MDEV-6676: Optimistic parallel replication

Implement a new mode for parallel replication. In this mode, all transactions
are optimistically attempted applied in parallel. In case of conflicts, the
offending transaction is rolled back and retried later non-parallel.

This is an early-release patch to facilitate testing, more changes to user
interface / options will be expected. The new mode is not enabled by default.

sql/rpl_rli.h

@@ -621,6 +621,8 @@ struct rpl_group_info
     counting one event group twice.
   */
   bool did_mark_start_commit;
+  /* Copy of flags2 from GTID event. */
+  uchar gtid_ev_flags2;
   enum {
     GTID_DUPLICATE_NULL=0,
     GTID_DUPLICATE_IGNORE=1,
@@ -652,6 +654,36 @@ struct rpl_group_info
   inuse_relaylog *relay_log;
   uint64 retry_start_offset;
   uint64 retry_event_count;
+  /*
+    If `speculation' is != SPECULATE_NO, then we are optimistically running
+    this transaction in parallel, even though it might not be safe (there may
+    be a conflict with a prior event group).
+
+    In this case, a conflict can cause other errors than deadlocks (like
+    duplicate key for example). So in case of _any_ error, we need to roll
+    back and retry the event group.
+  */
+  enum enum_speculation {
+    /*
+      This transaction was group-committed together on the master with the
+      other transactions with which it is replicated in parallel.
+    */
+    SPECULATE_NO,
+    /*
+      We will optimistically try to run this transaction in parallel with
+      other transactions, even though it is not known to be conflict free.
+      If we get a conflict, we will detect it as a deadlock, roll back and
+      retry.
+    */
+    SPECULATE_OPTIMISTIC,
+    /*
+      This transaction got a conflict during speculative parallel apply, or
+      it was marked on the master as likely to cause a conflict or unsafe to
+      speculate. So it will wait for the prior transaction to commit before
+      starting to replicate.
+    */
+    SPECULATE_WAIT
+  } speculation;
   bool killed_for_retry;
 
   rpl_group_info(Relay_log_info *rli_);