Don't let the PubSub internal offsets overflow + fix some warnings.

src/data/pubsub.rs

@@ -3,16 +3,28 @@
 use serde::export::PhantomData;
 use std::collections::VecDeque;
 
-/// The position of a subscriber on a pub-sub queue.
+/// A permanent subscription to a pub-sub queue.
 #[cfg_attr(feature = "serde-serialize", derive(Serialize, Deserialize))]
-pub struct PubSubCursor<T> {
-    // Index of the next message to read.
+pub struct Subscription<T> {
+    // Position on the cursor array.
     id: u32,
-    next: u32,
     _phantom: PhantomData<T>,
 }
 
-impl<T> PubSubCursor<T> {
+#[cfg_attr(feature = "serde-serialize", derive(Serialize, Deserialize))]
+struct PubSubCursor {
+    // Position on the offset array.
+    id: u32,
+    // Index of the next message to read.
+    // NOTE: Having this here is not actually necessary because
+    // this value is supposed to be equal to `offsets[self.id]`.
+    // However, we keep it because it lets us avoid one lookup
+    // on the `offsets` array inside of message-polling loops
+    // based on `read_ith`.
+    next: u32,
+}
+
+impl PubSubCursor {
     fn id(&self, num_deleted: u32) -> usize {
         (self.id - num_deleted) as usize
     }
@@ -29,6 +41,7 @@ pub struct PubSub<T> {
     deleted_offsets: u32,
     messages: VecDeque<T>,
     offsets: VecDeque<u32>,
+    cursors: Vec<PubSubCursor>,
 }
 
 impl<T> PubSub<T> {
@@ -39,9 +52,24 @@ impl<T> PubSub<T> {
             deleted_messages: 0,
             messages: VecDeque::new(),
             offsets: VecDeque::new(),
+            cursors: Vec::new(),
         }
     }
 
+    fn reset_shifts(&mut self) {
+        for offset in &mut self.offsets {
+            *offset -= self.deleted_messages;
+        }
+
+        for cursor in &mut self.cursors {
+            cursor.id -= self.deleted_offsets;
+            cursor.next -= self.deleted_messages;
+        }
+
+        self.deleted_offsets = 0;
+        self.deleted_messages = 0;
+    }
+
     /// Publish a new message.
     pub fn publish(&mut self, message: T) {
         if self.offsets.is_empty() {
@@ -55,25 +83,33 @@ impl<T> PubSub<T> {
     /// Subscribe to the queue.
     ///
     /// A subscription cannot be cancelled.
-    pub fn subscribe(&mut self) -> PubSubCursor<T> {
+    #[must_use]
+    pub fn subscribe(&mut self) -> Subscription<T> {
         let cursor = PubSubCursor {
             next: self.messages.len() as u32 + self.deleted_messages,
             id: self.offsets.len() as u32 + self.deleted_offsets,
+        };
+
+        let subscription = Subscription {
+            id: self.cursors.len() as u32,
             _phantom: PhantomData,
         };
 
         self.offsets.push_back(cursor.next);
-        cursor
+        self.cursors.push(cursor);
+        subscription
     }
 
     /// Read the i-th message not yet read by the given subsciber.
-    pub fn read_ith(&self, cursor: &PubSubCursor<T>, i: usize) -> Option<&T> {
+    pub fn read_ith(&self, sub: &Subscription<T>, i: usize) -> Option<&T> {
+        let cursor = &self.cursors[sub.id as usize];
         self.messages
             .get(cursor.next(self.deleted_messages) as usize + i)
     }
 
     /// Get the messages not yet read by the given subscriber.
-    pub fn read(&self, cursor: &PubSubCursor<T>) -> impl Iterator<Item = &T> {
+    pub fn read(&self, sub: &Subscription<T>) -> impl Iterator<Item = &T> {
+        let cursor = &self.cursors[sub.id as usize];
         let next = cursor.next(self.deleted_messages);
 
         // TODO: use self.queue.range(next..) once it is stabilised.
@@ -86,11 +122,14 @@ impl<T> PubSub<T> {
     /// Makes the given subscribe acknowledge all the messages in the queue.
     ///
     /// A subscriber cannot read acknowledged messages any more.
-    pub fn ack(&mut self, cursor: &mut PubSubCursor<T>) {
+    pub fn ack(&mut self, sub: &Subscription<T>) {
         // Update the cursor.
-        cursor.next = self.messages.len() as u32 + self.deleted_messages;
+        let cursor = &mut self.cursors[sub.id as usize];
+
         self.offsets[cursor.id(self.deleted_offsets)] = u32::MAX;
         cursor.id = self.offsets.len() as u32 + self.deleted_offsets;
+
+        cursor.next = self.messages.len() as u32 + self.deleted_messages;
         self.offsets.push_back(cursor.next);
 
         // Now clear the messages we don't need to
@@ -110,6 +149,12 @@ impl<T> PubSub<T> {
         }
 
         self.deleted_messages += num_to_delete;
+
+        if self.deleted_messages > u32::MAX / 2 || self.deleted_offsets > u32::MAX / 2 {
+            // Don't let the deleted_* shifts grow indefinitely otherwise
+            // they will end up overflowing, breaking everything.
+            self.reset_shifts();
+        }
     }
 }