fix: do not miss new messages while expunging the folder

This should fix flaky `test_verified_group_vs_delete_server_after`.

src/imap/idle.rs

@@ -29,9 +29,13 @@ impl Session {
     ) -> Result<Self> {
         use futures::future::FutureExt;
 
-        self.select_folder(context, folder).await?;
+        self.select_with_uidvalidity(context, folder).await?;
 
         if self.server_sent_unsolicited_exists(context)? {
+            self.new_mail = true;
+        }
+
+        if self.new_mail {
             return Ok(self);
         }
 
@@ -92,6 +96,9 @@ impl Session {
         session.as_mut().set_read_timeout(Some(IMAP_TIMEOUT));
         self.inner = session;
 
+        // Fetch mail once we exit IDLE.
+        self.new_mail = true;
+
         Ok(self)
     }
 }

src/imap/select_folder.rs

@@ -39,6 +39,7 @@ impl ImapSession {
                 info!(context, "close/expunge succeeded");
                 self.selected_folder = None;
                 self.selected_folder_needs_expunge = false;
+                self.new_mail = false;
             }
         }
         Ok(())
@@ -47,11 +48,7 @@ impl ImapSession {
     /// Selects a folder, possibly updating uid_validity and, if needed,
     /// expunging the folder to remove delete-marked messages.
     /// Returns whether a new folder was selected.
-    pub(crate) async fn select_folder(
-        &mut self,
-        context: &Context,
-        folder: &str,
-    ) -> Result<NewlySelected> {
+    async fn select_folder(&mut self, context: &Context, folder: &str) -> Result<NewlySelected> {
         // if there is a new folder and the new folder is equal to the selected one, there's nothing to do.
         // if there is _no_ new folder, we continue as we might want to expunge below.
         if let Some(selected_folder) = &self.selected_folder {
@@ -119,13 +116,14 @@ impl ImapSession {
     /// When selecting a folder for the first time, sets the uid_next to the current
     /// mailbox.uid_next so that no old emails are fetched.
     ///
-    /// Returns Result<new_emails> (i.e. whether new emails arrived),
-    /// if in doubt, returns new_emails=true so emails are fetched.
+    /// Updates `self.new_mail` if folder was previously unselected
+    /// and new mails are detected after selecting,
+    /// i.e. UIDNEXT advanced while the folder was closed.
     pub(crate) async fn select_with_uidvalidity(
         &mut self,
         context: &Context,
         folder: &str,
-    ) -> Result<bool> {
+    ) -> Result<()> {
         let newly_selected = self
             .select_or_create_folder(context, folder)
             .await
@@ -182,28 +180,26 @@ impl ImapSession {
         mailbox.uid_next = new_uid_next;
 
         if new_uid_validity == old_uid_validity {
-            let new_emails = if newly_selected == NewlySelected::No {
-                // The folder was not newly selected i.e. no SELECT command was run. This means that mailbox.uid_next
-                // was not updated and may contain an incorrect value. So, just return true so that
-                // the caller tries to fetch new messages (we could of course run a SELECT command now, but trying to fetch
-                // new messages is only one command, just as a SELECT command)
-                true
-            } else if let Some(new_uid_next) = new_uid_next {
-                if new_uid_next < old_uid_next {
-                    warn!(
-                        context,
-                        "The server illegally decreased the uid_next of folder {folder:?} from {old_uid_next} to {new_uid_next} without changing validity ({new_uid_validity}), resyncing UIDs...",
-                    );
-                    set_uid_next(context, folder, new_uid_next).await?;
-                    context.schedule_resync().await?;
-                }
-                new_uid_next != old_uid_next // If UIDNEXT changed, there are new emails
-            } else {
-                // We have no UIDNEXT and if in doubt, return true.
-                true
-            };
+            if newly_selected == NewlySelected::Yes {
+                if let Some(new_uid_next) = new_uid_next {
+                    if new_uid_next < old_uid_next {
+                        warn!(
+                            context,
+                            "The server illegally decreased the uid_next of folder {folder:?} from {old_uid_next} to {new_uid_next} without changing validity ({new_uid_validity}), resyncing UIDs...",
+                        );
+                        set_uid_next(context, folder, new_uid_next).await?;
+                        context.schedule_resync().await?;
+                    }
 
-            return Ok(new_emails);
+                    // If UIDNEXT changed, there are new emails.
+                    self.new_mail |= new_uid_next != old_uid_next;
+                } else {
+                    warn!(context, "Folder {folder} was just selected but we failed to determine UIDNEXT, assume that it has new mail.");
+                    self.new_mail = true;
+                }
+            }
+
+            return Ok(());
         }
 
         // UIDVALIDITY is modified, reset highest seen MODSEQ.
@@ -214,6 +210,7 @@ impl ImapSession {
         let new_uid_next = new_uid_next.unwrap_or_default();
         set_uid_next(context, folder, new_uid_next).await?;
         set_uidvalidity(context, folder, new_uid_validity).await?;
+        self.new_mail = true;
 
         // Collect garbage entries in `imap` table.
         context
@@ -236,7 +233,7 @@ impl ImapSession {
             old_uid_next,
             old_uid_validity,
         );
-        Ok(false)
+        Ok(())
     }
 }
 

src/imap/session.rs

@@ -40,6 +40,11 @@ pub(crate) struct Session {
     pub selected_mailbox: Option<Mailbox>,
 
     pub selected_folder_needs_expunge: bool,
+
+    /// True if currently selected folder has new messages.
+    ///
+    /// Should be false if no folder is currently selected.
+    pub new_mail: bool,
 }
 
 impl Deref for Session {
@@ -67,6 +72,7 @@ impl Session {
             selected_folder: None,
             selected_mailbox: None,
             selected_folder_needs_expunge: false,
+            new_mail: false,
         }
     }