Bug#38551 query cache can still consume [very little] cpu time even when it is off.

      
When the query cache is disabled, the server shouldn't attempt to take the 
query cache mutex.
                             
By using the command line option --query_cache_type=0, the user can disable
   
(backport from mysql-pe)


mysql-test/t/query_cache_disabled-master.opt:
  * added test case for bug38551
mysql-test/t/query_cache_disabled.test:
  * added test case for bug38551
sql/set_var.cc:
  * Added before-trigger to verify that query_cache_type wasn't turned off or on during
  runtime.
sql/set_var.h:
  * Changed order on how the enumeration is processed. By first projecting the
  character representation of the variable to a temporary integer we can have
  one function instead of two to check if the value is valid.
sql/share/errmsg-utf8.txt:
  * Added error message for query cache disabled state
sql/sql_cache.cc:
  * If the query cache is disabled at start up, shorten the execution path and avoid
  grabbing the query cache mutex each time the invalidate interface methods are called.
sql/sql_cache.h:
  * Added new methods to set the query cache into a disabled state.

1 files changed, 10 insertions, 4 deletions

diff --git a/sql/set_var.h b/sql/set_var.h
index a83de6425ae..c08097521d2 100644
--- a/sql/set_var.h
+++ b/sql/set_var.h
@@ -543,10 +543,16 @@ public:
   { chain_sys_var(chain); }
   bool check(THD *thd, set_var *var)
   {
-    int ret= 0;
-    if (check_func)
-      ret= (*check_func)(thd, var);
-    return ret ? ret : check_enum(thd, var, enum_names);
+    /*
+      check_enum fails if the character representation supplied was wrong
+      or that the integer value was wrong or missing.
+    */
+    if (check_enum(thd, var, enum_names))
+      return TRUE;
+    else if ((check_func && (*check_func)(thd, var)))
+      return TRUE;
+    else
+      return FALSE;
   }
   bool update(THD *thd, set_var *var);
   void set_default(THD *thd, enum_var_type type);