fix(imap): limit the rate of LOGIN attempts rather than connection attempts

As ratelimit was introduced to avoid reconnecting immediately after disconnecting
in case of bugs in IMAP protocol handling,
connection attempts should only be counted when IMAP is actually used,
i.e. when the first command (LOGIN) is sent.

src/imap.rs

@@ -94,6 +94,20 @@ pub struct Imap {
     login_failed_once: bool,
 
     pub(crate) connectivity: ConnectivityStore,
+
+    /// Rate limit for IMAP connection usage attempts.
+    ///
+    /// Rate limit is checked before connecting
+    /// and updated right before login attempt.
+    /// It does not limit the number of connection attempts
+    /// if the network is bad as only successful connections are counted.
+    ///
+    /// Main purpose of this rate limit is
+    /// to prevent busy loop in case
+    /// connection gets dropped over and over due to IMAP bug,
+    /// e.g. the server returning invalid response to SELECT command
+    /// immediately after logging in or returning an error in response to LOGIN command
+    /// due to internal server error.
     ratelimit: RwLock<Ratelimit>,
 }
 
@@ -256,7 +270,7 @@ impl Imap {
             session: None,
             login_failed_once: false,
             connectivity: Default::default(),
-            // 1 connection per minute + a burst of 2.
+            // 1 login per minute + a burst of 2.
             ratelimit: RwLock::new(Ratelimit::new(Duration::new(120, 0), 2.0)),
         };
 
@@ -306,6 +320,12 @@ impl Imap {
         }
 
         self.connectivity.set_connecting(context).await;
+
+        // Check rate limit before trying to connect
+        // to avoid connecting and not using the connection
+        // in case we are currently ratelimited.
+        // Otherwise connection may become unusable due to NAT forgetting about it
+        // before we attempt to actually login.
         let ratelimit_duration = self.ratelimit.read().await.until_can_send();
         if !ratelimit_duration.is_zero() {
             warn!(
@@ -316,10 +336,7 @@ impl Imap {
             tokio::time::sleep(ratelimit_duration).await;
         }
 
-        let oauth2 = self.config.lp.oauth2;
-
         info!(context, "Connecting to IMAP server");
-        self.ratelimit.write().await.send();
         let connection_res: Result<Client> = if self.config.lp.security == Socket::Starttls
             || self.config.lp.security == Socket::Plain
         {
@@ -369,11 +386,13 @@ impl Imap {
                 Client::connect_secure(context, imap_server, imap_port, config.strict_tls).await
             }
         };
-
         let client = connection_res?;
+        self.ratelimit.write().await.send();
+
         let config = &self.config;
         let imap_user: &str = config.lp.user.as_ref();
         let imap_pw: &str = config.lp.password.as_ref();
+        let oauth2 = self.config.lp.oauth2;
 
         let login_res = if oauth2 {
             info!(context, "Logging into IMAP server with OAuth 2");