MDEV-14586 Assertion `0' failed in retrieve_auto_increment ...

Problem:-
 If we create table using myisam/aria then this crashes the server.
  CREATE TABLE t1(a bit(1), b int auto_increment , index(a,b));
  insert into t1 values(1,1);
 Or this query
  CREATE TABLE t1 (b BIT(1), pk INTEGER AUTO_INCREMENT PRIMARY KEY);
  ALTER TABLE t1 ADD INDEX(b,pk);
  INSERT INTO t1 VALUES (1,b'1');
  ALTER TABLE t1 DROP PRIMARY KEY;

Reason:-
 The reason for this is
 1st- find_ref_key() finds what key an auto_increment field belongs to by
  comparing key_part->offset and field->ptr. But BIT fields might have
  zero length in the record, so a key might have many key parts with the
  same offset. That is, comparing offsets cannot uniquely identify the
  correct key part.
 2nd- Since next_number_key_offset is zero it myisam/aria will think that
  auto_increment is in first part of key.
 3nd- myisam/aria will call retrieve_auto_key which will see first key_part
  field as a bit field and call assert(0)

Solution:-
  Many key parts might have the same offset, but BIT fields do not
  support auto_increment. So, we can skip all key parts over BIT fields,
  and then comparing offsets will be unambiguous.

1 files changed, 4 insertions, 2 deletions

diff --git a/sql/key.cc b/sql/key.cc
index 110b13000ed..700bf6a05a6 100644
--- a/sql/key.cc
+++ b/sql/key.cc
@@ -62,7 +62,8 @@ int find_ref_key(KEY *key, uint key_count, uchar *record, Field *field,
        i < (int) key_count ;
        i++, key_info++)
   {
-    if (key_info->key_part[0].offset == fieldpos)
+    if (key_info->key_part[0].offset == fieldpos &&
+            key_info->key_part[0].field->type() != MYSQL_TYPE_BIT)
     {                                  		/* Found key. Calc keylength */
       *key_length= *keypart= 0;
       return i;                                 /* Use this key */
@@ -81,7 +82,8 @@ int find_ref_key(KEY *key, uint key_count, uchar *record, Field *field,
 	 j < key_info->key_parts ;
 	 j++, key_part++)
     {
-      if (key_part->offset == fieldpos)
+      if (key_part->offset == fieldpos &&
+            key_part->field->type() != MYSQL_TYPE_BIT)
       {
         *keypart= j;
         return i;                               /* Use this key */