From c98d91333c34b014c822e22bad20430a43d6593a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jochen=20M=C3=BCller?= <mail@jnmr.de>
Date: Sat, 27 Jul 2024 05:08:52 +0200
Subject: [PATCH 1/7] Restrict Env type to prepare using it in place of
 ConcreteEnv

---
 storage/database/src/env.rs | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/storage/database/src/env.rs b/storage/database/src/env.rs
index cae497330..e93e1be21 100644
--- a/storage/database/src/env.rs
+++ b/storage/database/src/env.rs
@@ -61,7 +61,7 @@ pub trait Env: Sized {
     // For `heed`, this is just `heed::Env`, for `redb` this is
     // `(redb::Database, redb::Durability)` as each transaction
     // needs the sync mode set during creation.
-    type EnvInner<'env>: EnvInner<'env>
+    type EnvInner<'env>: EnvInner<'env> + Sync
     where
         Self: 'env;
 
@@ -209,7 +209,7 @@ pub trait EnvInner<'env> {
     /// The read-only transaction type of the backend.
     ///
     /// `'tx` is the lifetime of the transaction itself.
-    type Ro<'tx>: TxRo<'tx>;
+    type Ro<'tx>: TxRo<'tx> + Send;
     /// The read-write transaction type of the backend.
     ///
     /// `'tx` is the lifetime of the transaction itself.

From 990e9f2353856777efe5caaa1051cc9433d7c4ee Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jochen=20M=C3=BCller?= <mail@jnmr.de>
Date: Sat, 27 Jul 2024 05:09:44 +0200
Subject: [PATCH 2/7] Make functions in service/read.rs generic

---
 storage/blockchain/src/service/read.rs | 30 +++++++++++++-------------
 1 file changed, 15 insertions(+), 15 deletions(-)

diff --git a/storage/blockchain/src/service/read.rs b/storage/blockchain/src/service/read.rs
index 3f0b26338..f73b64fba 100644
--- a/storage/blockchain/src/service/read.rs
+++ b/storage/blockchain/src/service/read.rs
@@ -177,7 +177,7 @@ impl tower::Service<BCReadRequest> for DatabaseReadHandle {
         let env = Arc::clone(&self.env);
         self.pool.spawn(move || {
             let _permit: OwnedSemaphorePermit = permit;
-            map_request(&env, request, response_sender);
+            map_request(&*env, request, response_sender);
         }); // drop(permit/env);
 
         InfallibleOneshotReceiver::from(receiver)
@@ -195,8 +195,8 @@ impl tower::Service<BCReadRequest> for DatabaseReadHandle {
 /// 1. `Request` is mapped to a handler function
 /// 2. Handler function is called
 /// 3. [`BCResponse`] is sent
-fn map_request(
-    env: &ConcreteEnv,               // Access to the database
+fn map_request<E: Env>(
+    env: &E,                         // Access to the database
     request: BCReadRequest,          // The request we must fulfill
     response_sender: ResponseSender, // The channel we must send the response back to
 ) {
@@ -299,7 +299,7 @@ macro_rules! get_tables {
 
 /// [`BCReadRequest::BlockExtendedHeader`].
 #[inline]
-fn block_extended_header(env: &ConcreteEnv, block_height: BlockHeight) -> ResponseResult {
+fn block_extended_header<E: Env>(env: &E, block_height: BlockHeight) -> ResponseResult {
     // Single-threaded, no `ThreadLocal` required.
     let env_inner = env.env_inner();
     let tx_ro = env_inner.tx_ro()?;
@@ -312,7 +312,7 @@ fn block_extended_header(env: &ConcreteEnv, block_height: BlockHeight) -> Respon
 
 /// [`BCReadRequest::BlockHash`].
 #[inline]
-fn block_hash(env: &ConcreteEnv, block_height: BlockHeight) -> ResponseResult {
+fn block_hash<E: Env>(env: &E, block_height: BlockHeight) -> ResponseResult {
     // Single-threaded, no `ThreadLocal` required.
     let env_inner = env.env_inner();
     let tx_ro = env_inner.tx_ro()?;
@@ -325,7 +325,7 @@ fn block_hash(env: &ConcreteEnv, block_height: BlockHeight) -> ResponseResult {
 
 /// [`BCReadRequest::FilterUnknownHashes`].
 #[inline]
-fn filter_unknown_hashes(env: &ConcreteEnv, mut hashes: HashSet<BlockHash>) -> ResponseResult {
+fn filter_unknown_hashes<E: Env>(env: &E, mut hashes: HashSet<BlockHash>) -> ResponseResult {
     // Single-threaded, no `ThreadLocal` required.
     let env_inner = env.env_inner();
     let tx_ro = env_inner.tx_ro()?;
@@ -353,8 +353,8 @@ fn filter_unknown_hashes(env: &ConcreteEnv, mut hashes: HashSet<BlockHash>) -> R
 
 /// [`BCReadRequest::BlockExtendedHeaderInRange`].
 #[inline]
-fn block_extended_header_in_range(
-    env: &ConcreteEnv,
+fn block_extended_header_in_range<E: Env>(
+    env: &E,
     range: std::ops::Range<BlockHeight>,
 ) -> ResponseResult {
     // Prepare tx/tables in `ThreadLocal`.
@@ -377,7 +377,7 @@ fn block_extended_header_in_range(
 
 /// [`BCReadRequest::ChainHeight`].
 #[inline]
-fn chain_height(env: &ConcreteEnv) -> ResponseResult {
+fn chain_height<E: Env>(env: &E) -> ResponseResult {
     // Single-threaded, no `ThreadLocal` required.
     let env_inner = env.env_inner();
     let tx_ro = env_inner.tx_ro()?;
@@ -393,7 +393,7 @@ fn chain_height(env: &ConcreteEnv) -> ResponseResult {
 
 /// [`BCReadRequest::GeneratedCoins`].
 #[inline]
-fn generated_coins(env: &ConcreteEnv) -> ResponseResult {
+fn generated_coins<E: Env>(env: &E) -> ResponseResult {
     // Single-threaded, no `ThreadLocal` required.
     let env_inner = env.env_inner();
     let tx_ro = env_inner.tx_ro()?;
@@ -410,7 +410,7 @@ fn generated_coins(env: &ConcreteEnv) -> ResponseResult {
 
 /// [`BCReadRequest::Outputs`].
 #[inline]
-fn outputs(env: &ConcreteEnv, outputs: HashMap<Amount, HashSet<AmountIndex>>) -> ResponseResult {
+fn outputs<E: Env>(env: &E, outputs: HashMap<Amount, HashSet<AmountIndex>>) -> ResponseResult {
     // Prepare tx/tables in `ThreadLocal`.
     let env_inner = env.env_inner();
     let tx_ro = thread_local(env);
@@ -451,7 +451,7 @@ fn outputs(env: &ConcreteEnv, outputs: HashMap<Amount, HashSet<AmountIndex>>) ->
 
 /// [`BCReadRequest::NumberOutputsWithAmount`].
 #[inline]
-fn number_outputs_with_amount(env: &ConcreteEnv, amounts: Vec<Amount>) -> ResponseResult {
+fn number_outputs_with_amount<E: Env>(env: &E, amounts: Vec<Amount>) -> ResponseResult {
     // Prepare tx/tables in `ThreadLocal`.
     let env_inner = env.env_inner();
     let tx_ro = thread_local(env);
@@ -496,7 +496,7 @@ fn number_outputs_with_amount(env: &ConcreteEnv, amounts: Vec<Amount>) -> Respon
 
 /// [`BCReadRequest::KeyImagesSpent`].
 #[inline]
-fn key_images_spent(env: &ConcreteEnv, key_images: HashSet<KeyImage>) -> ResponseResult {
+fn key_images_spent<E: Env>(env: &E, key_images: HashSet<KeyImage>) -> ResponseResult {
     // Prepare tx/tables in `ThreadLocal`.
     let env_inner = env.env_inner();
     let tx_ro = thread_local(env);
@@ -532,7 +532,7 @@ fn key_images_spent(env: &ConcreteEnv, key_images: HashSet<KeyImage>) -> Respons
 }
 
 /// [`BCReadRequest::CompactChainHistory`]
-fn compact_chain_history(env: &ConcreteEnv) -> ResponseResult {
+fn compact_chain_history<E: Env>(env: &E) -> ResponseResult {
     let env_inner = env.env_inner();
     let tx_ro = env_inner.tx_ro()?;
 
@@ -573,7 +573,7 @@ fn compact_chain_history(env: &ConcreteEnv) -> ResponseResult {
 /// `block_ids` must be sorted in chronological block order, or else
 /// the returned result is unspecified and meaningless, as this function
 /// performs a binary search.
-fn find_first_unknown(env: &ConcreteEnv, block_ids: &[BlockHash]) -> ResponseResult {
+fn find_first_unknown<E: Env>(env: &E, block_ids: &[BlockHash]) -> ResponseResult {
     let env_inner = env.env_inner();
     let tx_ro = env_inner.tx_ro()?;
 

From 732e8e1d1d1de32c518b66a382e14c99b38423cc Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jochen=20M=C3=BCller?= <mail@jnmr.de>
Date: Tue, 30 Jul 2024 02:10:34 +0200
Subject: [PATCH 3/7] Implement hot-swapping

---
 storage/blockchain/src/config/backend.rs | 22 +++---
 storage/blockchain/src/config/config.rs  |  6 +-
 storage/blockchain/src/config/mod.rs     |  3 +
 storage/blockchain/src/free.rs           |  6 +-
 storage/blockchain/src/service/free.rs   | 20 ++++-
 storage/blockchain/src/service/read.rs   | 97 ++++++++++++++----------
 storage/blockchain/src/service/write.rs  | 24 +++---
 storage/database/src/backend/mod.rs      | 17 ++++-
 storage/database/src/lib.rs              |  7 ++
 9 files changed, 131 insertions(+), 71 deletions(-)

diff --git a/storage/blockchain/src/config/backend.rs b/storage/blockchain/src/config/backend.rs
index ee72b3dfa..566a50f60 100644
--- a/storage/blockchain/src/config/backend.rs
+++ b/storage/blockchain/src/config/backend.rs
@@ -1,22 +1,22 @@
 //! SOMEDAY
 
 //---------------------------------------------------------------------------------------------------- Import
-use std::{
-    borrow::Cow,
-    num::NonZeroUsize,
-    path::{Path, PathBuf},
-};
+//use std::{
+//    borrow::Cow,
+//    num::NonZeroUsize,
+//    path::{Path, PathBuf},
+//};
 
 #[cfg(feature = "serde")]
 use serde::{Deserialize, Serialize};
 
-use cuprate_helper::fs::cuprate_blockchain_dir;
+//use cuprate_helper::fs::cuprate_blockchain_dir;
 
-use crate::{
-    config::{ReaderThreads, SyncMode},
-    constants::DATABASE_DATA_FILENAME,
-    resize::ResizeAlgorithm,
-};
+//use crate::{
+//    config::{ReaderThreads, SyncMode},
+//    constants::DATABASE_DATA_FILENAME,
+//    resize::ResizeAlgorithm,
+//};
 
 //---------------------------------------------------------------------------------------------------- Backend
 /// SOMEDAY: allow runtime hot-swappable backends.
diff --git a/storage/blockchain/src/config/config.rs b/storage/blockchain/src/config/config.rs
index c58e292a0..fbf5cb11a 100644
--- a/storage/blockchain/src/config/config.rs
+++ b/storage/blockchain/src/config/config.rs
@@ -9,7 +9,7 @@ use serde::{Deserialize, Serialize};
 use cuprate_database::{config::SyncMode, resize::ResizeAlgorithm};
 use cuprate_helper::fs::cuprate_blockchain_dir;
 
-use crate::config::ReaderThreads;
+use crate::config::{Backend, ReaderThreads};
 
 //---------------------------------------------------------------------------------------------------- ConfigBuilder
 /// Builder for [`Config`].
@@ -65,6 +65,7 @@ impl ConfigBuilder {
             .build();
 
         Config {
+            backend: Backend::default(),
             db_config,
             reader_threads,
         }
@@ -149,6 +150,9 @@ impl Default for ConfigBuilder {
 #[derive(Debug, Clone, PartialEq, PartialOrd)]
 #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
 pub struct Config {
+    /// The database backend.
+    pub backend: Backend,
+
     /// The database configuration.
     pub db_config: cuprate_database::config::Config,
 
diff --git a/storage/blockchain/src/config/mod.rs b/storage/blockchain/src/config/mod.rs
index 7ecc14c4c..2a81552de 100644
--- a/storage/blockchain/src/config/mod.rs
+++ b/storage/blockchain/src/config/mod.rs
@@ -45,3 +45,6 @@ pub use config::{Config, ConfigBuilder};
 
 mod reader_threads;
 pub use reader_threads::ReaderThreads;
+
+mod backend;
+pub use backend::Backend;
diff --git a/storage/blockchain/src/free.rs b/storage/blockchain/src/free.rs
index 8288e65f7..edd81ef56 100644
--- a/storage/blockchain/src/free.rs
+++ b/storage/blockchain/src/free.rs
@@ -1,7 +1,7 @@
 //! General free functions (related to the database).
 
 //---------------------------------------------------------------------------------------------------- Import
-use cuprate_database::{ConcreteEnv, Env, EnvInner, InitError, RuntimeError, TxRw};
+use cuprate_database::{Env, EnvInner, InitError, RuntimeError, TxRw};
 
 use crate::{config::Config, tables::OpenTables};
 
@@ -22,9 +22,9 @@ use crate::{config::Config, tables::OpenTables};
 /// - A table could not be created/opened
 #[cold]
 #[inline(never)] // only called once
-pub fn open(config: Config) -> Result<ConcreteEnv, InitError> {
+pub fn open<E: Env>(config: Config) -> Result<E, InitError> {
     // Attempt to open the database environment.
-    let env = <ConcreteEnv as Env>::open(config.db_config)?;
+    let env = E::open(config.db_config)?;
 
     /// Convert runtime errors to init errors.
     ///
diff --git a/storage/blockchain/src/service/free.rs b/storage/blockchain/src/service/free.rs
index 3701f66f0..bbd498208 100644
--- a/storage/blockchain/src/service/free.rs
+++ b/storage/blockchain/src/service/free.rs
@@ -3,14 +3,15 @@
 //---------------------------------------------------------------------------------------------------- Import
 use std::sync::Arc;
 
-use cuprate_database::InitError;
+use cuprate_database::{Env, InitError};
 
 use crate::{
-    config::Config,
+    config::{Backend, Config},
     service::{DatabaseReadHandle, DatabaseWriteHandle},
 };
 
 //---------------------------------------------------------------------------------------------------- Init
+#[allow(unreachable_patterns)]
 #[cold]
 #[inline(never)] // Only called once (?)
 /// Initialize a database & thread-pool, and return a read/write handle to it.
@@ -21,10 +22,22 @@ use crate::{
 /// # Errors
 /// This will forward the error if [`crate::open`] failed.
 pub fn init(config: Config) -> Result<(DatabaseReadHandle, DatabaseWriteHandle), InitError> {
+    match config.backend {
+        #[cfg(feature = "heed")]
+        Backend::Heed => do_init::<cuprate_database::HeedEnv>(config),
+        #[cfg(feature = "redb")]
+        Backend::Redb => do_init::<cuprate_database::RedbEnv>(config),
+        _ => panic!("Selected database backend not available in this build."),
+    }
+}
+
+#[cold]
+#[inline(never)] 
+fn do_init<E: Env + Send + Sync + 'static>(config: Config) -> Result<(DatabaseReadHandle, DatabaseWriteHandle), InitError> {
     let reader_threads = config.reader_threads;
 
     // Initialize the database itself.
-    let db = Arc::new(crate::open(config)?);
+    let db = Arc::new(crate::open::<E>(config)?);
 
     // Spawn the Reader thread pool and Writer.
     let readers = DatabaseReadHandle::init(&db, reader_threads);
@@ -33,6 +46,7 @@ pub fn init(config: Config) -> Result<(DatabaseReadHandle, DatabaseWriteHandle),
     Ok((readers, writer))
 }
 
+
 //---------------------------------------------------------------------------------------------------- Compact history
 /// Given a position in the compact history, returns the height offset that should be in that position.
 ///
diff --git a/storage/blockchain/src/service/read.rs b/storage/blockchain/src/service/read.rs
index f73b64fba..4c81ead5a 100644
--- a/storage/blockchain/src/service/read.rs
+++ b/storage/blockchain/src/service/read.rs
@@ -39,6 +39,54 @@ use crate::{
     types::{Amount, AmountIndex, BlockHash, BlockHeight, KeyImage, PreRctOutputId},
 };
 
+trait SpawnReadTask {
+    fn spawn(&self, request: BCReadRequest, permit: OwnedSemaphorePermit) -> ResponseReceiver;
+}
+
+#[derive(Clone)]
+struct GenericEnvSpawnReadTask<E: Env> {
+    /// Access to the database.
+    env: Arc<E>,
+
+    /// Handle to the custom `rayon` DB reader thread-pool.
+    ///
+    /// Requests are [`rayon::ThreadPool::spawn`]ed in this thread-pool,
+    /// and responses are returned via a channel we (the caller) provide.
+    pool: Arc<rayon::ThreadPool>,
+}
+
+impl<E: Env> GenericEnvSpawnReadTask<E> {
+    fn new(env: &Arc<E>, pool: Arc<rayon::ThreadPool>) -> Self {
+        Self {
+            env: Arc::clone(&env),
+            pool
+        }
+    }
+}
+
+impl<E: Env + Send + Sync + 'static> SpawnReadTask for GenericEnvSpawnReadTask<E> {
+    fn spawn(&self, request: BCReadRequest, permit: OwnedSemaphorePermit) -> ResponseReceiver { 
+        // Response channel we `.await` on.
+        let (response_sender, receiver) = oneshot::channel();
+
+        // Spawn the request in the rayon DB thread-pool.
+        //
+        // Note that this uses `self.pool` instead of `rayon::spawn`
+        // such that any `rayon` parallel code that runs within
+        // the passed closure uses the same `rayon` threadpool.
+        //
+        // INVARIANT:
+        // The below `DatabaseReader` function impl block relies on this behavior.
+        let env = Arc::clone(&self.env);
+        self.pool.spawn(move || {
+            let _permit: OwnedSemaphorePermit = permit;
+            map_request(&*env, request, response_sender);
+        }); // drop(permit/env);
+
+        InfallibleOneshotReceiver::from(receiver)
+    }
+}
+
 //---------------------------------------------------------------------------------------------------- DatabaseReadHandle
 /// Read handle to the database.
 ///
@@ -49,12 +97,6 @@ use crate::{
 /// will return an `async`hronous channel that can be `.await`ed upon
 /// to receive the corresponding [`BCResponse`].
 pub struct DatabaseReadHandle {
-    /// Handle to the custom `rayon` DB reader thread-pool.
-    ///
-    /// Requests are [`rayon::ThreadPool::spawn`]ed in this thread-pool,
-    /// and responses are returned via a channel we (the caller) provide.
-    pool: Arc<rayon::ThreadPool>,
-
     /// Counting semaphore asynchronous permit for database access.
     /// Each [`tower::Service::poll_ready`] will acquire a permit
     /// before actually sending a request to the `rayon` DB threadpool.
@@ -68,8 +110,7 @@ pub struct DatabaseReadHandle {
     /// the request, i.e., after [`map_request()`] finishes.
     permit: Option<OwnedSemaphorePermit>,
 
-    /// Access to the database.
-    env: Arc<ConcreteEnv>,
+    spawn: Arc<dyn SpawnReadTask>,
 }
 
 // `OwnedSemaphorePermit` does not implement `Clone`,
@@ -78,10 +119,9 @@ pub struct DatabaseReadHandle {
 impl Clone for DatabaseReadHandle {
     fn clone(&self) -> Self {
         Self {
-            pool: Arc::clone(&self.pool),
             semaphore: self.semaphore.clone(),
             permit: None,
-            env: Arc::clone(&self.env),
+            spawn: Arc::clone(&self.spawn),
         }
     }
 }
@@ -95,28 +135,26 @@ impl DatabaseReadHandle {
     /// Should be called _once_ per actual database.
     #[cold]
     #[inline(never)] // Only called once.
-    pub(super) fn init(env: &Arc<ConcreteEnv>, reader_threads: ReaderThreads) -> Self {
+    pub(super) fn init<E: Env + Send + Sync + 'static>(env: &Arc<E>, reader_threads: ReaderThreads) -> Self {
         // How many reader threads to spawn?
         let reader_count = reader_threads.as_threads().get();
 
         // Spawn `rayon` reader threadpool.
-        let pool = rayon::ThreadPoolBuilder::new()
+        let pool = Arc::new(rayon::ThreadPoolBuilder::new()
             .num_threads(reader_count)
             .thread_name(|i| format!("cuprate_helper::service::read::DatabaseReader{i}"))
             .build()
-            .unwrap();
+            .unwrap()
+        );
 
         // Create a semaphore with the same amount of
         // permits as the amount of reader threads.
         let semaphore = PollSemaphore::new(Arc::new(Semaphore::new(reader_count)));
 
+        let spawn = Arc::new(GenericEnvSpawnReadTask::new(env, pool));
+
         // Return a handle to the pool.
-        Self {
-            pool: Arc::new(pool),
-            semaphore,
-            permit: None,
-            env: Arc::clone(env),
-        }
+        Self { semaphore, permit: None, spawn }
     }
 
     /// Access to the actual database environment.
@@ -132,7 +170,7 @@ impl DatabaseReadHandle {
     /// in this manner has not been tested.
     #[inline]
     pub const fn env(&self) -> &Arc<ConcreteEnv> {
-        &self.env
+        unimplemented!();
     }
 }
 
@@ -163,24 +201,7 @@ impl tower::Service<BCReadRequest> for DatabaseReadHandle {
             .take()
             .expect("poll_ready() should have acquire a permit before calling call()");
 
-        // Response channel we `.await` on.
-        let (response_sender, receiver) = oneshot::channel();
-
-        // Spawn the request in the rayon DB thread-pool.
-        //
-        // Note that this uses `self.pool` instead of `rayon::spawn`
-        // such that any `rayon` parallel code that runs within
-        // the passed closure uses the same `rayon` threadpool.
-        //
-        // INVARIANT:
-        // The below `DatabaseReader` function impl block relies on this behavior.
-        let env = Arc::clone(&self.env);
-        self.pool.spawn(move || {
-            let _permit: OwnedSemaphorePermit = permit;
-            map_request(&*env, request, response_sender);
-        }); // drop(permit/env);
-
-        InfallibleOneshotReceiver::from(receiver)
+        self.spawn.spawn(request, permit)
     }
 }
 
diff --git a/storage/blockchain/src/service/write.rs b/storage/blockchain/src/service/write.rs
index 041ae7b66..958ecfef1 100644
--- a/storage/blockchain/src/service/write.rs
+++ b/storage/blockchain/src/service/write.rs
@@ -8,7 +8,7 @@ use std::{
 
 use futures::channel::oneshot;
 
-use cuprate_database::{ConcreteEnv, Env, EnvInner, RuntimeError, TxRw};
+use cuprate_database::{Env, EnvInner, RuntimeError, TxRw};
 use cuprate_helper::asynch::InfallibleOneshotReceiver;
 use cuprate_types::{
     blockchain::{BCResponse, BCWriteRequest},
@@ -46,7 +46,7 @@ impl DatabaseWriteHandle {
     /// Initialize the single `DatabaseWriter` thread.
     #[cold]
     #[inline(never)] // Only called once.
-    pub(super) fn init(env: Arc<ConcreteEnv>) -> Self {
+    pub(super) fn init<E: Env + Send + Sync + 'static>(env: Arc<E>) -> Self {
         // Initialize `Request/Response` channels.
         let (sender, receiver) = crossbeam::channel::unbounded();
 
@@ -87,7 +87,7 @@ impl tower::Service<BCWriteRequest> for DatabaseWriteHandle {
 
 //---------------------------------------------------------------------------------------------------- DatabaseWriter
 /// The single database writer thread.
-pub(super) struct DatabaseWriter {
+pub(super) struct DatabaseWriter<E: Env> {
     /// Receiver side of the database request channel.
     ///
     /// Any caller can send some requests to this channel.
@@ -96,16 +96,16 @@ pub(super) struct DatabaseWriter {
     receiver: crossbeam::channel::Receiver<(BCWriteRequest, ResponseSender)>,
 
     /// Access to the database.
-    env: Arc<ConcreteEnv>,
+    env: Arc<E>,
 }
 
-impl Drop for DatabaseWriter {
+impl<E: Env> Drop for DatabaseWriter<E> {
     fn drop(&mut self) {
         // TODO: log the writer thread has exited?
     }
 }
 
-impl DatabaseWriter {
+impl<E: Env> DatabaseWriter<E> {
     /// The `DatabaseWriter`'s main function.
     ///
     /// The writer just loops in this function, handling requests forever
@@ -130,10 +130,12 @@ impl DatabaseWriter {
                 return;
             };
 
+            // FIXME
+            #[allow(unused_doc_comments, non_snake_case)]
             /// How many times should we retry handling the request on resize errors?
             ///
             /// This is 1 on automatically resizing databases, meaning there is only 1 iteration.
-            const REQUEST_RETRY_LIMIT: usize = if ConcreteEnv::MANUAL_RESIZE { 3 } else { 1 };
+            let REQUEST_RETRY_LIMIT: usize = if E::MANUAL_RESIZE { 3 } else { 1 };
 
             // Map [`Request`]'s to specific database functions.
             //
@@ -151,11 +153,11 @@ impl DatabaseWriter {
                 // FIXME: will there be more than 1 write request?
                 // this won't have to be an enum.
                 let response = match &request {
-                    BCWriteRequest::WriteBlock(block) => write_block(&self.env, block),
+                    BCWriteRequest::WriteBlock(block) => write_block(&*self.env, block),
                 };
 
                 // If the database needs to resize, do so.
-                if ConcreteEnv::MANUAL_RESIZE && matches!(response, Err(RuntimeError::ResizeNeeded))
+                if E::MANUAL_RESIZE && matches!(response, Err(RuntimeError::ResizeNeeded))
                 {
                     // If this is the last iteration of the outer `for` loop and we
                     // encounter a resize error _again_, it means something is wrong.
@@ -183,7 +185,7 @@ impl DatabaseWriter {
 
                 // Automatically resizing databases should not be returning a resize error.
                 #[cfg(debug_assertions)]
-                if !ConcreteEnv::MANUAL_RESIZE {
+                if !E::MANUAL_RESIZE {
                     assert!(
                         !matches!(response, Err(RuntimeError::ResizeNeeded)),
                         "auto-resizing database returned a ResizeNeeded error"
@@ -218,7 +220,7 @@ impl DatabaseWriter {
 
 /// [`BCWriteRequest::WriteBlock`].
 #[inline]
-fn write_block(env: &ConcreteEnv, block: &VerifiedBlockInformation) -> ResponseResult {
+fn write_block<E: Env>(env: &E, block: &VerifiedBlockInformation) -> ResponseResult {
     let env_inner = env.env_inner();
     let tx_rw = env_inner.tx_rw()?;
 
diff --git a/storage/database/src/backend/mod.rs b/storage/database/src/backend/mod.rs
index 11ae40b8b..e27501ec1 100644
--- a/storage/database/src/backend/mod.rs
+++ b/storage/database/src/backend/mod.rs
@@ -1,13 +1,22 @@
 //! Database backends.
 
+#[cfg(feature = "heed")]
+mod heed;
+
+#[cfg(feature = "heed")]
+pub use heed::ConcreteEnv as HeedEnv;
+
+#[cfg(feature = "redb")]
+mod redb;
+
+#[cfg(feature = "redb")]
+pub use redb::ConcreteEnv as RedbEnv;
+
 cfg_if::cfg_if! {
-    // If both backends are enabled, fallback to `heed`.
-    // This is useful when using `--all-features`.
+    // TODO remove this block when all ConcreteEnv references are gone.
     if #[cfg(all(feature = "redb", not(feature = "heed")))] {
-        mod redb;
         pub use redb::ConcreteEnv;
     } else {
-        mod heed;
         pub use heed::ConcreteEnv;
     }
 }
diff --git a/storage/database/src/lib.rs b/storage/database/src/lib.rs
index da36b0d5b..e467234d0 100644
--- a/storage/database/src/lib.rs
+++ b/storage/database/src/lib.rs
@@ -121,6 +121,13 @@ pub mod config;
 pub mod resize;
 
 pub use backend::ConcreteEnv;
+
+#[cfg(feature="heed")]
+pub use backend::HeedEnv;
+
+#[cfg(feature="redb")]
+pub use backend::RedbEnv;
+
 pub use constants::{
     DATABASE_BACKEND, DATABASE_CORRUPT_MSG, DATABASE_DATA_FILENAME, DATABASE_LOCK_FILENAME,
 };

From 70511a7f196ae59ce686af8c4d33376d7fad08b6 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jochen=20M=C3=BCller?= <mail@jnmr.de>
Date: Wed, 31 Jul 2024 11:10:39 +0200
Subject: [PATCH 4/7] Fix tests

---
 storage/blockchain/README.md            |  8 ++++++--
 storage/blockchain/src/config/mod.rs    |  9 +++++++--
 storage/blockchain/src/ops/mod.rs       |  8 ++++++--
 storage/blockchain/src/service/free.rs  | 22 ++++++++++++++--------
 storage/blockchain/src/service/mod.rs   |  1 +
 storage/blockchain/src/service/tests.rs | 14 ++++++++++----
 6 files changed, 44 insertions(+), 18 deletions(-)

diff --git a/storage/blockchain/README.md b/storage/blockchain/README.md
index 480054696..594e78b76 100644
--- a/storage/blockchain/README.md
+++ b/storage/blockchain/README.md
@@ -66,7 +66,6 @@ For examples of the higher-level APIs, see:
 ```rust
 use cuprate_blockchain::{
     cuprate_database::{
-        ConcreteEnv,
         Env, EnvInner,
         DatabaseRo, DatabaseRw, TxRo, TxRw,
     },
@@ -74,6 +73,11 @@ use cuprate_blockchain::{
     tables::{Tables, TablesMut, OpenTables},
 };
 
+#[cfg(feature = "heed")]
+use cuprate_blockchain::cuprate_database::HeedEnv as ConcreteEnv;
+#[cfg(all(feature = "redb", not(feature = "heed")))]
+use cuprate_blockchain::cuprate_database::RedbEnv as ConcreteEnv;
+
 # fn main() -> Result<(), Box<dyn std::error::Error>> {
 // Create a configuration for the database environment.
 let tmp_dir = tempfile::tempdir()?;
@@ -83,7 +87,7 @@ let config = ConfigBuilder::new()
     .build();
 
 // Initialize the database environment.
-let env = cuprate_blockchain::open(config)?;
+let env = cuprate_blockchain::open::<ConcreteEnv>(config)?;
 
 // Open up a transaction + tables for writing.
 let env_inner = env.env_inner();
diff --git a/storage/blockchain/src/config/mod.rs b/storage/blockchain/src/config/mod.rs
index 2a81552de..fc9dbfc58 100644
--- a/storage/blockchain/src/config/mod.rs
+++ b/storage/blockchain/src/config/mod.rs
@@ -19,6 +19,11 @@
 //!     config::{ConfigBuilder, ReaderThreads},
 //! };
 //!
+//!#[cfg(feature = "heed")]
+//!use cuprate_blockchain::cuprate_database::HeedEnv as ConcreteEnv;
+//!#[cfg(all(feature = "redb", not(feature = "heed")))]
+//!use cuprate_blockchain::cuprate_database::RedbEnv as ConcreteEnv;
+//!
 //! # fn main() -> Result<(), Box<dyn std::error::Error>> {
 //! let tmp_dir = tempfile::tempdir()?;
 //! let db_dir = tmp_dir.path().to_owned();
@@ -34,9 +39,9 @@
 //!     .build();
 //!
 //! // Start a database `service` using this configuration.
-//! let (reader_handle, _) = cuprate_blockchain::service::init(config.clone())?;
+//! let (_, _, env) = cuprate_blockchain::service::do_init::<ConcreteEnv>(config.clone())?;
 //! // It's using the config we provided.
-//! assert_eq!(reader_handle.env().config(), &config.db_config);
+//! assert_eq!(env.config(), &config.db_config);
 //! # Ok(()) }
 //! ```
 
diff --git a/storage/blockchain/src/ops/mod.rs b/storage/blockchain/src/ops/mod.rs
index 2699fc82e..dcba72d42 100644
--- a/storage/blockchain/src/ops/mod.rs
+++ b/storage/blockchain/src/ops/mod.rs
@@ -57,7 +57,6 @@
 //! use cuprate_test_utils::data::block_v16_tx0;
 //! use cuprate_blockchain::{
 //!     cuprate_database::{
-//!         ConcreteEnv,
 //!         Env, EnvInner,
 //!         DatabaseRo, DatabaseRw, TxRo, TxRw,
 //!     },
@@ -66,6 +65,11 @@
 //!     ops::block::{add_block, pop_block},
 //! };
 //!
+//! #[cfg(feature = "heed")]
+//! use cuprate_blockchain::cuprate_database::HeedEnv as ConcreteEnv;
+//! #[cfg(all(feature = "redb",not(feature = "heed")))]
+//! use cuprate_blockchain::cuprate_database::RedbEnv as ConcreteEnv;
+//!
 //! # fn main() -> Result<(), Box<dyn std::error::Error>> {
 //! // Create a configuration for the database environment.
 //! let tmp_dir = tempfile::tempdir()?;
@@ -75,7 +79,7 @@
 //!     .build();
 //!
 //! // Initialize the database environment.
-//! let env = cuprate_blockchain::open(config)?;
+//! let env = cuprate_blockchain::open::<ConcreteEnv>(config)?;
 //!
 //! // Open up a transaction + tables for writing.
 //! let env_inner = env.env_inner();
diff --git a/storage/blockchain/src/service/free.rs b/storage/blockchain/src/service/free.rs
index bbd498208..107d22a5f 100644
--- a/storage/blockchain/src/service/free.rs
+++ b/storage/blockchain/src/service/free.rs
@@ -13,7 +13,7 @@ use crate::{
 //---------------------------------------------------------------------------------------------------- Init
 #[allow(unreachable_patterns)]
 #[cold]
-#[inline(never)] // Only called once (?)
+#[inline(never)] // Only called once
 /// Initialize a database & thread-pool, and return a read/write handle to it.
 ///
 /// Once the returned handles are [`Drop::drop`]ed, the reader
@@ -22,18 +22,24 @@ use crate::{
 /// # Errors
 /// This will forward the error if [`crate::open`] failed.
 pub fn init(config: Config) -> Result<(DatabaseReadHandle, DatabaseWriteHandle), InitError> {
+    let (reader, writer); 
+
     match config.backend {
         #[cfg(feature = "heed")]
-        Backend::Heed => do_init::<cuprate_database::HeedEnv>(config),
+        Backend::Heed => (reader, writer, _) = do_init::<cuprate_database::HeedEnv>(config)?,
         #[cfg(feature = "redb")]
-        Backend::Redb => do_init::<cuprate_database::RedbEnv>(config),
+        Backend::Redb => (reader, writer, _) = do_init::<cuprate_database::RedbEnv>(config)?,
         _ => panic!("Selected database backend not available in this build."),
-    }
+    };
+
+    Ok((reader, writer))
 }
 
 #[cold]
-#[inline(never)] 
-fn do_init<E: Env + Send + Sync + 'static>(config: Config) -> Result<(DatabaseReadHandle, DatabaseWriteHandle), InitError> {
+#[inline(never)]
+/// Like [`init()`], but generic over database backend and returning the database environment too.
+/// Only exported for use in tests.
+pub fn do_init<E: Env + Send + Sync + 'static>(config: Config) -> Result<(DatabaseReadHandle, DatabaseWriteHandle, Arc<E>), InitError> {
     let reader_threads = config.reader_threads;
 
     // Initialize the database itself.
@@ -41,9 +47,9 @@ fn do_init<E: Env + Send + Sync + 'static>(config: Config) -> Result<(DatabaseRe
 
     // Spawn the Reader thread pool and Writer.
     let readers = DatabaseReadHandle::init(&db, reader_threads);
-    let writer = DatabaseWriteHandle::init(db);
+    let writer = DatabaseWriteHandle::init(Arc::clone(&db));
 
-    Ok((readers, writer))
+    Ok((readers, writer, db))
 }
 
 
diff --git a/storage/blockchain/src/service/mod.rs b/storage/blockchain/src/service/mod.rs
index 1d9d10b4d..52209522e 100644
--- a/storage/blockchain/src/service/mod.rs
+++ b/storage/blockchain/src/service/mod.rs
@@ -126,6 +126,7 @@ pub use write::DatabaseWriteHandle;
 
 mod free;
 pub use free::init;
+pub use free::do_init;
 
 // Internal type aliases for `service`.
 mod types;
diff --git a/storage/blockchain/src/service/tests.rs b/storage/blockchain/src/service/tests.rs
index 4f3fbe49a..b8a1e7829 100644
--- a/storage/blockchain/src/service/tests.rs
+++ b/storage/blockchain/src/service/tests.rs
@@ -15,7 +15,14 @@ use std::{
 use pretty_assertions::assert_eq;
 use tower::{Service, ServiceExt};
 
-use cuprate_database::{ConcreteEnv, DatabaseIter, DatabaseRo, Env, EnvInner, RuntimeError};
+use cuprate_database::{DatabaseIter, DatabaseRo, Env, EnvInner, RuntimeError};
+
+#[cfg(all(feature = "redb", not(feature = "heed")))]
+use cuprate_database::RedbEnv as ConcreteEnv;
+
+#[cfg(feature = "heed")]
+use cuprate_database::HeedEnv as ConcreteEnv;
+
 use cuprate_test_utils::data::{block_v16_tx0, block_v1_tx2, block_v9_tx3};
 use cuprate_types::{
     blockchain::{BCReadRequest, BCResponse, BCWriteRequest},
@@ -29,7 +36,7 @@ use crate::{
         blockchain::chain_height,
         output::id_to_output_on_chain,
     },
-    service::{init, DatabaseReadHandle, DatabaseWriteHandle},
+    service::{do_init, DatabaseReadHandle, DatabaseWriteHandle},
     tables::{OpenTables, Tables, TablesIter},
     tests::AssertTableLen,
     types::{Amount, AmountIndex, PreRctOutputId},
@@ -48,8 +55,7 @@ fn init_service() -> (
         .db_directory(Cow::Owned(tempdir.path().into()))
         .low_power()
         .build();
-    let (reader, writer) = init(config).unwrap();
-    let env = reader.env().clone();
+    let (reader, writer, env) = do_init::<ConcreteEnv>(config).unwrap();
     (reader, writer, env, tempdir)
 }
 

From c59d342dc2179499262edd711bcb455edbe0b09d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jochen=20M=C3=BCller?= <mail@jnmr.de>
Date: Thu, 1 Aug 2024 09:37:07 +0200
Subject: [PATCH 5/7] cleanup1

---
 storage/blockchain/src/service/read.rs | 17 +----------------
 storage/blockchain/src/tests.rs        |  7 ++++++-
 storage/database/src/backend/mod.rs    |  9 ---------
 storage/database/src/lib.rs            |  2 --
 4 files changed, 7 insertions(+), 28 deletions(-)

diff --git a/storage/blockchain/src/service/read.rs b/storage/blockchain/src/service/read.rs
index 4c81ead5a..4bb5b70b5 100644
--- a/storage/blockchain/src/service/read.rs
+++ b/storage/blockchain/src/service/read.rs
@@ -13,7 +13,7 @@ use thread_local::ThreadLocal;
 use tokio::sync::{OwnedSemaphorePermit, Semaphore};
 use tokio_util::sync::PollSemaphore;
 
-use cuprate_database::{ConcreteEnv, DatabaseRo, Env, EnvInner, RuntimeError};
+use cuprate_database::{DatabaseRo, Env, EnvInner, RuntimeError};
 use cuprate_helper::{asynch::InfallibleOneshotReceiver, map::combine_low_high_bits_to_u128};
 use cuprate_types::{
     blockchain::{BCReadRequest, BCResponse},
@@ -157,21 +157,6 @@ impl DatabaseReadHandle {
         Self { semaphore, permit: None, spawn }
     }
 
-    /// Access to the actual database environment.
-    ///
-    /// # ⚠️ Warning
-    /// This function gives you access to the actual
-    /// underlying database connected to by `self`.
-    ///
-    /// I.e. it allows you to read/write data _directly_
-    /// instead of going through a request.
-    ///
-    /// Be warned that using the database directly
-    /// in this manner has not been tested.
-    #[inline]
-    pub const fn env(&self) -> &Arc<ConcreteEnv> {
-        unimplemented!();
-    }
 }
 
 impl tower::Service<BCReadRequest> for DatabaseReadHandle {
diff --git a/storage/blockchain/src/tests.rs b/storage/blockchain/src/tests.rs
index 65527e102..7c847f860 100644
--- a/storage/blockchain/src/tests.rs
+++ b/storage/blockchain/src/tests.rs
@@ -9,7 +9,12 @@ use std::{borrow::Cow, fmt::Debug};
 
 use pretty_assertions::assert_eq;
 
-use cuprate_database::{ConcreteEnv, DatabaseRo, Env, EnvInner};
+use cuprate_database::{DatabaseRo, Env, EnvInner};
+
+#[cfg(feature = "heed")]
+use cuprate_database::HeedEnv as ConcreteEnv;
+#[cfg(all(feature = "redb", not(feature = "heed")))]
+use cuprate_database::RedbEnv as ConcreteEnv;
 
 use crate::{
     config::ConfigBuilder,
diff --git a/storage/database/src/backend/mod.rs b/storage/database/src/backend/mod.rs
index e27501ec1..2b0499cc0 100644
--- a/storage/database/src/backend/mod.rs
+++ b/storage/database/src/backend/mod.rs
@@ -12,14 +12,5 @@ mod redb;
 #[cfg(feature = "redb")]
 pub use redb::ConcreteEnv as RedbEnv;
 
-cfg_if::cfg_if! {
-    // TODO remove this block when all ConcreteEnv references are gone.
-    if #[cfg(all(feature = "redb", not(feature = "heed")))] {
-        pub use redb::ConcreteEnv;
-    } else {
-        pub use heed::ConcreteEnv;
-    }
-}
-
 #[cfg(test)]
 mod tests;
diff --git a/storage/database/src/lib.rs b/storage/database/src/lib.rs
index e467234d0..2d992d3ac 100644
--- a/storage/database/src/lib.rs
+++ b/storage/database/src/lib.rs
@@ -120,8 +120,6 @@ mod transaction;
 pub mod config;
 pub mod resize;
 
-pub use backend::ConcreteEnv;
-
 #[cfg(feature="heed")]
 pub use backend::HeedEnv;
 

From 9ea39ad4056aaca03659063a194c2a48518892ae Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jochen=20M=C3=BCller?= <mail@jnmr.de>
Date: Thu, 1 Aug 2024 18:25:23 +0200
Subject: [PATCH 6/7] Cleanup

---
 storage/database/src/lib.rs | 10 ----------
 1 file changed, 10 deletions(-)

diff --git a/storage/database/src/lib.rs b/storage/database/src/lib.rs
index 2d992d3ac..b7ac9f139 100644
--- a/storage/database/src/lib.rs
+++ b/storage/database/src/lib.rs
@@ -144,13 +144,3 @@ pub(crate) mod tests;
 // Used inside public facing macros.
 #[doc(hidden)]
 pub use paste;
-
-//----------------------------------------------------------------------------------------------------
-// HACK: needed to satisfy the `unused_crate_dependencies` lint.
-cfg_if::cfg_if! {
-    if #[cfg(feature = "redb")]  {
-        use redb as _;
-    } else {
-        use heed as _;
-    }
-}

From 007e87606d0bfec6ada26fef9da87bf35a161990 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jochen=20M=C3=BCller?= <mail@jnmr.de>
Date: Thu, 1 Aug 2024 18:25:34 +0200
Subject: [PATCH 7/7] Restore doc tests

---
 storage/database/README.md            | 8 ++++++--
 storage/database/src/backend/tests.rs | 6 +++++-
 storage/database/src/config/mod.rs    | 7 ++++++-
 storage/database/src/env.rs           | 7 ++++++-
 storage/database/src/tables.rs        | 7 ++++++-
 storage/database/src/tests.rs         | 7 ++++++-
 6 files changed, 35 insertions(+), 7 deletions(-)

diff --git a/storage/database/README.md b/storage/database/README.md
index aed738eb9..256a05a4e 100644
--- a/storage/database/README.md
+++ b/storage/database/README.md
@@ -108,12 +108,16 @@ The below is an example of using `cuprate-database`.
 
 ```rust
 use cuprate_database::{
-    ConcreteEnv,
     config::ConfigBuilder,
     Env, EnvInner,
     DatabaseRo, DatabaseRw, TxRo, TxRw,
 };
 
+#[cfg(feature = "heed")]
+use cuprate_database::HeedEnv as ConcreteEnv;
+#[cfg(all(feature = "redb", not(feature = "heed")))]
+use cuprate_database::RedbEnv as ConcreteEnv;
+
 # fn main() -> Result<(), Box<dyn std::error::Error>> {
 // Create a configuration for the database environment.
 let tmp_dir = tempfile::tempdir()?;
@@ -153,4 +157,4 @@ let tx_ro = env_inner.tx_ro()?;
 let table = env_inner.open_db_ro::<Table>(&tx_ro)?;
 assert_eq!(table.first()?, (0, 1));
 # Ok(()) }
-```
\ No newline at end of file
+```
diff --git a/storage/database/src/backend/tests.rs b/storage/database/src/backend/tests.rs
index ac6b5927c..5350c02fa 100644
--- a/storage/database/src/backend/tests.rs
+++ b/storage/database/src/backend/tests.rs
@@ -21,9 +21,13 @@ use crate::{
     resize::ResizeAlgorithm,
     tests::{tmp_concrete_env, TestTable},
     transaction::{TxRo, TxRw},
-    ConcreteEnv,
 };
 
+#[cfg(feature = "heed")]
+use crate::HeedEnv as ConcreteEnv;
+#[cfg(all(feature = "redb", not(feature = "heed")))]
+use crate::RedbEnv as ConcreteEnv;
+
 //---------------------------------------------------------------------------------------------------- Tests
 /// Simply call [`Env::open`]. If this fails, something is really wrong.
 #[test]
diff --git a/storage/database/src/config/mod.rs b/storage/database/src/config/mod.rs
index 19a324e1f..d62919032 100644
--- a/storage/database/src/config/mod.rs
+++ b/storage/database/src/config/mod.rs
@@ -13,10 +13,15 @@
 //! # Example
 //! ```rust
 //! use cuprate_database::{
-//!     ConcreteEnv, Env,
+//!     Env,
 //!     config::{ConfigBuilder, SyncMode}
 //! };
 //!
+//! #[cfg(feature = "heed")]
+//! use cuprate_database::HeedEnv as ConcreteEnv;
+//! #[cfg(all(feature = "redb", not(feature = "heed")))]
+//! use cuprate_database::RedbEnv as ConcreteEnv;
+//!
 //! # fn main() -> Result<(), Box<dyn std::error::Error>> {
 //! let db_dir = tempfile::tempdir()?;
 //!
diff --git a/storage/database/src/env.rs b/storage/database/src/env.rs
index e93e1be21..5fd9fe393 100644
--- a/storage/database/src/env.rs
+++ b/storage/database/src/env.rs
@@ -237,11 +237,16 @@ pub trait EnvInner<'env> {
     ///
     /// ```rust
     /// # use cuprate_database::{
-    /// #     ConcreteEnv,
     /// #     config::ConfigBuilder,
     /// #     Env, EnvInner,
     /// #     DatabaseRo, DatabaseRw, TxRo, TxRw,
     /// # };
+    /// #
+    /// # #[cfg(feature = "heed")]
+    /// # use cuprate_database::HeedEnv as ConcreteEnv;
+    /// # #[cfg(all(feature = "redb", not(feature = "heed")))]
+    /// # use cuprate_database::RedbEnv as ConcreteEnv;
+    ///
     /// # fn main() -> Result<(), Box<dyn std::error::Error>> {
     /// # let tmp_dir = tempfile::tempdir()?;
     /// # let db_dir = tmp_dir.path().to_owned();
diff --git a/storage/database/src/tables.rs b/storage/database/src/tables.rs
index 83a00e16e..24bbcbeea 100644
--- a/storage/database/src/tables.rs
+++ b/storage/database/src/tables.rs
@@ -44,12 +44,17 @@
 /// An example:
 /// ```rust
 /// use cuprate_database::{
-///     ConcreteEnv, Table,
+///     Table,
 ///     config::ConfigBuilder,
 ///     Env, EnvInner,
 ///     DatabaseRo, DatabaseRw, TxRo, TxRw,
 /// };
 ///
+/// #[cfg(feature = "heed")]
+/// use cuprate_database::HeedEnv as ConcreteEnv;
+/// #[cfg(all(feature = "redb", not(feature = "heed")))]
+/// use cuprate_database::RedbEnv as ConcreteEnv;
+///
 /// // This generates `pub struct Table{1,2,3}`
 /// // where all those implement `Table` with
 /// // the defined name and key/value types.
diff --git a/storage/database/src/tests.rs b/storage/database/src/tests.rs
index 9c9317d2e..da7447272 100644
--- a/storage/database/src/tests.rs
+++ b/storage/database/src/tests.rs
@@ -7,7 +7,12 @@
 //---------------------------------------------------------------------------------------------------- Import
 use std::borrow::Cow;
 
-use crate::{config::ConfigBuilder, table::Table, ConcreteEnv, Env};
+use crate::{config::ConfigBuilder, table::Table, Env};
+
+#[cfg(feature = "heed")]
+use crate::HeedEnv as ConcreteEnv;
+#[cfg(all(feature = "redb",not(feature = "heed")))]
+use crate::RedbEnv as ConcreteEnv;
 
 //---------------------------------------------------------------------------------------------------- struct
 /// A test table.