transport: silence "broken pipe" and "connection reset by peer" errors

The code would already silence broken pipe exceptions since it's
expected when the other side closes the connection or when we shutdown the
socket during Scylla shutdown, but the code wouldn't handle the following:
1. "Connection reset by peer" errors: these can also happen in the
   aforementioned two scenarios; the conditions that determine which of
   the two types of errors occur are unclear.
2. The scenarios would sometimes result in a `seastar::nested_exception`,
   mainly during shutdown. The errors could happen once when trying to send
   a response to a request (`_write_buf.write(...)/flush(...)`) and then
   again when trying to close the connection in a `finally` block. These
   nested exceptions were not silenced.

The commit handles each of these cases.
Closes #7907.

Closes #7931

transport/server.cc

@@ -134,6 +134,16 @@ sstring to_string(const event::schema_change::target_type t) {
     assert(false && "unreachable");
 }
 
+static bool is_broken_pipe_or_connection_reset(std::exception_ptr ep) {
+    try {
+        std::rethrow_exception(ep);
+    } catch (const std::system_error& e) {
+        return e.code().category() == std::system_category()
+            && (e.code().value() == EPIPE || e.code().value() == ECONNRESET);
+    } catch (...) {}
+    return false;
+}
+
 event::event_type parse_event_type(const sstring& value)
 {
     if (value == "TOPOLOGY_CHANGE") {
@@ -320,14 +330,10 @@ cql_server::do_accepts(int which, bool keepalive, socket_address server_addr) {
                     --_stats.connections;
                     return unadvertise_connection(conn);
                 }).handle_exception([] (std::exception_ptr ep) {
-                    try {
-                        std::rethrow_exception(ep);
-                    } catch(std::system_error& serr) {
-                        if (serr.code().category() ==  std::system_category() &&
-                                serr.code().value() == EPIPE) {  // expected if another side closes a connection
-                            return;
-                        }
-                    } catch(...) {}
+                    if (is_broken_pipe_or_connection_reset(ep)) {
+                        // expected if another side closes a connection or we're shutting down
+                        return;
+                    }
                     clogger.info("exception while processing connection: {}", ep);
                 });
             });
@@ -628,8 +634,14 @@ future<> cql_server::connection::process()
     }).finally([this] {
         return _pending_requests_gate.close().then([this] {
             _server._notifier->unregister_connection(this);
-            return _ready_to_respond.finally([this] {
-                return _write_buf.close();
+            return _ready_to_respond.handle_exception([] (std::exception_ptr ep) {
+                if (is_broken_pipe_or_connection_reset(ep)) {
+                    // expected if another side closes a connection or we're shutting down
+                    return;
+                }
+                std::rethrow_exception(ep);
+            }).finally([this] {
+                 return _write_buf.close();
             });
         });
     });