Add database connection module

.vscode/settings.json

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+{
+	"rust-analyzer.showUnlinkedFileNotification": false
+}

src/common/database.rs

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+use diesel::r2d2::{self, ConnectionManager, PooledConnection};
+use once_cell::sync::OnceCell;
+
+#[cfg(feature = "database_postgres")]
+type DbCon = diesel::PgConnection;
+
+#[cfg(all(feature = "database_postgres", debug_assertions))]
+#[allow(dead_code)]
+pub type DieselBackend = diesel::pg::Pg;
+
+
+pub type Pool = r2d2::Pool<ConnectionManager<DbCon>>;
+pub type Connection = PooledConnection<ConnectionManager<DbCon>>;
+
+/// wrapper function for a database pool
+#[derive(Clone)]
+pub struct Database {
+    pub pool: &'static Pool,
+}
+
+impl Default for Database {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl Database {
+    pub fn new() -> Database {
+        Database {
+            pool: Self::get_or_init_pool(),
+        }
+    }
+
+    pub fn get_connection(&self) -> Connection {
+        self.pool.get().unwrap()
+    }
+
+    fn get_or_init_pool() -> &'static Pool {
+        #[cfg(debug_assertions)]
+        crate::load_env_vars();
+
+        static POOL: OnceCell<Pool> = OnceCell::new();
+
+        POOL.get_or_init(|| {
+            Pool::builder()
+                .connection_timeout(std::time::Duration::from_secs(5))
+                .build(ConnectionManager::<DbCon>::new(Self::connection_url()))
+                .unwrap()
+        })
+    }
+
+    pub fn connection_url() -> String {
+        std::env::var("DATABASE_URL").expect("DATABASE_URL environment variable expected.")
+    }
+}