feat: transparent cursor caching (ENG-2129)

CLAUDE.md

@@ -33,13 +33,28 @@ be mediated via the `TxAccess` trait.
 
 ## API Patterns
 
-### Cursor Creation
+### Cursor Creation and Caching
 
 ```rust
 let db = txn.open_db(None).unwrap();  // Returns Database (has dbi + flags)
 let cursor = txn.cursor(db).unwrap(); // Takes Database, NOT raw dbi
 ```
 
+Cursors are transparently cached within transactions. When a cursor is
+dropped, its raw pointer is returned to the transaction's cache. Subsequent
+`cursor()` calls reuse cached pointers, avoiding `mdbx_cursor_open`/
+`mdbx_cursor_close` overhead (~100 ns per cycle). The cache is drained
+and all pointers closed on commit or abort.
+
+`DbCache` (in `src/tx/cache.rs`) stores raw `*mut ffi::MDBX_cursor`
+pointers, which makes it `!Send + !Sync` by default. Explicit `unsafe impl
+Send + Sync for DbCache` is required because:
+- `SyncKind::Cache` requires `Cache + Send` (for `RefCell<DbCache>: Send`)
+- `SharedCache` uses `Arc<RwLock<DbCache>>` which requires `DbCache: Send + Sync`
+- This is sound because `Cursor` itself is already `unsafe impl Send + Sync`,
+  and all access to cached pointers is mediated by `RefCell` (unsync) or
+  `RwLock` (sync)
+
 ### Database Flags Validation
 
 DUP_SORT/DUP_FIXED methods validate flags at runtime:

src/tx/access.rs

@@ -1,13 +1,20 @@
 use crate::{
     Environment,
     sys::txn_manager::{Abort, RawTxPtr},
+    tx::{
+        cache::{Cache, DbCache},
+        ops,
+    },
 };
 use core::fmt;
 use parking_lot::{Mutex, MutexGuard};
-use std::sync::{
-    Arc,
-    atomic::{AtomicBool, Ordering},
-    mpsc::sync_channel,
+use std::{
+    cell::RefCell,
+    sync::{
+        Arc,
+        atomic::{AtomicBool, Ordering},
+        mpsc::sync_channel,
+    },
 };
 use tracing::debug_span;
 
@@ -29,8 +36,16 @@ mod sealed {
 /// are stored for read-only and read-write transactions. It ensures that
 /// the transaction pointer can be accessed safely, respecting timeouts
 /// and ownership semantics.
+///
+/// The associated [`Cache`] type co-locates the cursor cache with the
+/// transaction so that the cache lifetime is bound to the transaction
+/// pointer's: cached cursors are drained and closed exactly once, in the
+/// implementing type's `Drop`, before the transaction is aborted.
 #[allow(unreachable_pub)]
 pub trait TxPtrAccess: fmt::Debug + sealed::Sealed {
+    /// Cache type co-located with this transaction pointer.
+    type Cache: Cache;
+
     /// Create an instance of the implementing type from a raw transaction
     /// pointer.
     fn from_ptr_and_env(ptr: *mut ffi::MDBX_txn, env: Environment, is_read_only: bool) -> Self
@@ -45,6 +60,10 @@ pub trait TxPtrAccess: fmt::Debug + sealed::Sealed {
     /// Mark the transaction as committed.
     fn mark_committed(&self);
 
+    /// Returns a reference to the cursor/database cache associated with
+    /// this transaction.
+    fn cache(&self) -> &Self::Cache;
+
     /// Get the transaction ID by making a call into the MDBX C API.
     fn tx_id(&self) -> Option<usize> {
         let mut id = 0;
@@ -60,6 +79,8 @@ impl<T> TxPtrAccess for Arc<T>
 where
     T: TxPtrAccess,
 {
+    type Cache = T::Cache;
+
     fn from_ptr_and_env(ptr: *mut ffi::MDBX_txn, env: Environment, is_read_only: bool) -> Self
     where
         Self: Sized,
@@ -77,12 +98,17 @@ where
     fn mark_committed(&self) {
         self.as_ref().mark_committed();
     }
+
+    fn cache(&self) -> &Self::Cache {
+        self.as_ref().cache()
+    }
 }
 
-/// Wrapper for raw txn pointer for RW transactions.
+/// Wrapper for raw txn pointer for unsynchronized transactions.
 pub struct PtrUnsync {
     committed: AtomicBool,
     ptr: *mut ffi::MDBX_txn,
+    cache: RefCell<DbCache>,
 }
 
 impl fmt::Debug for PtrUnsync {
@@ -92,11 +118,13 @@ impl fmt::Debug for PtrUnsync {
 }
 
 impl TxPtrAccess for PtrUnsync {
+    type Cache = RefCell<DbCache>;
+
     fn from_ptr_and_env(ptr: *mut ffi::MDBX_txn, _env: Environment, _is_read_only: bool) -> Self
     where
         Self: Sized,
     {
-        Self { committed: AtomicBool::new(false), ptr }
+        Self { committed: AtomicBool::new(false), ptr, cache: RefCell::new(DbCache::default()) }
     }
 
     fn with_txn_ptr<F, R>(&self, f: F) -> R
@@ -111,13 +139,24 @@ impl TxPtrAccess for PtrUnsync {
         // Type is neither Sync nor Send, so no concurrent access is possible.
         unsafe { *self.committed.as_ptr() = true };
     }
+
+    fn cache(&self) -> &Self::Cache {
+        &self.cache
+    }
 }
 
 impl Drop for PtrUnsync {
     fn drop(&mut self) {
         // SAFETY:
         // We have exclusive ownership of this pointer.
         unsafe {
+            // Close any cached cursors before the txn ends. No-op if
+            // commit/abort already drained.
+            for cursor in self.cache.get_mut().drain_cursors() {
+                // SAFETY: cursor was returned from an earlier Cursor::drop
+                // bound to this still-live txn.
+                ops::cursor_close_raw(cursor);
+            }
             if !*self.committed.as_ptr() {
                 ffi::mdbx_txn_abort(self.ptr);
             }
@@ -139,9 +178,10 @@ pub struct PtrSync {
     /// Whether the transaction was committed.
     committed: AtomicBool,
 
-    /// Contains a lock to ensure exclusive access to the transaction.
-    /// The inner boolean indicates the timeout status.
-    lock: Mutex<()>,
+    /// Lock that serialises access to the transaction pointer **and**
+    /// guards the cursor/database cache. Held inside `with_txn_ptr`; also
+    /// taken implicitly via the `Cache` impl on `Mutex<DbCache>`.
+    lock: Mutex<DbCache>,
 
     /// The environment that owns the transaction.
     env: Environment,
@@ -159,7 +199,7 @@ unsafe impl Sync for PtrSync {}
 impl PtrSync {
     /// Acquires the inner transaction lock to guarantee exclusive access to the transaction
     /// pointer.
-    pub(crate) fn lock(&self) -> MutexGuard<'_, ()> {
+    pub(crate) fn lock(&self) -> MutexGuard<'_, DbCache> {
         if let Some(lock) = self.lock.try_lock() {
             lock
         } else {
@@ -176,13 +216,15 @@ impl PtrSync {
 }
 
 impl TxPtrAccess for PtrSync {
+    type Cache = Mutex<DbCache>;
+
     fn from_ptr_and_env(ptr: *mut ffi::MDBX_txn, env: Environment, is_read_only: bool) -> Self
     where
         Self: Sized,
     {
         Self {
             committed: AtomicBool::new(false),
-            lock: Mutex::new(()),
+            lock: Mutex::new(DbCache::default()),
             txn: ptr,
             env,
             is_read_only,
@@ -200,10 +242,24 @@ impl TxPtrAccess for PtrSync {
     fn mark_committed(&self) {
         self.committed.store(true, Ordering::SeqCst);
     }
+
+    fn cache(&self) -> &Self::Cache {
+        &self.lock
+    }
 }
 
 impl Drop for PtrSync {
     fn drop(&mut self) {
+        // Close any cached cursors before the transaction ends. Runs
+        // exactly once: this `Drop` only fires when the last `Arc<PtrSync>`
+        // is released, so there is no race with surviving `TxSync` clones.
+        // No-op if `commit_inner` already drained the cache.
+        for cursor in self.lock.get_mut().drain_cursors() {
+            // SAFETY: cursor was returned from an earlier Cursor::drop
+            // bound to this still-live txn.
+            unsafe { ops::cursor_close_raw(cursor) };
+        }
+
         if self.committed.load(Ordering::SeqCst) {
             return;
         }