enable self referential rtxn

heed/src/env.rs

@@ -748,6 +748,7 @@ impl Env {
     ///
     /// You can make this transaction `Send`able between threads by
     /// using the `read-txn-no-tls` crate feature.
+    /// See [`Self::static_read_txn`] if you want the txn to own the environment.
     ///
     /// ## LMDB Limitations
     ///
@@ -771,6 +772,35 @@ impl Env {
         RoTxn::new(self)
     }
 
+    /// Create a transaction with read-only access for use with the environment.
+    /// Contrary to [`Self::read_txn`], this version **owns** the environment, which
+    /// means you won't be able to close the environment while this transaction is alive.
+    ///
+    /// You can make this transaction `Send`able between threads by
+    /// using the `read-txn-no-tls` crate feature.
+    ///
+    /// ## LMDB Limitations
+    ///
+    /// It's possible to have multiple read transactions in the same environment
+    /// while there is a write transaction ongoing.
+    ///
+    /// But read transactions prevent reuse of pages freed by newer write transactions,
+    /// thus the database can grow quickly. Write transactions prevent other write transactions,
+    /// since writes are serialized.
+    ///
+    /// So avoid long-lived read transactions.
+    ///
+    /// ## Errors
+    ///
+    /// * [`crate::MdbError::Panic`]: A fatal error occurred earlier, and the environment must be shut down
+    /// * [`crate::MdbError::MapResized`]: Another process wrote data beyond this [`Env`] mapsize and this env
+    /// map must be resized
+    /// * [`crate::MdbError::ReadersFull`]: a read-only transaction was requested, and the reader lock table is
+    /// full
+    pub fn static_read_txn(self) -> Result<RoTxn<'static>> {
+        RoTxn::static_read_txn(self)
+    }
+
     /// Copy an LMDB environment to the specified path, with options.
     ///
     /// This function may be used to make a backup of an existing environment.

heed/src/txn.rs

@@ -1,3 +1,4 @@
+use std::borrow::Cow;
 use std::ops::Deref;
 use std::ptr;
 
@@ -26,7 +27,7 @@ use crate::{Env, Result};
 /// You may increase the limit by editing it **at your own risk**: `/Library/LaunchDaemons/sysctl.plist`
 pub struct RoTxn<'e> {
     pub(crate) txn: *mut ffi::MDB_txn,
-    env: &'e Env,
+    env: Cow<'e, Env>,
 }
 
 impl<'e> RoTxn<'e> {
@@ -42,7 +43,22 @@ impl<'e> RoTxn<'e> {
             ))?
         };
 
-        Ok(RoTxn { txn, env })
+        Ok(RoTxn { txn, env: Cow::Borrowed(env) })
+    }
+
+    pub(crate) fn static_read_txn(env: Env) -> Result<RoTxn<'static>> {
+        let mut txn: *mut ffi::MDB_txn = ptr::null_mut();
+
+        unsafe {
+            mdb_result(ffi::mdb_txn_begin(
+                env.env_mut_ptr(),
+                ptr::null_mut(),
+                ffi::MDB_RDONLY,
+                &mut txn,
+            ))?
+        };
+
+        Ok(RoTxn { txn, env: Cow::Owned(env) })
     }
 
     pub(crate) fn env_mut_ptr(&self) -> *mut ffi::MDB_env {
@@ -110,7 +126,7 @@ impl<'p> RwTxn<'p> {
 
         unsafe { mdb_result(ffi::mdb_txn_begin(env.env_mut_ptr(), ptr::null_mut(), 0, &mut txn))? };
 
-        Ok(RwTxn { txn: RoTxn { txn, env } })
+        Ok(RwTxn { txn: RoTxn { txn, env: Cow::Borrowed(env) } })
     }
 
     pub(crate) fn nested(env: &'p Env, parent: &'p mut RwTxn) -> Result<RwTxn<'p>> {
@@ -119,7 +135,7 @@ impl<'p> RwTxn<'p> {
 
         unsafe { mdb_result(ffi::mdb_txn_begin(env.env_mut_ptr(), parent_ptr, 0, &mut txn))? };
 
-        Ok(RwTxn { txn: RoTxn { txn, env } })
+        Ok(RwTxn { txn: RoTxn { txn, env: Cow::Borrowed(env) } })
     }
 
     pub(crate) fn env_mut_ptr(&self) -> *mut ffi::MDB_env {