log message when BackoffGuard is dropped without recording result

src/service/server_backoff.rs

@@ -6,7 +6,7 @@ use std::{
 
 use rand::{thread_rng, Rng};
 use ruma::{OwnedServerName, ServerName};
-use tracing::{debug, info, instrument};
+use tracing::{debug, error, info, instrument};
 
 use crate::{observability::METRICS, services, Error, Result};
 
@@ -52,6 +52,7 @@ pub(crate) struct Service {
 /// to it.
 #[must_use]
 pub(crate) struct BackoffGuard {
+    result_recorded: bool,
     backoff: Arc<RwLock<BackoffState>>,
     /// Store the last failure timestamp observed when this request started. If
     /// there was another failure recorded since the request started, do not
@@ -127,6 +128,7 @@ impl Service {
         };
 
         Ok(BackoffGuard {
+            result_recorded: false,
             backoff: state,
             last_failure,
         })
@@ -239,7 +241,9 @@ impl BackoffState {
 impl BackoffGuard {
     /// Record a successful request.
     #[instrument(skip(self))]
-    pub(crate) fn success(self) {
+    pub(crate) fn success(mut self) {
+        self.result_recorded = true;
+
         let mut state = self.backoff.write().unwrap();
         let was_online = state.is_online();
 
@@ -266,7 +270,9 @@ impl BackoffGuard {
     /// Examples of failures in this category are a timeout, a 500 status, or
     /// a 404 from an endpoint that is not specced to return 404.
     #[instrument(skip(self))]
-    pub(crate) fn hard_failure(self) {
+    pub(crate) fn hard_failure(mut self) {
+        self.result_recorded = true;
+
         let config = &services().globals.config.federation.backoff;
 
         let mut state = self.backoff.write().unwrap();
@@ -299,9 +305,18 @@ impl BackoffGuard {
     /// An example of a failure in this category is 404 from querying a user
     /// profile. This might occur if the server no longer exists, but will also
     /// occur if the userid doesn't exist.
-    // Taking `self` here is intentional, to allow callers to destroy the guard
-    // without triggering the `must_use` warning.
-    #[allow(clippy::unused_self)]
     #[instrument(skip(self))]
-    pub(crate) fn soft_failure(self) {}
+    pub(crate) fn soft_failure(mut self) {
+        self.result_recorded = true;
+    }
+}
+
+impl Drop for BackoffGuard {
+    fn drop(&mut self) {
+        if !self.result_recorded {
+            error!(
+                "BackoffGuard dropped without recording result. This is a bug."
+            );
+        }
+    }
 }