Bug#11753363 (bug#44793) CHARACTER SETS: CASE CLAUSE, UCS2 OR UTF32, FAILURE

Problem: in case of string CASE/WHEN arguments with different
character sets, Item_func_case::find_item() called comparator
cmp_items[x] on mixed character set Items, so a 8-bit value could
be errouneously referenced to as being utf16/utf32 value,
which led to crash on DBUG_ASSERT() because of wrong value length.
This was wrong, as string comparator expects arguments in the same
character set.

Fix: modify Item_func_case's argument list after calling
agg_arg_charsets_for_comparison() - put the Items in "agg" array
back to "args", because some of the Items in the "agg" array might
have been changed to character set converters:
- to Item_func_conv_charset for non-constant items
- to Item_string for constant items

In other words, perform the same substitution which is done in
all other operations string comparison or string result operations:

Replace
  CASE         latin1_item              WHEN utf16_item THEN ... END
to
  CASE CONVERT(latin1_item USING utf16) WHEN utf16_item THEN ... END

Replace
  CASE utf16_item WHEN         latin1_item              THEN ... END
to
  CASE utf16_item WHEN CONVERT(latin1_item USING utf16) THEN ... END


  @ mysql-test/r/ctype_utf16.result
  @ mysql-test/r/ctype_utf32.result
  @ mysql-test/t/ctype_utf16.test
  @ mysql-test/t/ctype_utf32.test
  Adding tests

  @ sql/item_cmpfunc.cc
  Put "agg" back to "args".

  @ sql/sql_string.cc
  Backporting a fix for String::set_or_copy_aligned() from 5.6,
  for better test coverage:
  "SELECT _utf16 0x61" should expand the string to 0x0061 rather
  than to 0x000061.
  This fix was made in 5.6 under terms of "WL#4616 Implement UTF16-LE".

1 files changed, 42 insertions, 3 deletions

diff --git a/sql/item_cmpfunc.cc b/sql/item_cmpfunc.cc
index df541f603ee..68c63285693 100644
--- a/sql/item_cmpfunc.cc
+++ b/sql/item_cmpfunc.cc
@@ -3054,20 +3054,59 @@ void Item_func_case::fix_length_and_dec()
     agg[0]= args[first_expr_num];
     left_result_type= agg[0]->result_type();
 
+    /*
+      As the first expression and WHEN expressions
+      are intermixed in args[] array THEN and ELSE items,
+      extract the first expression and all WHEN expressions into 
+      a temporary array, to process them easier.
+    */
     for (nagg= 0; nagg < ncases/2 ; nagg++)
       agg[nagg+1]= args[nagg*2];
     nagg++;
     if (!(found_types= collect_cmp_types(agg, nagg)))
       return;
+    if (found_types & (1 << STRING_RESULT))
+    {
+      /*
+        If we'll do string comparison, we also need to aggregate
+        character set and collation for first/WHEN items and
+        install converters for some of them to cmp_collation when necessary.
+        This is done because cmp_item compatators cannot compare
+        strings in two different character sets.
+        Some examples when we install converters:
+
+        1. Converter installed for the first expression:
+
+           CASE         latin1_item              WHEN utf16_item THEN ... END
+
+        is replaced to:
+
+           CASE CONVERT(latin1_item USING utf16) WHEN utf16_item THEN ... END
+
+        2. Converter installed for the left WHEN item:
 
+          CASE utf16_item WHEN         latin1_item              THEN ... END
+
+        is replaced to:
+
+           CASE utf16_item WHEN CONVERT(latin1_item USING utf16) THEN ... END
+      */
+      if (agg_arg_charsets_for_comparison(cmp_collation, agg, nagg))
+        return;
+      /*
+        Now copy first expression and all WHEN expressions back to args[]
+        arrray, because some of the items might have been changed to converters
+        (e.g. Item_func_conv_charset, or Item_string for constants).
+      */
+      args[first_expr_num]= agg[0];
+      for (nagg= 0; nagg < ncases / 2; nagg++)
+        args[nagg * 2]= agg[nagg + 1];
+    }
     for (i= 0; i <= (uint)DECIMAL_RESULT; i++)
     {
       if (found_types & (1 << i) && !cmp_items[i])
       {
         DBUG_ASSERT((Item_result)i != ROW_RESULT);
-        if ((Item_result)i == STRING_RESULT &&
-            agg_arg_charsets_for_comparison(cmp_collation, agg, nagg))
-          return;
         if (!(cmp_items[i]=
             cmp_item::get_comparator((Item_result)i,
                                      cmp_collation.collation)))