
-            ClaimResult::Cycle { same_thread } => WaitForResult::Cycle { same_thread },
+            ClaimResult::Cycle => WaitForResult::Cycle,
             ClaimResult::Claimed(_) => WaitForResult::Available,
         }
     }

src/function/maybe_changed_after.rs

@@ -267,7 +267,7 @@ where
     ///   cycle heads have all been finalized.
     /// * provisional memos that have been created in the same revision and iteration and are part of the same cycle.
     #[inline]
-    pub(super) fn validate_may_be_provisional(
+    fn validate_may_be_provisional(
         &self,
         zalsa: &Zalsa,
         zalsa_local: &ZalsaLocal,
@@ -342,7 +342,7 @@ where
     /// If this is a provisional memo, validate that it was cached in the same iteration of the
     /// same cycle(s) that we are still executing. If so, it is valid for reuse. This avoids
     /// runaway re-execution of the same queries within a fixpoint iteration.
-    pub(super) fn validate_same_iteration(
+    fn validate_same_iteration(
         &self,
         zalsa: &Zalsa,
         zalsa_local: &ZalsaLocal,
@@ -369,15 +369,42 @@ where
                         .find(|query| query.database_key_index == cycle_head.database_key_index)
                         .map(|query| query.iteration_count())
                         .or_else(|| {
-                            // If this is a cycle head is owned by another thread that is blocked by this ingredient,
-                            // check if it has the same iteration count.
+                            // If the cycle head isn't on our stack because:
+                            //
+                            // * another thread holds the lock on the cycle head (but it waits for the current query to complete)
+                            // * we're in `maybe_changed_after` because `maybe_changed_after` doesn't modify the cycle stack
+                            //
+                            // check if the latest memo has the same iteration count.
+
+                            // However, we've to be careful to skip over fixpoint initial values:
+                            // If the head is the memo we're trying to validate, always return `None`
+                            // to force a re-execution of the query. This is necessary because the query
+                            // has obviously not completed its iteration yet.
+                            //
+                            // This should be rare but the `cycle_panic` test fails on some platforms (mainly GitHub actions)
+                            // without this check. What happens there is that:
+                            //
+                            // * query a blocks on query b
+                            // * query b tries to claim a, fails to do so and inserts the fixpoint initial value
+                            // * query b completes and has `a` as head. It returns its query result Salsa blocks query b from
+                            //   exiting inside `block_on` (or the thread would complete before the cycle iteration is complete)
+                            // * query a resumes but panics because of the fixpoint iteration function
+                            // * query b resumes. It rexecutes its own query which then tries to fetch a (which depends on itself because it's a fixpoint initial value).
+                            //   Without this check, `validate_same_iteration` would return `true` because the latest memo for `a` is the fixpoint initial value.
+                            //   But it should return `false` so that query b's thread re-executes `a` (which then also causes the panic).
+                            //
+                            // That's why we always return `None` if the cycle head is the same as the current database key index.
+                            if cycle_head.database_key_index == database_key_index {
+                                return None;
+                            }
+
                             let ingredient = zalsa.lookup_ingredient(
                                 cycle_head.database_key_index.ingredient_index(),
                             );
                             let wait_result = ingredient
                                 .wait_for(zalsa, cycle_head.database_key_index.key_index());
 
-                            if !wait_result.is_cycle_with_other_thread() {
+                            if !wait_result.is_cycle() {
                                 return None;
                             }
 

src/function/sync.rs

@@ -20,7 +20,7 @@ pub(crate) enum ClaimResult<'a> {
     /// Can't claim the query because it is running on an other thread.
     Running(Running<'a>),
     /// Claiming the query results in a cycle.
-    Cycle { same_thread: bool },
+    Cycle,
     /// Successfully claimed the query.
     Claimed(ClaimGuard<'a>),
 }
@@ -62,7 +62,7 @@ impl SyncTable {
                     write,
                 ) {
                     BlockResult::Running(blocked_on) => ClaimResult::Running(blocked_on),
-                    BlockResult::Cycle { same_thread } => ClaimResult::Cycle { same_thread },
+                    BlockResult::Cycle => ClaimResult::Cycle,
                 }
             }
             std::collections::hash_map::Entry::Vacant(vacant_entry) => {

src/ingredient.rs

@@ -250,12 +250,11 @@ pub(crate) fn fmt_index(debug_name: &str, id: Id, fmt: &mut fmt::Formatter<'_>)
 pub enum WaitForResult<'me> {
     Running(Running<'me>),
     Available,
-    Cycle { same_thread: bool },
+    Cycle,
 }
 
 impl WaitForResult<'_> {
-    /// Returns `true` if waiting for this input results in a cycle with another thread.
-    pub const fn is_cycle_with_other_thread(&self) -> bool {
-        matches!(self, WaitForResult::Cycle { same_thread: false })
+    pub const fn is_cycle(&self) -> bool {
+        matches!(self, WaitForResult::Cycle)
     }
 }

src/runtime.rs

@@ -51,7 +51,7 @@ pub(crate) enum BlockResult<'me> {
     ///
     /// The lock is hold by the current thread or there's another thread that is waiting on the current thread,
     /// and blocking this thread on the other thread would result in a deadlock/cycle.
-    Cycle { same_thread: bool },
+    Cycle,
 }
 
 pub struct Running<'me>(Box<BlockedOnInner<'me>>);
@@ -230,14 +230,14 @@ impl Runtime {
         let thread_id = thread::current().id();
         // Cycle in the same thread.
         if thread_id == other_id {
-            return BlockResult::Cycle { same_thread: true };
+            return BlockResult::Cycle;
         }
 
         let dg = self.dependency_graph.lock();
 
         if dg.depends_on(other_id, thread_id) {
             crate::tracing::debug!("block_on: cycle detected for {database_key:?} in thread {thread_id:?} on {other_id:?}");
-            return BlockResult::Cycle { same_thread: false };
+            return BlockResult::Cycle;
         }
 
         BlockResult::Running(Running(Box::new(BlockedOnInner {

tests/common/mod.rs

@@ -34,6 +34,10 @@ pub trait LogDatabase: HasLogger + Database {
         self.logger().logs.lock().unwrap().push(string);
     }
 
+    fn clear_logs(&self) {
+        std::mem::take(&mut *self.logger().logs.lock().unwrap());
+    }
+
     /// Asserts what the (formatted) logs should look like,
     /// clearing the logged events. This takes `&mut self` because
     /// it is meant to be run from outside any tracked functions.

tests/cycle.rs

@@ -1025,3 +1025,42 @@ fn repeat_provisional_query() {
             "salsa_event(WillExecute { database_key: min_panic(Id(2)) })",
         ]"#]]);
 }
+
+#[test]
+fn repeat_provisional_query_incremental() {
+    let mut db = ExecuteValidateLoggerDatabase::default();
+    let a_in = Inputs::new(&db, vec![]);
+    let b_in = Inputs::new(&db, vec![]);
+    let c_in = Inputs::new(&db, vec![]);
+    let a = Input::MinIterate(a_in);
+    let b = Input::MinPanic(b_in);
+    let c = Input::MinPanic(c_in);
+    a_in.set_inputs(&mut db).to(vec![value(59), b.clone()]);
+    b_in.set_inputs(&mut db)
+        .to(vec![value(60), c.clone(), c.clone(), c]);
+    c_in.set_inputs(&mut db).to(vec![a.clone()]);
+
+    a.assert_value(&db, 59);
+
+    db.clear_logs();
+
+    c_in.set_inputs(&mut db).to(vec![a.clone()]);
+
+    a.assert_value(&db, 59);
+
+    // `min_panic(Id(2)) should only twice:
+    // * Once before iterating
+    // * Once as part of iterating
+    //
+    // If it runs more than once before iterating, than this suggests that
+    // `validate_same_iteration` incorrectly returns `false`.
+    db.assert_logs(expect![[r#"
+        [
+            "salsa_event(WillExecute { database_key: min_panic(Id(2)) })",
+            "salsa_event(WillExecute { database_key: min_panic(Id(1)) })",
+            "salsa_event(WillExecute { database_key: min_iterate(Id(0)) })",
+            "salsa_event(WillIterateCycle { database_key: min_iterate(Id(0)), iteration_count: IterationCount(1), fell_back: false })",
+            "salsa_event(WillExecute { database_key: min_panic(Id(1)) })",
+            "salsa_event(WillExecute { database_key: min_panic(Id(2)) })",
+        ]"#]]);
+}