Add option to drop pending updates w/ polling

Former-commit-id: 612f47d242

src/dispatching/update_listeners/polling.rs

@@ -27,6 +27,7 @@ pub struct PollingBuilder<R> {
     timeout: Option<Duration>,
     limit: Option<u8>,
     allowed_updates: Option<Vec<AllowedUpdate>>,
+    drop_pending_updates: bool,
 }
 
 impl<R> PollingBuilder<R>
@@ -74,6 +75,11 @@ where
         Self { allowed_updates: Some(allowed_updates), ..self }
     }
 
+    /// Drops pending updates.
+    pub fn drop_pending_updates(self) -> Self {
+        Self { drop_pending_updates: true, ..self }
+    }
+
     /// Deletes webhook if it was set up.
     pub async fn delete_webhook(self) -> Self {
         delete_webhook_if_setup(&self.bot).await;
@@ -86,9 +92,9 @@ where
     ///
     /// See also: [`polling_default`], [`Polling`].
     pub fn build(self) -> Polling<R> {
-        let Self { bot, timeout, limit, allowed_updates } = self;
+        let Self { bot, timeout, limit, allowed_updates, drop_pending_updates } = self;
         let (token, flag) = AsyncStopToken::new_pair();
-        Polling { bot, timeout, limit, allowed_updates, flag, token }
+        Polling { bot, timeout, limit, allowed_updates, drop_pending_updates, flag, token }
     }
 }
 
@@ -98,7 +104,13 @@ where
     R: Requester + Send + 'static,
     <R as Requester>::GetUpdates: Send,
 {
-    PollingBuilder { bot, timeout: None, limit: None, allowed_updates: None }
+    PollingBuilder {
+        bot,
+        timeout: None,
+        limit: None,
+        allowed_updates: None,
+        drop_pending_updates: false,
+    }
 }
 
 /// Returns a long polling update listener with `timeout` of 10 seconds.
@@ -232,6 +244,7 @@ pub struct Polling<B: Requester> {
     timeout: Option<Duration>,
     limit: Option<u8>,
     allowed_updates: Option<Vec<AllowedUpdate>>,
+    drop_pending_updates: bool,
     flag: AsyncStopFlag,
     token: AsyncStopToken,
 }
@@ -241,6 +254,9 @@ pub struct PollingStream<'a, B: Requester> {
     /// Parent structure
     polling: &'a mut Polling<B>,
 
+    /// Whatever to drop pending updates or not.
+    drop_pending_updates: bool,
+
     /// Timeout parameter for normal `get_updates()` calls.
     timeout: Option<u32>,
     /// Allowed updates parameter for the first `get_updates()` call.
@@ -285,8 +301,10 @@ impl<'a, B: Requester + Send + 'a> AsUpdateStream<'a, B::Err> for Polling<B> {
     fn as_stream(&'a mut self) -> Self::Stream {
         let timeout = self.timeout.map(|t| t.as_secs().try_into().expect("timeout is too big"));
         let allowed_updates = self.allowed_updates.clone();
+        let drop_pending_updates = self.drop_pending_updates;
         PollingStream {
             polling: self,
+            drop_pending_updates,
             timeout,
             allowed_updates,
             offset: 0,
@@ -326,7 +344,10 @@ impl<B: Requester> Stream for PollingStream<'_, B> {
                         *this.offset = upd.id + 1;
                     }
 
-                    *this.buffer = updates.into_iter();
+                    match *this.drop_pending_updates {
+                        false => *this.buffer = updates.into_iter(),
+                        true => *this.drop_pending_updates = false,
+                    }
                 }
                 Err(err) => return Ready(Some(Err(err))),
             }
@@ -337,13 +358,18 @@ impl<B: Requester> Stream for PollingStream<'_, B> {
             return Ready(Some(Ok(upd)));
         }
 
-        // When stopping we set `timeout = 0` and `limit = 1` so that `get_updates()`
-        // set last seen update (offset) and return immediately
-        let (timeout, limit) = if this.polling.flag.is_stopped() {
-            *this.stopping = true;
-            (Some(0), Some(1))
-        } else {
-            (*this.timeout, this.polling.limit)
+        *this.stopping = this.polling.flag.is_stopped();
+        let (offset, limit, timeout) = match (this.stopping, this.drop_pending_updates) {
+            // Normal `get_updates()` call
+            (false, false) => (*this.offset, this.polling.limit, *this.timeout),
+            // Graceful shutdown `get_updates()` call (shutdown takes priority over dropping pending
+            // updates)
+            //
+            // When stopping we set `timeout = 0` and `limit = 1` so that `get_updates()`
+            // set last seen update (offset) and return immediately
+            (true, _) => (*this.offset, Some(1), Some(0)),
+            // Drop pending updates
+            (_, true) => (-1, Some(1), Some(0)),
         };
 
         let req = this
@@ -351,7 +377,7 @@ impl<B: Requester> Stream for PollingStream<'_, B> {
             .bot
             .get_updates()
             .with_payload_mut(|pay| {
-                pay.offset = Some(*this.offset);
+                pay.offset = Some(offset);
                 pay.timeout = timeout;
                 pay.limit = limit;
                 pay.allowed_updates = this.allowed_updates.take();