Merge switching SQLite connection pool from FIFO to LIFO

src/sql/pool.rs

@@ -1,10 +1,18 @@
-//! Connection pool.
+//! # SQLite connection pool.
+//!
+//! The connection pool holds a number of SQLite connections and allows to allocate them.
+//! When allocated connection is dropped, underlying connection is returned back to the pool.
+//!
+//! The pool is organized as a stack. It always allocates the most recently used connection.
+//! Each SQLite connection has its own page cache, so allocating recently used connections
+//! improves the performance compared to, for example, organizing the pool as a queue
+//! and returning the least recently used connection each time.
 
 use std::ops::{Deref, DerefMut};
 use std::sync::{Arc, Weak};
 
 use anyhow::{Context, Result};
-use crossbeam_queue::ArrayQueue;
+use parking_lot::Mutex;
 use rusqlite::Connection;
 use tokio::sync::{OwnedSemaphorePermit, Semaphore};
 
@@ -12,7 +20,7 @@ use tokio::sync::{OwnedSemaphorePermit, Semaphore};
 #[derive(Debug)]
 struct InnerPool {
     /// Available connections.
-    connections: ArrayQueue<Connection>,
+    connections: Mutex<Vec<Connection>>,
 
     /// Counts the number of available connections.
     semaphore: Arc<Semaphore>,
@@ -23,7 +31,9 @@ impl InnerPool {
     ///
     /// The connection could be new or returned back.
     fn put(&self, connection: Connection) {
-        self.connections.force_push(connection);
+        let mut connections = self.connections.lock();
+        connections.push(connection);
+        drop(connections);
     }
 }
 
@@ -74,22 +84,19 @@ pub struct Pool {
 impl Pool {
     /// Creates a new connection pool.
     pub fn new(connections: Vec<Connection>) -> Self {
+        let semaphore = Arc::new(Semaphore::new(connections.len()));
         let inner = Arc::new(InnerPool {
-            connections: ArrayQueue::new(connections.len()),
-            semaphore: Arc::new(Semaphore::new(connections.len())),
+            connections: Mutex::new(connections),
+            semaphore,
         });
-        for connection in connections {
-            inner.connections.force_push(connection);
-        }
         Pool { inner }
     }
 
     /// Retrieves a connection from the pool.
     pub async fn get(&self) -> Result<PooledConnection> {
         let permit = self.inner.semaphore.clone().acquire_owned().await?;
-        let conn = self
-            .inner
-            .connections
+        let mut connections = self.inner.connections.lock();
+        let conn = connections
             .pop()
             .context("got a permit when there are no connections in the pool")?;
         let conn = PooledConnection {