Replace r2d2 with an own connection pool

New connection pool does not use threads
and does not remove idle connections
or create new connections at runtime.

src/sql/connection_manager.rs

@@ -1,73 +0,0 @@
-use std::path::PathBuf;
-use std::time::Duration;
-
-use r2d2::ManageConnection;
-use rusqlite::{Connection, Error, OpenFlags};
-
-#[derive(Debug)]
-pub struct ConnectionManager {
-    /// Database file path.
-    path: PathBuf,
-
-    /// SQLite open flags.
-    flags: rusqlite::OpenFlags,
-
-    /// SQLCipher database passphrase.
-    /// Empty string if database is not encrypted.
-    passphrase: String,
-}
-
-impl ConnectionManager {
-    /// Creates new connection manager.
-    pub fn new(path: PathBuf, passphrase: String) -> Self {
-        let mut flags = OpenFlags::SQLITE_OPEN_NO_MUTEX;
-        flags.insert(OpenFlags::SQLITE_OPEN_READ_WRITE);
-        flags.insert(OpenFlags::SQLITE_OPEN_CREATE);
-
-        Self {
-            path,
-            flags,
-            passphrase,
-        }
-    }
-}
-
-impl ManageConnection for ConnectionManager {
-    type Connection = Connection;
-    type Error = Error;
-
-    fn connect(&self) -> Result<Connection, Error> {
-        let conn = Connection::open_with_flags(&self.path, self.flags)?;
-        conn.execute_batch(&format!(
-            "PRAGMA cipher_memory_security = OFF; -- Too slow on Android
-             PRAGMA secure_delete=on;
-             PRAGMA busy_timeout = {};
-             PRAGMA temp_store=memory; -- Avoid SQLITE_IOERR_GETTEMPPATH errors on Android
-             PRAGMA foreign_keys=on;
-             ",
-            Duration::from_secs(60).as_millis()
-        ))?;
-        conn.pragma_update(None, "key", &self.passphrase)?;
-        // Try to enable auto_vacuum. This will only be
-        // applied if the database is new or after successful
-        // VACUUM, which usually happens before backup export.
-        // When auto_vacuum is INCREMENTAL, it is possible to
-        // use PRAGMA incremental_vacuum to return unused
-        // database pages to the filesystem.
-        conn.pragma_update(None, "auto_vacuum", "INCREMENTAL".to_string())?;
-
-        conn.pragma_update(None, "journal_mode", "WAL".to_string())?;
-        // Default synchronous=FULL is much slower. NORMAL is sufficient for WAL mode.
-        conn.pragma_update(None, "synchronous", "NORMAL".to_string())?;
-
-        Ok(conn)
-    }
-
-    fn is_valid(&self, _conn: &mut Connection) -> Result<(), Error> {
-        Ok(())
-    }
-
-    fn has_broken(&self, _conn: &mut Connection) -> bool {
-        false
-    }
-}

src/sql/pool.rs

@@ -0,0 +1,105 @@
+//! Connection pool.
+
+use std::fmt;
+use std::ops::{Deref, DerefMut};
+use std::sync::{Arc, Weak};
+
+use parking_lot::{Condvar, Mutex};
+use rusqlite::Connection;
+
+/// Inner connection pool.
+struct InnerPool {
+    /// Available connections.
+    connections: Mutex<Vec<Connection>>,
+
+    /// Conditional variable to notify about added connections.
+    ///
+    /// Used to wait for available connection when the pool is empty.
+    cond: Condvar,
+}
+
+impl InnerPool {
+    /// Puts a connection into the pool.
+    ///
+    /// The connection could be new or returned back.
+    fn put(&self, connection: Connection) {
+        let mut connections = self.connections.lock();
+        connections.push(connection);
+        drop(connections);
+        self.cond.notify_one();
+    }
+}
+
+/// Pooled connection.
+pub struct PooledConnection {
+    /// Weak reference to the pool used to return the connection back.
+    pool: Weak<InnerPool>,
+
+    /// Only `None` right after moving the connection back to the pool.
+    conn: Option<Connection>,
+}
+
+impl Drop for PooledConnection {
+    fn drop(&mut self) {
+        // Put the connection back unless the pool is already dropped.
+        if let Some(pool) = self.pool.upgrade() {
+            if let Some(conn) = self.conn.take() {
+                pool.put(conn);
+            }
+        }
+    }
+}
+
+impl Deref for PooledConnection {
+    type Target = Connection;
+
+    fn deref(&self) -> &Connection {
+        self.conn.as_ref().unwrap()
+    }
+}
+
+impl DerefMut for PooledConnection {
+    fn deref_mut(&mut self) -> &mut Connection {
+        self.conn.as_mut().unwrap()
+    }
+}
+
+/// Connection pool.
+#[derive(Clone)]
+pub struct Pool {
+    /// Reference to the actual connection pool.
+    inner: Arc<InnerPool>,
+}
+
+impl fmt::Debug for Pool {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        write!(fmt, "Pool")
+    }
+}
+
+impl Pool {
+    /// Creates a new connection pool.
+    pub fn new(connections: Vec<Connection>) -> Self {
+        let inner = Arc::new(InnerPool {
+            connections: Mutex::new(connections),
+            cond: Condvar::new(),
+        });
+        Pool { inner }
+    }
+
+    /// Retrieves a connection from the pool.
+    pub fn get(&self) -> PooledConnection {
+        let mut connections = self.inner.connections.lock();
+
+        loop {
+            if let Some(conn) = connections.pop() {
+                return PooledConnection {
+                    pool: Arc::downgrade(&self.inner),
+                    conn: Some(conn),
+                };
+            }
+
+            self.inner.cond.wait(&mut connections);
+        }
+    }
+}