fix #1039 #1030: add shared-mode lock in query ReadMigrationRangeValues to avoid losing data

[wangzihuacool]

related #1039 #1030 ..
This PR is to avoid losing data when gh-ost in MySQL with semi-sync replication enabled.

Reason
In mysql with a semi-sync slave, transactions may be in uncommitted state with 'Waiting for semi-sync ACK from slave' wait, though its related binlog event has already been handled and discarded by gh-ost BinlogStreamer. If gh-ost ReadMigrationRangeValues executes before the transactions finnally committed, the uncommitted rows is lost.

Fix
Add a shared mode lock in the query of ReadMigrationRangeValues. If min/max value of the uniquekey was changed by another transaction that has not yet committed, query just waits until that transaction ends and then uses the latest values.
Thus ReadMigrationRangeValues returns the accurate value.

Thank you !

[dikang123]

so quick，nubility！

[shaohk]

#1054
No need to use lock.
And add a heartbeat between the addDMLEventsListener and ReadMigrationRangeValues to fix this bug.

[Fanduzi]

Any progress?

[timvaillancourt]

@Fanduzi the core issue was probably resolved in another PR. Please test and let me know

I'll leave this open in case this locking is still needed. For now I've avoided it because the impact of the lock is hard to quantify

[Fanduzi]

@Fanduzi the core issue was probably resolved in another PR. Please test and let me know

I'll leave this open in case this locking is still needed. For now I've avoided it because the impact of the lock is hard to quantify

In my opinion, this lock is no longer needed. #1141 solves the problem perfectly