fix: Don't fall into infinite loop if the folder is missing (#8021)

Previously, if the mvbox_move folder is missing, then core will loop
infinitely, because `new_mail` is never set to false.

The fix is to first set `new_mail` to false, then return if the folder
is missing.

This is the bug @hpk42 experienced when commenting in
https://github.com/chatmail/core/issues/7989

---------

Co-authored-by: holger krekel <holger@merlinux.eu>

src/imap.rs

@@ -561,16 +561,19 @@ impl Imap {
             .select_with_uidvalidity(context, folder)
             .await
             .with_context(|| format!("Failed to select folder {folder:?}"))?;
-        if !folder_exists {
-            return Ok(false);
-        }
 
         if !session.new_mail {
             info!(context, "No new emails in folder {folder:?}.");
             return Ok(false);
         }
+        // Make sure not to return before setting new_mail to false
+        // Otherwise, we will skip IDLE and go into an infinite loop
         session.new_mail = false;
 
+        if !folder_exists {
+            return Ok(false);
+        }
+
         let mut read_cnt = 0;
         loop {
             let (n, fetch_more) = self
@@ -1237,6 +1240,7 @@ impl Session {
             // have been modified while our request was in progress.
             // We may or may not have these new flags as a part of the response,
             // so better skip next IDLE and do another round of flag synchronization.
+            info!(context, "Got unsolicited fetch, will skip idle");
             self.new_mail = true;
         }