Implement a total cached message cap, alongside the per-channel cap

src/cache/mod.rs

@@ -25,6 +25,7 @@
 
 use std::collections::{HashSet, VecDeque};
 use std::hash::Hash;
+use std::sync::atomic::AtomicUsize;
 #[cfg(feature = "temp_cache")]
 use std::sync::Arc;
 #[cfg(feature = "temp_cache")]
@@ -216,6 +217,10 @@ pub struct Cache {
     /// cache. When a maximum number of messages are in a channel's cache, we can pop the front and
     /// remove that ID from the cache.
     pub(crate) message_queue: DashMap<ChannelId, VecDeque<MessageId>, BuildHasher>,
+    /// Total number of messages stored across all channels.
+    ///
+    /// Must not exceed [`Settings::max_messages_total`]
+    pub(crate) total_messages: Option<AtomicUsize>,
 
     // Miscellanous fixed-size data
     // ---
@@ -282,6 +287,7 @@ impl Cache {
 
             messages: DashMap::default(),
             message_queue: DashMap::default(),
+            total_messages: settings.max_messages_total.map(|_| AtomicUsize::new(0)),
 
             shard_data: RwLock::new(CachedShardData {
                 total: 1,

src/cache/settings.rs

@@ -1,3 +1,4 @@
+use std::num::NonZeroUsize;
 use std::time::Duration;
 
 /// Settings for the cache.
@@ -20,6 +21,12 @@ pub struct Settings {
     ///
     /// Defaults to 0.
     pub max_messages: usize,
+    /// The maximum number of messages to store in the message cache, in total.
+    ///
+    /// See [`Self::max_messages`] to disable the cache entirely.
+    ///
+    /// Defaults to None, meaning no limit set.
+    pub max_messages_total: Option<NonZeroUsize>,
     /// How long temporarily-cached data should be stored before being thrown out.
     ///
     /// Defaults to one hour.
@@ -42,6 +49,7 @@ impl Default for Settings {
     fn default() -> Self {
         Self {
             max_messages: 0,
+            max_messages_total: None,
             time_to_live: Duration::from_secs(60 * 60),
             cache_guilds: true,
             cache_channels: true,