feat: disallow online scaling for shared source

src/meta/src/stream/scale.rs

@@ -441,6 +441,8 @@ pub struct ScaleController {
 
     pub env: MetaSrvEnv,
 
+    /// We will acquire lock during DDL to prevent scaling operations on jobs that are in the creating state.
+    /// e.g., a MV cannot be rescheduled during foreground backfill.
     pub reschedule_lock: RwLock<()>,
 }
 

src/stream/src/executor/source/source_backfill_executor.rs

@@ -705,6 +705,7 @@ impl<S: StateStore> SourceBackfillExecutorInner<S> {
                     yield Message::Barrier(barrier);
                 }
                 Message::Chunk(chunk) => {
+                    // FIXME: consider SourceCatchingUp here?
                     yield Message::Chunk(chunk);
                 }
                 Message::Watermark(watermark) => {
@@ -716,19 +717,15 @@ impl<S: StateStore> SourceBackfillExecutorInner<S> {
 
     /// All splits finished backfilling.
     ///
-    /// We check all splits for the source, including other actors' splits here, before going to the forward stage.
-    /// Otherwise if we break early, but after rescheduling, an unfinished split is migrated to
-    /// this actor, we still need to backfill it.
+    /// Note: we don't need to consider split migration (online scaling) here, so we can just check the splits assigned to this actor.
+    /// - For foreground DDL, scaling is not allowed during backfilling.
+    /// - For background DDL, scaling is skipped when backfilling is not finished, and can be triggered by recreating actors during recovery.
+    ///
+    /// See <https://github.com/risingwavelabs/risingwave/issues/18300> for more details.
     async fn backfill_finished(&self, states: &BackfillStates) -> StreamExecutorResult<bool> {
         Ok(states
             .values()
-            .all(|state| matches!(state, BackfillState::Finished))
-            && self
-                .backfill_state_store
-                .scan()
-                .await?
-                .into_iter()
-                .all(|state| matches!(state, BackfillState::Finished)))
+            .all(|state| matches!(state, BackfillState::Finished)))
     }
 
     /// For newly added splits, we do not need to backfill and can directly forward from upstream.
@@ -787,7 +784,10 @@ impl<S: StateStore> SourceBackfillExecutorInner<S> {
                     }
                     Some(backfill_state) => {
                         // Migrated split. Backfill if unfinished.
-                        // TODO: disallow online scaling during backfilling.
+                        debug_assert!(
+                            false,
+                            "split migration during backfill stage should not happen"
+                        );
                         target_state.insert(split_id, backfill_state);
                     }
                 }
@@ -878,6 +878,7 @@ impl<S: StateStore> SourceBackfillExecutorInner<S> {
                     }
                     Some(backfill_state) => {
                         // Migrated split. It should also be finished since we are in forwarding stage.
+                        // FIXME: it's still possible to have SourceCatchingUp here if we want to consider it as finished..?
                         match backfill_state {
                             BackfillState::Finished => {}
                             _ => {

src/stream/src/executor/source/source_backfill_state_table.rs

@@ -55,30 +55,6 @@ impl<S: StateStore> BackfillStateTableHandler<S> {
             .map_err(StreamExecutorError::from)
     }
 
-    /// XXX: we might get stale data for other actors' writes, but it's fine?
-    pub async fn scan(&self) -> StreamExecutorResult<Vec<BackfillState>> {
-        let sub_range: &(Bound<OwnedRow>, Bound<OwnedRow>) = &(Bound::Unbounded, Bound::Unbounded);
-
-        let state_table_iter = self
-            .state_store
-            .iter_with_prefix(None::<OwnedRow>, sub_range, Default::default())
-            .await?;
-        pin_mut!(state_table_iter);
-
-        let mut ret = vec![];
-        while let Some(item) = state_table_iter.next().await {
-            let row = item?.into_owned_row();
-            let state = match row.datum_at(1) {
-                Some(ScalarRefImpl::Jsonb(jsonb_ref)) => {
-                    BackfillState::restore_from_json(jsonb_ref.to_owned_scalar())?
-                }
-                _ => unreachable!(),
-            };
-            ret.push(state);
-        }
-        Ok(ret)
-    }
-
     async fn set(&mut self, key: SplitId, state: BackfillState) -> StreamExecutorResult<()> {
         let row = [
             Some(Self::string_to_scalar(key.as_ref())),