feat: Ratelimit IMAP connections (#4940)

Limit the number of IMAP connections to 1 per minute regardless of the reason of reconnection, but
allow one immediate retry. This is more reliable than ratelimiting only in error conditions because
ratelimiting can't be skipped by mistake. Anyway connections shouldn't be frequent in normal
operation mode.

src/imap.rs

@@ -9,6 +9,7 @@ use std::{
     collections::{BTreeMap, BTreeSet, HashMap},
     iter::Peekable,
     mem::take,
+    time::Duration,
 };
 
 use anyhow::{bail, format_err, Context as _, Result};
@@ -16,6 +17,8 @@ use async_channel::Receiver;
 use async_imap::types::{Fetch, Flag, Name, NameAttribute, UnsolicitedResponse};
 use futures::{StreamExt, TryStreamExt};
 use num_traits::FromPrimitive;
+use ratelimit::Ratelimit;
+use tokio::sync::RwLock;
 
 use crate::chat::{self, ChatId, ChatIdBlocked};
 use crate::config::Config;
@@ -38,7 +41,7 @@ use crate::scheduler::connectivity::ConnectivityStore;
 use crate::socks::Socks5Config;
 use crate::sql;
 use crate::stock_str;
-use crate::tools::create_id;
+use crate::tools::{create_id, duration_to_str};
 
 pub(crate) mod capabilities;
 mod client;
@@ -91,6 +94,7 @@ pub struct Imap {
     login_failed_once: bool,
 
     pub(crate) connectivity: ConnectivityStore,
+    ratelimit: RwLock<Ratelimit>,
 }
 
 #[derive(Debug)]
@@ -252,6 +256,8 @@ impl Imap {
             session: None,
             login_failed_once: false,
             connectivity: Default::default(),
+            // 1 connection per minute + a burst of 2.
+            ratelimit: RwLock::new(Ratelimit::new(Duration::new(120, 0), 2.0)),
         };
 
         Ok(imap)
@@ -300,10 +306,20 @@ impl Imap {
         }
 
         self.connectivity.set_connecting(context).await;
+        let ratelimit_duration = self.ratelimit.read().await.until_can_send();
+        if !ratelimit_duration.is_zero() {
+            warn!(
+                context,
+                "IMAP got rate limited, waiting for {} until can connect",
+                duration_to_str(ratelimit_duration),
+            );
+            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
         {