sql: use semaphore to limit access to the connection pool

This ensures that if multiple connections are returned
to the pool at the same time, waiters get them in the order
they were placed in the queue.

src/sql/pool.rs

@@ -3,9 +3,10 @@
 use std::ops::{Deref, DerefMut};
 use std::sync::{Arc, Weak};
 
+use anyhow::{Context, Result};
 use crossbeam_queue::ArrayQueue;
 use rusqlite::Connection;
-use tokio::sync::Notify;
+use tokio::sync::{OwnedSemaphorePermit, Semaphore};
 
 /// Inner connection pool.
 #[derive(Debug)]
@@ -13,10 +14,8 @@ struct InnerPool {
     /// Available connections.
     connections: ArrayQueue<Connection>,
 
-    /// Notifies about added connections.
-    ///
-    /// Used to wait for available connection when the pool is empty.
-    notify: Notify,
+    /// Counts the number of available connections.
+    semaphore: Arc<Semaphore>,
 }
 
 impl InnerPool {
@@ -25,7 +24,6 @@ impl InnerPool {
     /// The connection could be new or returned back.
     fn put(&self, connection: Connection) {
         self.connections.force_push(connection);
-        self.notify.notify_one();
     }
 }
 
@@ -36,6 +34,9 @@ pub struct PooledConnection {
 
     /// Only `None` right after moving the connection back to the pool.
     conn: Option<Connection>,
+
+    /// Semaphore permit, dropped after returning the connection to the pool.
+    _permit: OwnedSemaphorePermit,
 }
 
 impl Drop for PooledConnection {
@@ -75,7 +76,7 @@ impl Pool {
     pub fn new(connections: Vec<Connection>) -> Self {
         let inner = Arc::new(InnerPool {
             connections: ArrayQueue::new(connections.len()),
-            notify: Notify::new(),
+            semaphore: Arc::new(Semaphore::new(connections.len())),
         });
         for connection in connections {
             inner.connections.force_push(connection);
@@ -84,16 +85,18 @@ impl Pool {
     }
 
     /// Retrieves a connection from the pool.
-    pub async fn get(&self) -> PooledConnection {
-        loop {
-            if let Some(conn) = self.inner.connections.pop() {
-                return PooledConnection {
-                    pool: Arc::downgrade(&self.inner),
-                    conn: Some(conn),
-                };
-            }
-
-            self.inner.notify.notified().await;
-        }
+    pub async fn get(&self) -> Result<PooledConnection> {
+        let permit = self.inner.semaphore.clone().acquire_owned().await?;
+        let conn = self
+            .inner
+            .connections
+            .pop()
+            .context("got a permit when there are no connections in the pool")?;
+        let conn = PooledConnection {
+            pool: Arc::downgrade(&self.inner),
+            conn: Some(conn),
+            _permit: permit,
+        };
+        Ok(conn)
     }
 }