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The patch detects if the processed PREPARE command is first part of the pipelined DIRECT+EXECUTE commands with stmt id -1 for execution. For that the vio class has been extended with peek method that lets us verify that the server has already received next command, it is EXECUTE with stmt id -1, and the packet is minimally complete. If it is so, the execution is forwarded to the fused prepare+execute path as if it was a single command. It lets a statement's execute leg run before any real prepare/check_prepared_statement() has happened and the prepare leg does basically query parsing only. Several places downstream assumed that had already happened, so give Query_arena a single predicate, is_direct_exec_unresolved() - true only for the one-shot fused pass that never went through a real prepare - and fix each broken assumption against it: * Query cache: a cached response is replayed without ever running mysql_execute_command(), so the buffered prepare-response hook never fires; conversely, populating the cache captures that hook's bytes too (net_real_write() sees the whole flushed buffer), poisoning the entry for unrelated later clients. Direct-exec statements now opt out of the query cache entirely (query_cache_is_applicable / lex->safe_to_cache_query, saved and restored around just that one execution). Covered by test_mdev_16670_qc_hit_hang and test_mdev_16670_qc_poison. * Reprepare: a mid-execute reprepare() (e.g. ER_NEED_REPREPARE) swaps in a lex that DID go through a real prepare, so the statement is no longer "unresolved" once that happens - tracked via a new IS_DIRECT_EXEC_REPREPARED flag so the direct-exec special-casing only applies to the original pass. * System versioning / periods: skip_setup_conds() treated the fused pass as an ordinary re-execution rather than a first resolution, so vers_setup_conds()/period_setup_conds() never ran - a versioned table or FOR PORTION OF statement used the wrong predicate. Now counted as "first execute" via is_direct_exec_unresolved(). Covered by test_mdev_16670_versioned_rows and test_mdev_16670_trigger_versioned. * Unsupported statements check used to be done as part of the check_prepared_statement, has been moved to the new check_stmt_command_supported function called from the Prepared_statement::prepare method. Covered by test_mdev_16670_unsupported_ps. * CALL statements and queries referencing a stored function are excluded from the optimized path (skip_direct_exec_optimization()): their nested sub-statements run with thd->cur_stmt still pointing at the outer statement, which would otherwise misfire the buffered prepare-response hook. Also, the stored function use would cause guaranteed re-prepare. The flag is reset for them after the query parsing. Covered by test_mdev_16670_demoted_function_call. Also adds regression tests for the reply-pair on parse error (test_mdev_16670_parse_error_reply_pair), SQLSTATE/errno preservation through the execute-leg fallback (test_mdev_16670_fallback_sqlstate), general_log coverage (test_mdev_16670_general_log), a statement killed mid-fallback (test_mdev_16670_kill_during_fallback), and prebind usage of mariadb_stmt_execute_direct() with real bound parameters (test_mdev_16670_prebind_direct). It's possible to compile with global variable opt_exec_direct that controls if the optimization should be used. Its existence can be switched on using cmake option WITH_DIRECT_EXEC_OPTIMIZE_SWITCH. By default it does not exist, as it is believed to be unneeded in the general case. create_and_prepare_stmt() helper added to perform common statement creation routine, and erase_stmt_safely() helper - for cleaning in create_and_prepare_stmt()'s prepare-failure cleanup, direct_exec_prepare_ok_fallback()'s OOM branch, and mysqld_stmt_direct_execute()'s packet_error branch. Assisted-by: Claude Sonnet 5