PQconnectPoll can file descriptor (FD) during the connection process. We need to check whether FD has changed and, if so, update it in the thread's poll array.

lib/PgSQL_Connection.cpp

@@ -884,7 +884,7 @@ void PgSQL_Connection::connect_cont(short event) {
 	case PGRES_POLLING_WRITING:
 		async_exit_status = PG_EVENT_WRITE;
 		break;
-	case PGRES_POLLING_ACTIVE:
+	case PGRES_POLLING_ACTIVE: // Not used
 	case PGRES_POLLING_READING:
 		async_exit_status = PG_EVENT_READ;
 		break;
@@ -897,7 +897,13 @@ void PgSQL_Connection::connect_cont(short event) {
 		const PGresult* result = PQgetResultFromPGconn(pgsql_conn);
 		set_error_from_result(result);
 		proxy_error("Connect failed. %s\n", get_error_code_with_message().c_str());
-		return;
+	}
+	int current_fd = PQsocket(pgsql_conn);
+	if (current_fd != fd) {
+		proxy_warning("PgSQL Connection FD has been changed by PQconnectPoll(). oldFD:%d newFD:%d\n", fd, current_fd);
+		proxy_debug(PROXY_DEBUG_MYSQL_CONNECTION, 5, "PgSQL Connection FD has been changed by PQconnectPoll()"
+			"Session=%p, Conn=%p, myds=%p, oldFD=%d, newFD=%d\n", myds->sess, this, myds, fd, current_fd);
+		fd = current_fd;
 	}
 }
 

lib/PgSQL_Session.cpp

@@ -1744,10 +1744,21 @@ bool PgSQL_Session::handler_again___status_CONNECTING_SERVER(int* _rc) {
 		if (myds->mypolls == NULL) {
 			// connection yet not in mypolls
 			myds->assign_fd_from_mysql_conn();
-			thread->mypolls.add(POLLIN | POLLOUT, mybe->server_myds->fd, mybe->server_myds, curtime);
+			thread->mypolls.add(POLLIN | POLLOUT, myds->fd, myds, curtime);
 			if (mirror) {
 				PROXY_TRACE();
 			}
+		} else {
+			// See Issue#4919 (https://github.com/sysown/proxysql/issues/4919)
+			// File descriptor was already set previously. Let's verify if it has changed
+			if (myds->fd != myconn->fd)
+			{
+				// PQconnectPoll has changed the file descriptor (FD) during the connection process.
+				// We need to update the new FD in mypolls, replacing the old one,
+				// Note: previous FD is closed by PQconnectPoll
+				myds->assign_fd_from_mysql_conn();
+				thread->mypolls.update_fd_at_index(myds->poll_fds_idx, myds->fd);
+			}
 		}
 		switch (rc) {
 		case 0: