zhuqi-lucas commented on code in PR #22460:
URL: https://github.com/apache/datafusion/pull/22460#discussion_r3296061149
##########
datafusion/physical-expr/src/expressions/dynamic_filters/mod.rs:
##########
@@ -326,6 +329,31 @@ impl DynamicFilterPhysicalExpr {
.await;
}
+ /// Returns `true` if this filter has been marked complete via
+ /// [`Self::mark_complete`] and will therefore never change again.
+ pub(crate) fn is_complete(&self) -> bool {
+ self.inner.read().is_complete
+ }
+
+ /// Subscribe to this filter's updates for cheap, synchronous change
+ /// detection.
+ ///
+ /// The returned [`DynamicFilterSubscription`] lets a consumer poll whether
+ /// the filter's expression has advanced since it last looked, without
+ /// re-walking a predicate tree or re-deriving a generation on every check.
+ /// This is the building block used by [`DynamicFilterTracker`] to watch
+ /// every dynamic filter inside a (possibly composite) predicate.
+ pub(crate) fn subscribe(&self) -> DynamicFilterSubscription {
+ let mut receiver = self.state_watch.subscribe();
+ // Mark the current state as already-seen so the first `observe()` only
+ // reports updates that happen *after* subscription.
+ let last_generation = receiver.borrow_and_update().generation();
Review Comment:
Minor:
Missing test for the load-bearing `borrow_and_update()` in `subscribe()`:
tracker constructed *after* an `update()` should report `changed() == false` on
first call. Worth pinning as a regression test.
##########
datafusion/physical-expr/src/expressions/dynamic_filters/mod.rs:
##########
@@ -522,6 +550,66 @@ impl PhysicalExpr for DynamicFilterPhysicalExpr {
}
}
+/// The result of polling a [`DynamicFilterSubscription`].
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub(crate) struct DynamicFilterChange {
+ /// The filter's expression advanced since the previous observation.
+ pub(crate) changed: bool,
+ /// The filter has been marked complete; it will never change again and the
+ /// subscription can be dropped.
+ pub(crate) complete: bool,
+}
+
+/// A cheap, synchronous handle for observing updates to a single
+/// [`DynamicFilterPhysicalExpr`].
+///
+/// Obtained via [`DynamicFilterPhysicalExpr::subscribe`]. Steady-state polling
+/// via [`Self::observe`] is a single atomic load (the underlying
+/// [`tokio::sync::watch`] version counter); the lock is only taken when the
+/// filter has actually been updated.
+#[derive(Debug)]
+pub(crate) struct DynamicFilterSubscription {
+ receiver: watch::Receiver<FilterState>,
+ /// Last generation we reported as "seen". Used to distinguish a real
+ /// expression update from a bare
[`DynamicFilterPhysicalExpr::mark_complete`]
+ /// (which re-broadcasts the current generation without changing the
+ /// expression).
+ last_generation: u64,
+}
+
+impl DynamicFilterSubscription {
+ /// Observe the latest state of the filter.
+ ///
+ /// Reports whether the filter's expression advanced since the previous
call
+ /// and whether it has since been marked complete. Cheap when nothing has
+ /// changed: a single atomic comparison with no lock acquisition.
+ pub(crate) fn observe(&mut self) -> DynamicFilterChange {
+ match self.receiver.has_changed() {
+ Ok(true) => {
+ let state = *self.receiver.borrow_and_update();
+ let changed = state.generation() > self.last_generation;
+ if changed {
+ self.last_generation = state.generation();
+ }
+ DynamicFilterChange {
+ changed,
+ complete: matches!(state, FilterState::Complete { .. }),
+ }
+ }
+ Ok(false) => DynamicFilterChange {
+ changed: false,
+ complete: false,
+ },
+ // The sender was dropped: no further updates are possible, so
treat
+ // the subscription as complete.
+ Err(_) => DynamicFilterChange {
Review Comment:
`observe()` here treats receiver `Err(_)` (sender dropped) as `complete:
true`. The sender is kept alive by the predicate `Arc` so this branch shouldn't
be reachable in practice — consider a `debug_assert!` / `log::warn!` so a
future bug doesn't get masked.
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