From 0b592098d899cfc52b6a7fca04fb5892d6a7e564 Mon Sep 17 00:00:00 2001
From: Carl Meyer <carl@oddbird.net>
Date: Thu, 31 Oct 2024 15:56:07 -0700
Subject: [PATCH] better fix for multi-revision bug

---
 src/function/execute.rs             | 18 +++++++++---------
 src/function/fetch.rs               |  5 ++++-
 src/function/maybe_changed_after.rs |  8 ++------
 src/zalsa_local.rs                  |  4 ++--
 4 files changed, 17 insertions(+), 18 deletions(-)

diff --git a/src/function/execute.rs b/src/function/execute.rs
index 89d1a93a4..158f94a02 100644
--- a/src/function/execute.rs
+++ b/src/function/execute.rs
@@ -57,15 +57,6 @@ where
             let mut new_value = C::execute(db, C::id_to_input(db, id));
             let mut revisions = active_query.pop();
 
-            // If the new value is equal to the old one, then it didn't
-            // really change, even if some of its inputs have. So we can
-            // "backdate" its `changed_at` revision to be the same as the
-            // old value.
-            if let Some(old_memo) = &opt_old_memo {
-                self.backdate_if_appropriate(old_memo, &mut revisions, &new_value);
-                self.diff_outputs(db, database_key_index, old_memo, &mut revisions);
-            }
-
             // Did the new result we got depend on our own provisional value, in a cycle?
             if revisions.cycle_heads.contains(&database_key_index) {
                 let opt_owned_last_provisional;
@@ -145,6 +136,15 @@ where
 
             tracing::debug!("{database_key_index:?}: execute: result.revisions = {revisions:#?}");
 
+            // If the new value is equal to the old one, then it didn't
+            // really change, even if some of its inputs have. So we can
+            // "backdate" its `changed_at` revision to be the same as the
+            // old value.
+            if let Some(old_memo) = &opt_old_memo {
+                self.backdate_if_appropriate(old_memo, &mut revisions, &new_value);
+                self.diff_outputs(db, database_key_index, old_memo, &mut revisions);
+            }
+
             return self.insert_memo(
                 zalsa,
                 id,
diff --git a/src/function/fetch.rs b/src/function/fetch.rs
index ae4c6c376..6dcc4e3ac 100644
--- a/src/function/fetch.rs
+++ b/src/function/fetch.rs
@@ -90,7 +90,10 @@ where
                             Memo::new(
                                 Some(initial_value),
                                 zalsa.current_revision(),
-                                QueryRevisions::fixpoint_initial(database_key_index),
+                                QueryRevisions::fixpoint_initial(
+                                    database_key_index,
+                                    zalsa.current_revision(),
+                                ),
                             ),
                         )
                     })
diff --git a/src/function/maybe_changed_after.rs b/src/function/maybe_changed_after.rs
index ad55bcf26..e0b33eb8c 100644
--- a/src/function/maybe_changed_after.rs
+++ b/src/function/maybe_changed_after.rs
@@ -96,12 +96,8 @@ where
         // the value has not logically changed.
         if old_memo.value.is_some() {
             let memo = self.execute(db, database_key_index, Some(old_memo));
-            // If we get back a memo that's part of a cycle and requires further iteration to
-            // resolve, we can't backdate that.
-            if !memo.revisions.cycle_ignore {
-                let changed_at = memo.revisions.changed_at;
-                return Some(changed_at > revision);
-            }
+            let changed_at = memo.revisions.changed_at;
+            return Some(changed_at > revision);
         }
 
         // Otherwise, nothing for it: have to consider the value to have changed.
diff --git a/src/zalsa_local.rs b/src/zalsa_local.rs
index 1ba09b0c2..4ef67f64d 100644
--- a/src/zalsa_local.rs
+++ b/src/zalsa_local.rs
@@ -371,10 +371,10 @@ pub(crate) struct QueryRevisions {
 }
 
 impl QueryRevisions {
-    pub(crate) fn fixpoint_initial(query: DatabaseKeyIndex) -> Self {
+    pub(crate) fn fixpoint_initial(query: DatabaseKeyIndex, revision: Revision) -> Self {
         let cycle_heads = FxHashSet::from_iter([query]);
         Self {
-            changed_at: Revision::start(),
+            changed_at: revision,
             durability: Durability::MAX,
             origin: QueryOrigin::FixpointInitial,
             tracked_struct_ids: Default::default(),