From 2fbcdfd4b4a9ccd792254cf11e2a7a73d7c9fdb7 Mon Sep 17 00:00:00 2001
From: Dario Mirovic <dario.mirovic@scylladb.com>
Date: Tue, 15 Jul 2025 20:25:20 +0200
Subject: [PATCH] transport/server: avoid exception-throw overhead in
 handle_error

Previously, connection::handle_error always called f.get() inside a try/catch,
forcing every failed future to throw and immediately catch an exception just to
classify it. This change eliminates that extra throw/catch cycle by first checking
f.failed(), getting the stored std::exception_ptr via f.get_exception(), and
then dispatching on its type via utils::try_catch<T>(eptr).

The error-response logic is not changed - cassandra_exception, std::exception,
and unknown exceptions are caught and processed, and any exceptions thrown by
write_response while handling those exceptions continues to escape handle_error.

Refs: #24567
Fixes: #25273
(cherry picked from commit 30d424e0d3468a8566adcd669f5f02bd5c011bd0)
---
 transport/server.cc | 24 ++++++++++++++----------
 1 file changed, 14 insertions(+), 10 deletions(-)

diff --git a/transport/server.cc b/transport/server.cc
index 870f6d18d7..598dd1e1c9 100644
--- a/transport/server.cc
+++ b/transport/server.cc
@@ -664,17 +664,21 @@ thread_local cql_server::connection::execution_stage_type
         cql_server::connection::_process_request_stage{"transport", &connection::process_request_one};
 
 void cql_server::connection::handle_error(future<>&& f) {
-    try {
-        f.get();
-    } catch (const exceptions::cassandra_exception& ex) {
-        clogger.debug("{}: connection error, code {}, message [{}]", _client_state.get_remote_address(), ex.code(), ex.what());
-        try { ++_server._stats.errors[ex.code()]; } catch(...) {}
-        write_response(make_error(0, ex.code(), ex.what(), tracing::trace_state_ptr()));
-    } catch (std::exception& ex) {
-        clogger.debug("{}: connection error, message [{}]", _client_state.get_remote_address(), ex.what());
+    if (!f.failed()) {
+        return;
+    }
+
+    std::exception_ptr eptr = f.get_exception();
+
+    if (auto* ex = try_catch<exceptions::cassandra_exception>(eptr)) {
+        clogger.debug("{}: connection error, code {}, message [{}]", _client_state.get_remote_address(), ex->code(), ex->what());
+        try { ++_server._stats.errors[ex->code()]; } catch(...) {}
+        write_response(make_error(0, ex->code(), ex->what(), tracing::trace_state_ptr()));
+    } else if (auto* ex = try_catch<std::exception>(eptr)) {
+        clogger.debug("{}: connection error, message [{}]", _client_state.get_remote_address(), ex->what());
         try { ++_server._stats.errors[exceptions::exception_code::SERVER_ERROR]; } catch(...) {}
-        write_response(make_error(0, exceptions::exception_code::SERVER_ERROR, ex.what(), tracing::trace_state_ptr()));
-    } catch (...) {
+        write_response(make_error(0, exceptions::exception_code::SERVER_ERROR, ex->what(), tracing::trace_state_ptr()));
+    } else {
         clogger.debug("{}: connection error, unknown error", _client_state.get_remote_address());
         try { ++_server._stats.errors[exceptions::exception_code::SERVER_ERROR]; } catch(...) {}
         write_response(make_error(0, exceptions::exception_code::SERVER_ERROR, "unknown error", tracing::trace_state_ptr()));