Group Replication uses transactions write-set and database version
to check if parallel transactions from different members do
conflict, on which case the ordered first is the one allowed to
commit and the other(s) are rollback.
The write-set is the list of the primary key equivalents that can
unequivocally identify a row on a table.
The database version is the GTID_EXECUTED on top of which the
transaction was executed.
These information combined with the Database State Machine (DBSM)
approach allow us to check for conflicts between transactions.

When two transactions write on the same rows on the same server,
there was a race between InnoDB release locks and GTID_EXECUTED
read:

  T1: W(A)
                                      T2: W(A) -- blocks
  T1: PREPARE (innodb)
  T1: before_commit hook
  T1: Read GTID_EXECUTED
  T1: FLUSH (binlog)
  T1: SYNC (binlog)
  T1: COMMIT (innodb)
  T1: release innodb locks (unblocks T2)
                                     T2: PREPARE (innodb)
                                     T2: before_commit hook
                                     T2: read GTID_EXECUTED
  T1: update GTID_EXECUTED

The "T2: read GTID_EXECUTED" operation would have missed the fact
that T1 had already committed, thence would treat it as conflicting.

There was a race condition on GTID_EXECUTED read that could make
transactions to read not updated GTID_EXECUTED, despite the binary
log was already updated, that is, there was a small windows between
write to binary log and update GTID_EXECUTED on which parallel
threads could read a GTID_EXECUTED value which did not include the
last logged transaction to binary log.
When operating on single member, this window could cause a parallel
transactions to be marked as conflicting despite it was not, since
it was serialized be the server. The problem was the outdated
GTID_EXECUTED value that this transaction has read.

To eliminate this opportunity window, now GTID_EXECUTED is read
under the protection of MYSQL_BIN_LOG::LOCK_commit to ensure that we
when GTID_EXECUTED is read all previous transactions are committed
and GTID_EXECUTED updated:

  T1: W(A)
                                      T2: W(A) -- blocks
  T1: PREPARE (innodb)
  T1: before_commit hook
  T1: Read GTID_EXECUTED
  T1: FLUSH (binlog)
  T1: SYNC (binlog)
  T1: COMMIT (innodb)
  T1: update GTID_EXECUTED
  T1: release innodb locks (unblocks T2)
                                     T2: PREPARE (innodb)
                                     T2: before_commit hook
                                     T2: read GTID_EXECUTED