fix: Prefetch messages in limited batches (#6915)

I have logs from a user where messages are prefetched for long minutes, and while it's not a problem
on its own, we can't rely that the connection overlives such a period, so make
`fetch_new_messages()` prefetch (and then actually download) messages in batches of 500 messages.

src/imap.rs

@@ -566,10 +566,38 @@ impl Imap {
         }
         session.new_mail = false;
 
+        let mut read_cnt = 0;
+        loop {
+            let (n, fetch_more) = self
+                .fetch_new_msg_batch(context, session, folder, folder_meaning)
+                .await?;
+            read_cnt += n;
+            if !fetch_more {
+                return Ok(read_cnt > 0);
+            }
+        }
+    }
+
+    /// Returns number of messages processed and whether the function should be called again.
+    async fn fetch_new_msg_batch(
+        &mut self,
+        context: &Context,
+        session: &mut Session,
+        folder: &str,
+        folder_meaning: FolderMeaning,
+    ) -> Result<(usize, bool)> {
         let uid_validity = get_uidvalidity(context, folder).await?;
         let old_uid_next = get_uid_next(context, folder).await?;
+        info!(
+            context,
+            "fetch_new_msg_batch({folder}): UIDVALIDITY={uid_validity}, UIDNEXT={old_uid_next}."
+        );
 
-        let msgs = session.prefetch(old_uid_next).await.context("prefetch")?;
+        let uids_to_prefetch = 500;
+        let msgs = session
+            .prefetch(old_uid_next, uids_to_prefetch)
+            .await
+            .context("prefetch")?;
         let read_cnt = msgs.len();
 
         let download_limit = context.download_limit().await?;
@@ -729,7 +757,8 @@ impl Imap {
             largest_uid_fetched
         };
 
-        let actually_download_messages_future = async move {
+        let actually_download_messages_future = async {
+            let sender = sender;
             let mut uids_fetch_in_batch = Vec::with_capacity(max(uids_fetch.len(), 1));
             let mut fetch_partially = false;
             uids_fetch.push((0, !uids_fetch.last().unwrap_or(&(0, false)).1));
@@ -764,14 +793,17 @@ impl Imap {
         // if the message has arrived after selecting mailbox
         // and determining its UIDNEXT and before prefetch.
         let mut new_uid_next = largest_uid_fetched + 1;
-        if fetch_res.is_ok() {
+        let fetch_more = fetch_res.is_ok() && {
+            let prefetch_uid_next = old_uid_next + uids_to_prefetch;
             // If we have successfully fetched all messages we planned during prefetch,
             // then we have covered at least the range between old UIDNEXT
             // and UIDNEXT of the mailbox at the time of selecting it.
-            new_uid_next = max(new_uid_next, mailbox_uid_next);
+            new_uid_next = max(new_uid_next, min(prefetch_uid_next, mailbox_uid_next));
 
             new_uid_next = max(new_uid_next, largest_uid_skipped.unwrap_or(0) + 1);
-        }
+
+            prefetch_uid_next < mailbox_uid_next
+        };
         if new_uid_next > old_uid_next {
             set_uid_next(context, folder, new_uid_next).await?;
         }
@@ -788,7 +820,7 @@ impl Imap {
         // establish a new session if this one is broken.
         fetch_res?;
 
-        Ok(read_cnt > 0)
+        Ok((read_cnt, fetch_more))
     }
 
     /// Read the recipients from old emails sent by the user and add them as contacts.

src/imap/session.rs

@@ -110,14 +110,16 @@ impl Session {
         Ok(list)
     }
 
-    /// Prefetch all messages greater than or equal to `uid_next`. Returns a list of fetch results
+    /// Prefetch `n_uids` messages starting from `uid_next`. Returns a list of fetch results in the
-    /// in the order of ascending delivery time to the server (INTERNALDATE).
+    /// order of ascending delivery time to the server (INTERNALDATE).
     pub(crate) async fn prefetch(
         &mut self,
         uid_next: u32,
+        n_uids: u32,
     ) -> Result<Vec<(u32, async_imap::types::Fetch)>> {
+        let uid_last = uid_next.saturating_add(n_uids - 1);
         // fetch messages with larger UID than the last one seen
-        let set = format!("{uid_next}:*");
+        let set = format!("{uid_next}:{uid_last}");
         let mut list = self
             .uid_fetch(set, PREFETCH_FLAGS)
             .await
@@ -126,16 +128,7 @@ impl Session {
         let mut msgs = BTreeMap::new();
         while let Some(msg) = list.try_next().await? {
             if let Some(msg_uid) = msg.uid {
-                // If the mailbox is not empty, results always include
+                msgs.insert((msg.internal_date(), msg_uid), msg);
-                // at least one UID, even if last_seen_uid+1 is past
-                // the last UID in the mailbox.  It happens because
-                // uid:* is interpreted the same way as *:uid.
-                // See <https://tools.ietf.org/html/rfc3501#page-61> for
-                // standard reference. Therefore, sometimes we receive
-                // already seen messages and have to filter them out.
-                if msg_uid >= uid_next {
-                    msgs.insert((msg.internal_date(), msg_uid), msg);
-                }
             }
         }