Fix bug MDEV-4895 Valgrind warnings (Conditional jump or move depends on uninitialised value) in Field_datetime::get_date on GREATEST(..) IS NULL

Analysis:
The cause of the valgrind warning was an attempt to evaluate a Field that was not yet read.
The reason was that on one hand Item_func_isnotnull was marked as constant by
Item_func_isnotnull::update_used_tables, and this allowed eval_const_cond() to be called.
On the other hand Item_func_isnotnull::val_int() evaluated its argument as if it was not
constant.

Solution:
The fix make sure that Item_func_isnotnull::val_int() doesn't evaluate its argument when
it is constant and cannot be NULL, because the result is known in this case. 

1 files changed, 4 insertions, 0 deletions

diff --git a/sql/item_cmpfunc.cc b/sql/item_cmpfunc.cc
index de34a937262..fd688181c92 100644
--- a/sql/item_cmpfunc.cc
+++ b/sql/item_cmpfunc.cc
@@ -4621,6 +4621,8 @@ Item *and_expressions(Item *a, Item *b, Item **org_item)
 longlong Item_func_isnull::val_int()
 {
   DBUG_ASSERT(fixed == 1);
+  if (const_item() && !args[0]->maybe_null)
+    return 0;
   return args[0]->is_null() ? 1: 0;
 }
 
@@ -4628,6 +4630,8 @@ longlong Item_is_not_null_test::val_int()
 {
   DBUG_ASSERT(fixed == 1);
   DBUG_ENTER("Item_is_not_null_test::val_int");
+  if (const_item() && !args[0]->maybe_null)
+    DBUG_RETURN(1);
   if (args[0]->is_null())
   {
     DBUG_PRINT("info", ("null"));