Fix for Bug#56138 "valgrind errors about overlapping memory when double-assigning same variable",

and related small fixes.

mysql-test/t/user_var.test:
  test for bug
sql/field_conv.cc:
  From the C standard, memcpy() has undefined behaviour if to->ptr==from->ptr
sql/item_func.cc:
  In the case of BUG#56138, entry->value==ptr in which case memcpy()
  has undefined results per the C standard.
sql/sql_select.cc:
  Work around a bug in old gcc

1 files changed, 2 insertions, 5 deletions

diff --git a/sql/field_conv.cc b/sql/field_conv.cc
index 0bffde9671a..a4fca6f8ad7 100644
--- a/sql/field_conv.cc
+++ b/sql/field_conv.cc
@@ -786,11 +786,8 @@ int field_conv(Field *to,Field *from)
          ((Field_varstring*)from)->length_bytes ==
           ((Field_varstring*)to)->length_bytes))
     {						// Identical fields
-#ifdef HAVE_purify
-      /* This may happen if one does 'UPDATE ... SET x=x' */
-      if (to->ptr != from->ptr)
-#endif
-        memcpy(to->ptr,from->ptr,to->pack_length());
+      // to->ptr==from->ptr may happen if one does 'UPDATE ... SET x=x'
+      memmove(to->ptr, from->ptr, to->pack_length());
       return 0;
     }
   }