support some deprecations

src/database.rs

@@ -42,8 +42,8 @@ use self::proxy::ProxyConfig;
 pub struct Config {
     server_name: Box<ServerName>,
     database_path: String,
-    #[serde(default = "default_db_cache_capacity")]
-    db_cache_capacity: u32,
+    cache_capacity: Option<u32>, // deprecated
+    db_cache_capacity: Option<u32>,
     #[serde(default = "default_sqlite_read_pool_size")]
     sqlite_read_pool_size: usize,
     #[serde(default = "false_fn")]
@@ -73,6 +73,30 @@ pub struct Config {
     pub log: String,
 }
 
+macro_rules! deprecate_with {
+    ($self:expr ; $from:ident -> $to:ident) => {
+        if let Some(v) = $self.$from {
+            let from = stringify!($from);
+            let to = stringify!($to);
+            log::warn!("{} is deprecated, use {} instead", from, to);
+
+            $self.$to.get_or_insert(v);
+        }
+    };
+    ($self:expr ; $from:ident -> $to:ident or $default:expr) => {
+        deprecate_with!($self ; $from -> $to);
+        $self.$to.get_or_insert_with($default);
+    };
+}
+
+impl Config {
+    pub fn fallbacks(mut self) -> Self {
+        // TODO: have a proper way handle into above struct (maybe serde supports something like this?)
+        deprecate_with!(self ; cache_capacity -> db_cache_capacity or default_db_cache_capacity);
+        self
+    }
+}
+
 fn false_fn() -> bool {
     false
 }

src/database/abstraction/sqlite.rs

@@ -128,7 +128,7 @@ impl DatabaseEngine for Engine {
         let pool = Pool::new(
             Path::new(&config.database_path).join("conduit.db"),
             config.sqlite_read_pool_size,
-            config.db_cache_capacity / 1024, // bytes -> kb
+            config.db_cache_capacity.expect("fallbacks hasn't been called") / 1024, // bytes -> kb
         )?;
 
         pool.write_lock()

src/main.rs

@@ -196,7 +196,8 @@ async fn main() {
 
     let config = raw_config
         .extract::<Config>()
-        .expect("It looks like your config is invalid. Please take a look at the error");
+        .expect("It looks like your config is invalid. Please take a look at the error")
+        .fallbacks();
 
     let db = Database::load_or_create(config.clone())
         .await