query cache sysvar fixes

sql/share/errmsg-utf8.txt:
  correct the error message, as query_cache_type variable is not read-ony anymore
sql/sql_cache.cc:
  the caller should verify that query cache resize
  is possible, before trying it
sql/sys_vars.cc:
  * test if qc resize is possible in the sysvar on_check() funntion,
    not in the on_update() function.
  * use the error message that better describes the problem

sql/share/errmsg-utf8.txt

@@ -6270,8 +6270,7 @@ ER_SLAVE_IGNORE_SERVER_IDS
   eng "The requested server id %d clashes with the slave startup option --replicate-same-server-id"
   ger "Die angeforderte Server-ID %d steht im Konflikt mit der Startoption --replicate-same-server-id für den Slave"
 ER_QUERY_CACHE_DISABLED
-  eng "Query cache is disabled; restart the server with query_cache_type=1 to enable it"
-  ger "Abfragen-Cache ist deaktiviert. Starten Sie den Server neu mit query_cache_type=1, um ihn zu aktivieren"
+  eng "Query cache is disabled; set query_cache_type to ON or DEMAND to enable it"
 ER_SAME_NAME_PARTITION_FIELD
   eng "Duplicate partition field name '%-.192s'"
   ger "Partitionsfeld '%-.192s' ist ein Duplikat"

sql/sql_cache.cc

@@ -1292,6 +1292,7 @@ ulong Query_cache::resize(ulong query_cache_size_arg)
 
   if (global_system_variables.query_cache_type == 0)
   {
+    DBUG_ASSERT(query_cache_size_arg == 0);
     if (query_cache_size_arg != 0)
       my_error(ER_QUERY_CACHE_IS_DISABLED, MYF(0));
     DBUG_RETURN(0);

sql/sys_vars.cc

@@ -1819,6 +1819,17 @@ static Sys_var_enum Sys_thread_handling(
        thread_handling_names, DEFAULT(0));
 
 #ifdef HAVE_QUERY_CACHE
+static bool check_query_cache_size(sys_var *self, THD *thd, set_var *var)
+{
+  if (global_system_variables.query_cache_type == 0 &&
+      var->value && var->value->val_int() != 0)
+  {
+    my_error(ER_QUERY_CACHE_DISABLED, MYF(0));
+    return true;
+  }
+
+  return false;
+}
 static bool fix_query_cache_size(sys_var *self, THD *thd, enum_var_type type)
 {
   ulong new_cache_size= query_cache.resize(query_cache_size);
@@ -1839,7 +1850,7 @@ static Sys_var_ulonglong Sys_query_cache_size(
        "The memory allocated to store results from old queries",
        GLOBAL_VAR(query_cache_size), CMD_LINE(REQUIRED_ARG),
        VALID_RANGE(0, ULONGLONG_MAX), DEFAULT(0), BLOCK_SIZE(1024),
-       NO_MUTEX_GUARD, NOT_IN_BINLOG, ON_CHECK(0),
+       NO_MUTEX_GUARD, NOT_IN_BINLOG, ON_CHECK(check_query_cache_size),
        ON_UPDATE(fix_query_cache_size));
 
 static Sys_var_ulong Sys_query_cache_limit(
@@ -1867,7 +1878,7 @@ static bool check_query_cache_type(sys_var *self, THD *thd, set_var *var)
 {
   if (query_cache.is_disable_in_progress())
   {
-    my_error(ER_QUERY_CACHE_DISABLED, MYF(0));
+    my_error(ER_QUERY_CACHE_IS_DISABLED, MYF(0));
     return true;
   }
   if (var->type != OPT_GLOBAL &&