MDEV-31022: SIGSEGV in maria_create from create_internal_tmp_tablebb-11.0-MDEV-31022-variant3

The code in create_internal_tmp_table() didn't take into account that
now temporary (derived) tables may have multiple indexes:

- one index due to duplicate removal
   = In this example created by conversion of big-IN(...) into subquery
   = this index might be converted into a "unique constraint" if the field(s)
     length is too large.
- one index added by derived_with_keys optimization.

Make it handle multiple indexes.

As a consequence of this, remove TABLE_SHARE::uniques and replace it with
per-index HA_UNIQUE_HASH flag.
- TABLE_SHARE::uniques made sense when it referred to the only index in the
  table.
- Now, we need to know which of table indexes should be a unique constraint,
  so we indicate that as an index flag.

This patch is based on Monty's patch:
Co-Author: Monty <monty@mariadb.org>

1 files changed, 4 insertions, 2 deletions

diff --git a/sql/opt_subselect.cc b/sql/opt_subselect.cc
index 1d0b1ff1874..94adae02a17 100644
--- a/sql/opt_subselect.cc
+++ b/sql/opt_subselect.cc
@@ -4903,11 +4903,13 @@ SJ_TMP_TABLE::create_sj_weedout_tmp_table(THD *thd)
   {
     DBUG_PRINT("info",("Creating group key in temporary table"));
     share->keys=1;
-    share->uniques= MY_TEST(using_unique_constraint);
     table->key_info= share->key_info= keyinfo;
     keyinfo->key_part=key_part_info;
-    keyinfo->flags=HA_NOSAME;
+    keyinfo->flags= HA_NOSAME | (using_unique_constraint ? HA_UNIQUE_HASH : 0);
+    keyinfo->ext_key_flags= keyinfo->flags;
     keyinfo->usable_key_parts= keyinfo->user_defined_key_parts= 1;
+    keyinfo->ext_key_parts= 1;
+    share->key_parts= 1;
     keyinfo->key_length=0;
     keyinfo->rec_per_key=0;
     keyinfo->algorithm= HA_KEY_ALG_UNDEF;