WL#7318: Delayed Replication: GTID based and relative to immediate master commit

Affects: Server-8.0   —   Status: Complete

Delayed replication was introduced by WL#344. Currently there are following 
drawbacks with the implementation:
 1. In a chained replication topology, the delay is measured relative to the
    originating master. If two upstream masters are making updates, and there is
    a lag between the masters, then this will cause the actual delay relative to
    each master to fluctuate.
 2. The delay is relative to when the transaction started, not to when it
    was committed.
 3. The delay is checked per event, not per transaction.

We introduce a new timestamp in the binary log: the immediate_commit_timestamp
which is the number of microseconds since epoch when the transaction was
written to the binary log of the immediate master.
Delayed replication will be measured relative to this timestamp if both the
immediate master and current server (slave) support immediate_commit_timestamp.
If any one of immediate master or slave doesn't, we keep the old implementation
of delayed replication.

Related Bugs:

F1) The delayed replication should be expressed with reference to immediate
F2) The delay begins at commit on master, not beginning of transaction
    as was the case before this worklog.
F3) At slave, the delay happens at beginning of transaction and not events.
    So all events in the transaction either wait or apply without
    waiting the delay period in between.
    Precisely the wait happens at GTID_LOG_EVENT or
    ANONYMOUS_LOG_EVENT whichever be the case.
F4) The delay configuration and status should be available in
    SHOW SLAVE STATUS as well as replication P_S tables.
F5) The delay is expressed in seconds. This applies to:
    F5.6) Replication P_S tables.
F6) 5.7->8.0 delayed replication should work similar to 5.7->5.7.


NF1) Delayed replication should be allowed on every slave in any
     replication topology.
There are no changes in the interface except for those mentioned in the 
functional requirements. The interfaces specification of delayed replication 
should be as follows:

1) configuration to be done with CHANGE MASTER TO MASTER_DELAY= XYZ;
   where time is expressed in seconds.

2) The delay expressed (as in (1)) is measured with respect to commit at
   immediate master. Talking of the whole 2 PC commit, we assume that
   commit timestamp = time of writing to binlog (MYSQL_BIN_LOG::write_gtid())   

2) The delay happens per transaction (not event).
   So the delay is imposed only on gtid_log_event or anonymous_gtid_log_event.
   The other events in the transaction always follow these event without
   any waiting time imposed on them.

3) The delay is interpreted as follows:

   3.1: gtid_log_event case:

   (time of execution of  gtid_log_event of the transaction on slave) - 
   (commit timestamp at immediate master).

   3.2: anonymous_gtid_log_event case:
   For the purpose of delay calculations gtid_log_event can just be replaced by
   anonymous_gtid_log_event in (3.1).

4) The monitoring is achieved via:
       4.1.1 SQL_Delay
       4.1.2 SQL_Remaining_Delay
   4.2 Replication P_S Tables
       4.2.1 Table: replication_execute_configuration
             column: DESIRED_DELAY
       4.2.2 Table: replication_execute_status
             column: REMAINING_DELAY

   The unit of time is in seconds in all monitoring infrastructure.


The 8.0 slave needs commit timestamp from the master to work in the way this 
worklog describes. Since 5.7 master doesn’t send the commit timestamps we can
not use the new code to handle this code. So the delayed replication from a 5.7 
master to a 8.0 slave works the same way as 5.7->5.7 delayed replication. 
L1) This worklog builds on top of the patch for WL#7319.

The new delayed replication works as follows:

L2) Makes changes so that wait happens at GTID_LOG_EVENT or
L3) The time delay is now calculated as:

     time_t sql_delay_end=
       Gtid_log_event->(immediate_commit_timestamp / 1000000) + sql_delay;

We don't remove the old code because we have to use it in 5.7->8.0 delayed 
replication. We split delayed replication in two branches:

Branch 1 handles the 5.8->8.0 delayed replication (new logic as in L2 and L3 

Branch 2 handles the 5.7->8.0 replication (keep old code).