fix: rolling nested groups deadlock (pola-rs#13835)

crates/polars-lazy/src/physical_plan/expressions/rolling.rs

@@ -20,35 +20,26 @@ impl PhysicalExpr for RollingExpr {
     fn evaluate(&self, df: &DataFrame, state: &ExecutionState) -> PolarsResult<Series> {
         let groups_key = format!("{:?}", &self.options);
 
-        let mut groups_map = state.group_tuples.read().unwrap();
+        let groups_map = state.group_tuples.read().unwrap();
         // Groups must be set by expression runner.
         let groups = groups_map.get(&groups_key);
 
-        let mut groups_map_write;
-
         // There can be multiple rolling expressions in a single expr.
         // E.g. `min().rolling() + max().rolling()`
         // So if we hit that we will compute them here.
         let groups = match groups {
-            Some(groups) => groups,
+            Some(groups) => Cow::Borrowed(groups),
             None => {
-                drop(groups_map);
+                // We cannot cache those as mutexes under rayon can deadlock.
+                // TODO! precompute all groups up front.
                 let (_time_key, _keys, groups) = df.group_by_rolling(vec![], &self.options)?;
-                groups_map_write = state.group_tuples.write().unwrap();
-                groups_map_write.entry_ref(&groups_key).or_insert(groups);
-
-                drop(groups_map_write);
-
-                // Get a reference to the read guard so that other threads
-                // can continue
-                groups_map = state.group_tuples.read().unwrap();
-                groups_map.get(&groups_key).expect("impl error")
+                Cow::Owned(groups)
             },
         };
 
         let mut out = self
             .phys_function
-            .evaluate_on_groups(df, groups, state)?
+            .evaluate_on_groups(df, &groups, state)?
             .finalize();
         polars_ensure!(out.len() == groups.len(), agg_len = out.len(), groups.len());
         if let Some(name) = &self.out_name {