feat: Mark servers as online/offline based on transaction responses

src/service/sending/mod.rs

@@ -488,6 +488,8 @@ pub fn hit_unhealthy(&self, server_name: OwnedServerName) {
 	///
 	/// TODO: flush senders too
 	pub fn mark_healthy(&self, server_name: &ServerName) {
+		// TODO: We need to make sure the sender flush DOESN'T trigger if this is called
+		// by the senders themselves.
 		let mut map = self.remote_health.write();
 		map.remove(server_name);
 	}

src/service/sending/sender.rs

@@ -142,7 +142,14 @@ async fn handle_response<'a>(
 	) {
 		match response {
 			| Ok(dest) => self.handle_response_ok(&dest, futures, statuses).await,
-			| Err((dest, e)) => Self::handle_response_err(dest, statuses, &e),
+			| Err((dest, e)) => {
+				if e.status_code().is_server_error()
+					&& let Destination::Federation(dest) = &dest
+				{
+					self.hit_unhealthy(dest.clone());
+				}
+				Self::handle_response_err(dest, statuses, &e);
+			},
 		}
 	}
 
@@ -171,6 +178,9 @@ async fn handle_response_ok<'a>(
 	) {
 		let _cork = self.db.db.cork();
 		self.db.delete_all_active_requests_for(dest).await;
+		if let Destination::Federation(server_name) = dest {
+			self.mark_healthy(server_name);
+		}
 
 		// Find events that have been added since starting the last request
 		let new_events = self