Improve update handler (long unique keys on blobs)

MDEV-21606 Improve update handler (long unique keys on blobs)
MDEV-21470 MyISAM and Aria start_bulk_insert doesn't work with long unique
MDEV-21606 Bug fix for previous version of this code
MDEV-21819 2 Assertion `inited == NONE || update_handler != this'

- Move update_handler from TABLE to handler
- Move out initialization of update handler from ha_write_row() to
  prepare_for_insert()
- Fixed that INSERT DELAYED works with update handler
- Give an error if using long unique with an autoincrement column
- Added handler function to check if table has long unique hash indexes
- Disable write cache in MyISAM and Aria when using update_handler as
  if cache is used, the row will not be inserted until end of statement
  and update_handler would not find conflicting rows.
- Removed not used handler argument from
  check_duplicate_long_entries_update()
- Syntax cleanups
  - Indentation fixes
  - Don't use single character indentifiers for arguments

1 files changed, 19 insertions, 0 deletions

diff --git a/mysql-test/main/long_unique_delayed.result b/mysql-test/main/long_unique_delayed.result
new file mode 100644
index 00000000000..52365249097
--- /dev/null
+++ b/mysql-test/main/long_unique_delayed.result
@@ -0,0 +1,19 @@
+#
+# Test insert delayed with long unique keys
+#
+create table t1(a blob unique) engine=myisam;
+insert delayed into t1 values(1),(2),(3),(56),('sachin'),('maria'),(123456789034567891),(null),(null),(123456789034567890),('maria');
+insert delayed into t1 values(1),(9);
+flush tables t1;
+select count(*) from t1;
+count(*)
+11
+drop table t1;
+#
+# MDEV-19848
+# Server crashes in check_vcol_forward_refs upon INSERT DELAYED into
+# table with long blob key
+#
+CREATE  TABLE t1 (a BLOB, UNIQUE(a)) ENGINE=MyISAM;
+INSERT DELAYED t1 () VALUES ();
+DROP TABLE t1;