MDEV-27306: SET STATEMENT optimizer_trace=1 Doesn't save the trace

In mysql_execute_command(), move optimizer trace initialization to be
after run_set_statement_if_requested() call.

Unfortunately, mysql_execute_command() code uses "goto error" a lot, and
this means optimizer trace code cannot use RAII objects. Work this around
by:
- Make Opt_trace_start a non-RAII object, add init() method.
- Move the code that writes the top-level object and array into
  Opt_trace_start::init().

mysql-test/main/opt_trace.result

@@ -9248,5 +9248,22 @@ json_detailed(json_extract(trace, '$**.best_join_order'))
     ]
 ]
 DROP TABLE t1;
+#
+# MDEV-27306: SET STATEMENT optimizer_trace=1 Doesn't save the trace
+#
+set optimizer_trace=0;
+set statement optimizer_trace=1 for select * from seq_1_to_10 where seq<2;
+seq
+1
+# The trace must not be empty:
+select left(trace, 100) from information_schema.optimizer_trace;
+left(trace, 100)
+{
+  "steps": [
+    {
+      "join_preparation": {
+        "select_id": 1,
+        "steps": [
+        
 # End of 10.6 tests
 set optimizer_trace='enabled=off';

mysql-test/main/opt_trace.test

@@ -865,5 +865,13 @@ select json_detailed(json_extract(trace, '$**.best_join_order'))
     from information_schema.OPTIMIZER_TRACE;
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-27306: SET STATEMENT optimizer_trace=1 Doesn't save the trace
+--echo #
+set optimizer_trace=0;
+set statement optimizer_trace=1 for select * from seq_1_to_10 where seq<2;
+--echo # The trace must not be empty:
+select left(trace, 100) from information_schema.optimizer_trace;
+
 --echo # End of 10.6 tests
 set optimizer_trace='enabled=off';

sql/opt_trace.cc

@@ -471,12 +471,14 @@ void Opt_trace_context::end()
   current_trace= NULL;
 }
 
-Opt_trace_start::Opt_trace_start(THD *thd, TABLE_LIST *tbl,
+
+void Opt_trace_start::init(THD *thd,
+                           TABLE_LIST *tbl,
                            enum enum_sql_command sql_command,
                            List<set_var_base> *set_vars,
                            const char *query,
                            size_t query_length,
-                  const CHARSET_INFO *query_charset):ctx(&thd->opt_trace)
+                           const CHARSET_INFO *query_charset)
 {
   /*
     if optimizer trace is enabled and the statment we have is traceable,
@@ -496,6 +498,9 @@ Opt_trace_start::Opt_trace_start(THD *thd, TABLE_LIST *tbl,
     ctx->set_query(query, query_length, query_charset);
     traceable= TRUE;
     opt_trace_disable_if_no_tables_access(thd, tbl);
+    Json_writer *w= ctx->get_current_json();
+    w->start_object();
+    w->add_member("steps").start_array();
   }
 }
 
@@ -503,6 +508,9 @@ Opt_trace_start::~Opt_trace_start()
 {
   if (traceable)
   {
+    Json_writer *w= ctx->get_current_json();
+    w->end_array();
+    w->end_object();
     ctx->end();
     traceable= FALSE;
   }

sql/opt_trace.h

@@ -72,14 +72,18 @@ struct Opt_trace_info
 */
 
 
-class Opt_trace_start {
+class Opt_trace_start
+{
  public:
-  Opt_trace_start(THD *thd_arg, TABLE_LIST *tbl,
+  Opt_trace_start(THD *thd_arg): ctx(&thd_arg->opt_trace), traceable(false) {}
+
+  void init(THD *thd, TABLE_LIST *tbl,
             enum enum_sql_command sql_command,
             List<set_var_base> *set_vars,
             const char *query,
             size_t query_length,
             const CHARSET_INFO *query_charset);
+
   ~Opt_trace_start();
 
  private:

sql/sp_head.cc

@@ -3489,10 +3489,9 @@ sp_lex_keeper::reset_lex_and_exec_core(THD *thd, uint *nextp,
     thd->lex->safe_to_cache_query= 0;
 #endif
 
-  Opt_trace_start ots(thd,  m_lex->query_tables,
-                        SQLCOM_SELECT, &m_lex->var_list,
-                        NULL, 0,
-                        thd->variables.character_set_client);
+  Opt_trace_start ots(thd);
+  ots.init(thd, m_lex->query_tables, SQLCOM_SELECT, &m_lex->var_list,
+           NULL, 0, thd->variables.character_set_client);
 
   Json_writer_object trace_command(thd);
   Json_writer_array trace_command_steps(thd, "steps");

sql/sql_parse.cc

@@ -3644,12 +3644,7 @@ mysql_execute_command(THD *thd, bool is_called_from_prepared_stmt)
 #ifdef HAVE_REPLICATION
   } /* endif unlikely slave */
 #endif
-  Opt_trace_start ots(thd, all_tables, lex->sql_command, &lex->var_list,
-                      thd->query(), thd->query_length(),
-                      thd->variables.character_set_client);
-
-  Json_writer_object trace_command(thd);
-  Json_writer_array trace_command_steps(thd, "steps");
+  Opt_trace_start ots(thd);
 
   /* store old value of binlog format */
   enum_binlog_format orig_binlog_format,orig_current_stmt_binlog_format;
@@ -3715,6 +3710,10 @@ mysql_execute_command(THD *thd, bool is_called_from_prepared_stmt)
   if (run_set_statement_if_requested(thd, lex))
     goto error;
 
+  /* After SET STATEMENT is done, we can initialize the Optimizer Trace: */
+  ots.init(thd, all_tables, lex->sql_command, &lex->var_list, thd->query(),
+           thd->query_length(), thd->variables.character_set_client);
+
   if (thd->lex->mi.connection_name.str == NULL)
       thd->lex->mi.connection_name= thd->variables.default_master_connection;
 

sql/sql_prepare.cc

@@ -2437,9 +2437,9 @@ static bool check_prepared_statement(Prepared_statement *stmt)
     For the optimizer trace, this is the symmetric, for statement preparation,
     of what is done at statement execution (in mysql_execute_command()).
   */
-  Opt_trace_start ots(thd, tables, lex->sql_command, &lex->var_list,
-                      thd->query(), thd->query_length(),
-                      thd->variables.character_set_client);
+  Opt_trace_start ots(thd);
+  ots.init(thd, tables, lex->sql_command, &lex->var_list, thd->query(),
+           thd->query_length(), thd->variables.character_set_client);
 
   Json_writer_object trace_command(thd);
   Json_writer_array trace_command_steps(thd, "steps");