feat: impl Any for MemoryPool

datafusion/execution/src/memory_pool/mod.rs

@@ -19,6 +19,7 @@
 //! help with allocation accounting.
 
 use datafusion_common::{Result, internal_datafusion_err};
+use std::any::Any;
 use std::fmt::Display;
 use std::hash::{Hash, Hasher};
 use std::{cmp::Ordering, sync::Arc, sync::atomic};
@@ -182,7 +183,7 @@ pub use pool::*;
 ///
 /// * [`TrackConsumersPool`]: Wraps another [`MemoryPool`] and tracks consumers,
 ///   providing better error messages on the largest memory users.
-pub trait MemoryPool: Send + Sync + std::fmt::Debug + Display {
+pub trait MemoryPool: Any + Send + Sync + std::fmt::Debug + Display {
     /// Return pool name
     fn name(&self) -> &str;
 
@@ -224,6 +225,18 @@ pub trait MemoryPool: Send + Sync + std::fmt::Debug + Display {
     }
 }
 
+impl dyn MemoryPool {
+    /// Returns `true` if this pool is of type `T`.
+    pub fn is<T: MemoryPool>(&self) -> bool {
+        (self as &dyn Any).is::<T>()
+    }
+
+    /// Attempts to downcast this pool to a concrete type `T`.
+    pub fn downcast_ref<T: MemoryPool>(&self) -> Option<&T> {
+        (self as &dyn Any).downcast_ref()
+    }
+}
+
 /// Memory limit of `MemoryPool`
 pub enum MemoryLimit {
     Infinite,
@@ -603,6 +616,18 @@ mod tests {
         assert_eq!(pool.reserved(), 28);
     }
 
+    #[test]
+    fn test_downcast() {
+        let pool: Arc<dyn MemoryPool> = Arc::new(GreedyMemoryPool::new(50));
+
+        assert!(pool.is::<GreedyMemoryPool>());
+        assert!(!pool.is::<UnboundedMemoryPool>());
+
+        let greedy: &GreedyMemoryPool = pool.downcast_ref().unwrap();
+        assert_eq!(greedy.reserved(), 0);
+        assert!(pool.downcast_ref::<UnboundedMemoryPool>().is_none());
+    }
+
     #[test]
     fn test_try_shrink() {
         let pool = Arc::new(GreedyMemoryPool::new(100)) as _;

docs/source/library-user-guide/upgrading/54.0.0.md

@@ -497,3 +497,57 @@ impl Default for MyTreeNode {
     }
 }
 ```
+
+### `MemoryPool` now requires `'static` (adds `Any` as a supertrait)
+
+To enable downcasting of `dyn MemoryPool` to concrete pool types (via
+`is::<T>()` / `downcast_ref::<T>()`), the `MemoryPool` trait now has `Any`
+as a supertrait:
+
+```rust,ignore
+// Before
+pub trait MemoryPool: Send + Sync + std::fmt::Debug + Display { ... }
+
+// After
+pub trait MemoryPool: Any + Send + Sync + std::fmt::Debug + Display { ... }
+```
+
+Because `Any` is only implemented for `'static` types, this implicitly adds a
+`'static` bound to every `MemoryPool` implementor.
+
+**Who is affected:**
+
+- Users who implement a custom `MemoryPool` whose type carries a lifetime
+  parameter or borrows state (e.g. `struct MyPool<'a> { inner: &'a State }`).
+  Existing implementations that are already `'static` (the common case) need
+  no changes.
+
+**Migration guide:**
+
+Replace borrowed references with owned handles so the pool type becomes
+`'static`. The typical fix is to swap `&'a T` for `Arc<T>` (or `Rc<T>`, or an
+owned value):
+
+```rust,ignore
+// Before — not 'static, no longer compiles
+struct MyPool<'a> {
+    inner: &'a SomeState,
+}
+
+impl<'a> MemoryPool for MyPool<'a> { ... }
+
+// After — owned handle makes MyPool: 'static
+struct MyPool {
+    inner: Arc<SomeState>,
+}
+
+impl MemoryPool for MyPool { ... }
+```
+
+If the borrowed state truly cannot be made `'static`, you can wrap the
+borrowed pool in a `'static` adapter that the pool consumer owns — for
+example, store the underlying state in an `Arc` owned by the adapter, or
+move the borrow behind an interior-mutability primitive such as `Arc<Mutex<_>>`
+or `Arc<OnceLock<_>>`.
+
+See [PR #21803](https://github.com/apache/datafusion/pull/21803) for details.