refactor: remove Mutex around rate limiter

Rate limiter handles all the necessary locking internally.

filtermail/src/outbound.rs

@@ -10,12 +10,12 @@ use async_trait::async_trait;
 use governor::{DefaultKeyedRateLimiter, Quota, RateLimiter};
 use lettre::{AsyncSmtpTransport, AsyncTransport, Tokio1Executor};
 use mailparse::{MailHeaderMap, parse_mail};
-use std::sync::{Arc, Mutex};
+use std::sync::Arc;
 
 /// Handler for outgoing SMTP messages.
 pub struct OutgoingBeforeQueueHandler {
     config: Arc<Config>,
-    send_rate_limiter: Arc<Mutex<DefaultKeyedRateLimiter<String>>>,
+    send_rate_limiter: DefaultKeyedRateLimiter<String>,
 }
 
 impl OutgoingBeforeQueueHandler {
@@ -23,7 +23,7 @@ impl OutgoingBeforeQueueHandler {
         let quota = Quota::per_minute(config.max_user_send_per_minute);
         Self {
             config: Arc::new(config),
-            send_rate_limiter: Arc::new(Mutex::new(RateLimiter::keyed(quota))),
+            send_rate_limiter: RateLimiter::keyed(quota),
         }
     }
 }
@@ -38,11 +38,7 @@ impl SmtpHandler for OutgoingBeforeQueueHandler {
             return Err(format!("500 Invalid from address <{}>", address));
         }
 
-        let Ok(limiter) = self.send_rate_limiter.lock() else {
-            log::error!("send_rate_limiter lock panicked!");
-            return Err("451 Temporary server error".to_string());
-        };
-        if let Err(e) = limiter.check_key(&address.to_string()) {
+        if let Err(e) = self.send_rate_limiter.check_key(&address.to_string()) {
             // "<example@example.org> rate limited until: ..."
             log::debug!("<{address}> {e}");
             return Err(format!("450 4.7.1: Too much mail from <{address}>, {e}"));
@@ -55,7 +51,7 @@ impl SmtpHandler for OutgoingBeforeQueueHandler {
         // In the future, in case of higher traffic this can be further optimized by e.g. calling it
         // every N messages or in a separate task every N minutes.
         // Time complexity is O(n) where n is the number of unique senders in the last minute.
-        limiter.retain_recent();
+        self.send_rate_limiter.retain_recent();
 
         Ok(())
     }