refactor(imap): remove Session from Imap structure

Connection establishment now happens only in one place in each IMAP loop.
Now all connection establishment happens in one place
and is limited by the ratelimit.

Backoff was removed from fake_idle
as it does not establish connections anymore.
If connection fails, fake_idle will return an error.
We then drop the connection and get back to the beginning of IMAP
loop.

Backoff may be still nice to have to delay retries
in case of constant connection failures
so we don't immediately hit ratelimit if the network is unusable
and returns immediate error on each connection attempt
(e.g. ICMP network unreachable error),
but adding backoff for connection failures is out of scope for this change.

src/imap/scan_folders.rs

@@ -4,14 +4,18 @@ use anyhow::{Context as _, Result};
 
 use super::{get_folder_meaning_by_attrs, get_folder_meaning_by_name};
 use crate::config::Config;
-use crate::imap::Imap;
+use crate::imap::{session::Session, Imap};
 use crate::log::LogExt;
 use crate::tools::{self, time_elapsed};
 use crate::{context::Context, imap::FolderMeaning};
 
 impl Imap {
     /// Returns true if folders were scanned, false if scanning was postponed.
-    pub(crate) async fn scan_folders(&mut self, context: &Context) -> Result<bool> {
+    pub(crate) async fn scan_folders(
+        &mut self,
+        context: &Context,
+        session: &mut Session,
+    ) -> Result<bool> {
         // First of all, debounce to once per minute:
         let mut last_scan = context.last_full_folder_scan.lock().await;
         if let Some(last_scan) = *last_scan {
@@ -26,8 +30,6 @@ impl Imap {
         }
         info!(context, "Starting full folder scan");
 
-        self.prepare(context).await?;
-        let session = self.session.as_mut().context("No IMAP session")?;
         let folders = session.list_folders().await?;
         let watched_folders = get_watched_folders(context).await?;
 
@@ -64,18 +66,16 @@ impl Imap {
                 && folder_meaning != FolderMeaning::Drafts
                 && folder_meaning != FolderMeaning::Trash
             {
-                let session = self.session.as_mut().context("no session")?;
                 // Drain leftover unsolicited EXISTS messages
                 session.server_sent_unsolicited_exists(context)?;
 
                 loop {
-                    self.fetch_move_delete(context, folder.name(), folder_meaning)
+                    self.fetch_move_delete(context, session, folder.name(), folder_meaning)
                         .await
                         .context("Can't fetch new msgs in scanned folder")
                         .log_err(context)
                         .ok();
 
-                    let session = self.session.as_mut().context("no session")?;
                     // If the server sent an unsocicited EXISTS during the fetch, we need to fetch again
                     if !session.server_sent_unsolicited_exists(context)? {
                         break;