WL#7464: InnoDB: provide a way to do non-locking reads

Affects: Server-8.0   —   Status: Complete   —   Priority: Medium

New Data Dictionary allows INFORMATION_SCHEMA queries to get their data
directly from DD tables (see WL#6599 for more information). The main point
is that I_S tables will be represented as views, thus a query from I_S
table will be a query from a view.

I_S queries can be executed under different isolation levels that is set by
user. Hence it is possible that I_S queries getting blocked by a parallel
DDL operation. This will be a huge compatibility issue.

The proposed behavior is to request InnoDB to do lockless read operation
from the underlying DD-tables.

In order to do that, there should be a way to request non-locking read on
per handler (in terms of the Handler API) instance. This WL is about
providing such a way.
NF1: No user visible changes.
MySQL server should convey InnoDB to do lock-less reads for some DD table
handles. Note that I_S queries directly read DD tables and do not use DD API's.
So, the server should have some way to identify all the DD table handles
that are part of I_S query.

Once the handler that belong to DD table in a I_S query is identified, 
MySQL server can use following way to tell InnoDB to do lock-less reads.

1) Add new enum,
	enum ha_extra_function {
		...
		HA_EXTRA_SKIP_SERIALIZABLE_DD_VIEW
	};

2) Call handler::extra(HA_EXTRA_SKIP_SERIALIZABLE_DD_VIEW) for all the DD table 
handlers.

3) Within ha_innobase::extra() mark prebuilt->skip_serializable_dd_view = true

The blocking of I_S query happens only in Serializable transaction isolation
level. With other transaction isolation levels, it is a consistent read.

How serializable transaction isolation level is achieved in InnoDB:
(comment in source)
/* To get serializable execution, we let InnoDB
   conceptually add 'LOCK IN SHARE MODE' to all SELECTs
   which otherwise would have been consistent reads...
   .. */

i.e. We use prebuilt->select_lock_type = LOCK_S

So, we add an exception to the above with
extra(HA_EXTRA_SKIP_SERIALIZABLE_DD_VIEW)and the select lock type
remains as LOCK_NONE.

After the select is done, we reset the flag using ha_innobase::reset()