sql: organize connection pool as a stack rather than a queue

When connection pool is organized as a stack,
it always returns most recently used connection.
Because each connection has its own page cache,
using the connection with fresh cache improves performance.

I commented out `oauth2::tests::test_oauth_from_mx`
because it requires network connection,
turned off the Wi-Fi and ran the tests.

Before the change, with a queue:
```
$ hyperfine "cargo test"
Benchmark 1: cargo test
  Time (mean ± σ):     56.424 s ±  4.515 s    [User: 183.181 s, System: 128.156 s]
  Range (min … max):   52.123 s … 68.193 s    10 runs
```

With a stack:
```
$ hyperfine "cargo test"
Benchmark 1: cargo test
  Time (mean ± σ):     29.887 s ±  1.377 s    [User: 101.226 s, System: 45.573 s]
  Range (min … max):   26.591 s … 31.010 s    10 runs
```

On version 1.107.1:
```
$ hyperfine "cargo test"
Benchmark 1: cargo test
  Time (mean ± σ):     43.658 s ±  1.079 s    [User: 202.582 s, System: 50.723 s]
  Range (min … max):   41.531 s … 45.170 s    10 runs
```

CHANGELOG.md

@@ -4,6 +4,8 @@
 
 ### Changes
 - use transaction in `Contact::add_or_lookup()` #4059
+- Organize the connection pool as a stack rather than a queue to ensure that
+  connection page cache is reused more often. #4065
 
 ### Fixes
 

Cargo.lock

@@ -848,7 +848,6 @@ dependencies = [
  "bitflags",
  "chrono",
  "criterion",
- "crossbeam-queue",
  "deltachat_derive",
  "email",
  "encoded-words",
@@ -870,6 +869,7 @@ dependencies = [
  "num-traits",
  "num_cpus",
  "once_cell",
+ "parking_lot",
  "percent-encoding",
  "pgp",
  "pretty_env_logger",

Cargo.toml

@@ -38,7 +38,6 @@ backtrace = "0.3"
 base64 = "0.21"
 bitflags = "1.3"
 chrono = { version = "0.4", default-features=false, features = ["clock", "std"] }
-crossbeam-queue = "0.3"
 email = { git = "https://github.com/deltachat/rust-email", branch = "master" }
 encoded-words = { git = "https://github.com/async-email/encoded-words", branch = "master" }
 escaper = "0.1"
@@ -58,6 +57,7 @@ num-derive = "0.3"
 num-traits = "0.2"
 once_cell = "1.17.0"
 percent-encoding = "2.2"
+parking_lot = "0.12"
 pgp = { version = "0.9", default-features = false }
 pretty_env_logger = { version = "0.4", optional = true }
 qrcodegen = "1.7.0"

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()),
+            connections: Mutex::new(connections),
-            semaphore: Arc::new(Semaphore::new(connections.len())),
+            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
+        let mut connections = self.inner.connections.lock();
-            .inner
+        let conn = connections
-            .connections
             .pop()
             .context("got a permit when there are no connections in the pool")?;
         let conn = PooledConnection {