perf(qwp): retain websocket size-hint scratch

Author: nwoolmer

questdb-rs/src/ingress/buffer/qwp.rs

@@ -2529,8 +2529,11 @@ impl QwpWsSizeHint {
     }
 
     fn len(&mut self, tables: &[QwpWsTableBuffer]) -> usize {
-        let dirty_tables = std::mem::take(&mut self.dirty_tables);
-        for table_idx in dirty_tables {
+        // Preserve the dirty-index allocation across size queries. Taking and
+        // dropping this vector allocated once per row after every buffer clear,
+        // even though the buffer itself is explicitly reusable.
+        let mut dirty_tables = std::mem::take(&mut self.dirty_tables);
+        for table_idx in dirty_tables.drain(..) {
             if table_idx >= tables.len() || table_idx >= self.tables.len() {
                 continue;
             }
@@ -2548,6 +2551,8 @@ impl QwpWsSizeHint {
                 self.recomputed_tables += 1;
             }
         }
+        debug_assert!(dirty_tables.is_empty());
+        self.dirty_tables = dirty_tables;
 
         QWP_MESSAGE_HEADER_SIZE
             + 2
@@ -2847,6 +2852,15 @@ impl QwpWsColumnarBuffer {
             .recomputed_tables
     }
 
+    #[cfg(test)]
+    fn size_hint_dirty_capacity(&self) -> usize {
+        self.size_hint
+            .lock()
+            .unwrap_or_else(|poisoned| poisoned.into_inner())
+            .dirty_tables
+            .capacity()
+    }
+
     pub(crate) fn row_count(&self) -> usize {
         self.state.row_count
     }
@@ -10111,6 +10125,33 @@ mod tests {
         assert_eq!(buf.size_hint_recomputed_tables(), recomputed + 1);
     }
 
+    #[cfg(feature = "_sender-qwp-ws")]
+    #[test]
+    fn qwp_ws_cached_size_hint_retains_dirty_index_capacity() {
+        let mut buf = QwpWsColumnarBuffer::new(127);
+        buf.table("trades")
+            .unwrap()
+            .column_i64("value", 1)
+            .unwrap()
+            .at_now()
+            .unwrap();
+        let _ = buf.len();
+        let capacity = buf.size_hint_dirty_capacity();
+        assert!(capacity >= 1);
+
+        for value in 2..=10 {
+            buf.clear();
+            buf.table("trades")
+                .unwrap()
+                .column_i64("value", value)
+                .unwrap()
+                .at_now()
+                .unwrap();
+            let _ = buf.len();
+            assert_eq!(buf.size_hint_dirty_capacity(), capacity);
+        }
+    }
+
     #[cfg(feature = "_sender-qwp-ws")]
     #[test]
     fn qwp_ws_columnar_clear_bookmark_drops_only_current_snapshot() {