MDEV-20699 followup.

Normally we disable caching of routines in "SHOW CREATE".
Introduce an exception, if debug_dbug="+d,cache_sp_in_show_create".
lock_sync.test needs a way to populate the cache without side effects,
or else it runs into debug_sync timeouts.

So, this possibility to cache will be remain only for very special tests.

mysql-test/main/lock_sync.result

@@ -205,6 +205,8 @@ end|
 # called below.
 #
 connection con1;
+set @save_dbug=@@debug_dbug;
+set debug_dbug="+d,cache_sp_in_show_create";
 # Cache all functions used in the tests below so statements
 # calling them won't need to open and lock mysql.proc table
 # and we can assume that each statement locks its tables
@@ -229,6 +231,7 @@ show create function f14;
 show create function f15;
 show create function f16;
 show create function f17;
+set debug_dbug=@save_dbug;
 connection default;
 #
 # 1. Statements that read tables and do not use subqueries.

mysql-test/main/lock_sync.test

@@ -232,6 +232,8 @@ let $con_aux2= con2;
 let $table= t1;
 
 connection con1;
+set @save_dbug=@@debug_dbug;
+set debug_dbug="+d,cache_sp_in_show_create";
 --echo # Cache all functions used in the tests below so statements
 --echo # calling them won't need to open and lock mysql.proc table
 --echo # and we can assume that each statement locks its tables
@@ -257,6 +259,7 @@ show create function f14;
 show create function f15;
 show create function f16;
 show create function f17;
+set debug_dbug=@save_dbug;
 --enable_result_log
 connection default;
 

sql/sp.cc

@@ -1903,6 +1903,14 @@ Sp_handler::sp_show_create_routine(THD *thd,
     of the binary log or the query cache, which currently it does not.
   */
   sp_head *sp= 0;
+
+  DBUG_EXECUTE_IF("cache_sp_in_show_create",
+    /* Some tests need just need a way to cache SP without other side-effects.*/
+    sp_cache_routine(thd, name, false, &sp);
+    sp->show_create_routine(thd, this);
+    DBUG_RETURN(false);
+  );
+
   bool free_sp= db_find_routine(thd, name, &sp) == SP_OK;
   bool ret= !sp || sp->show_create_routine(thd, this);
   if (ret)