MDEV-7825: Parallel replication race condition on gco->flags, possibly resulting in slave hang

The patch for optimistic parallel replication as a memory optimisation moved
the gco->installed field into a bit in gco->flags. However, that is just plain
wrong. The gco->flags field is owned by the SQL driver thread, but
gco->installed is used by the worker threads, so this will cause a race
condition.

The user-visible problem might be conflicts between transactions and/or slave
threads hanging.

So revert this part of the optimistic parallel replication patch, going back
to using a separate field gco->installed like in 10.0.

1 files changed, 4 insertions, 3 deletions

diff --git a/sql/rpl_parallel.h b/sql/rpl_parallel.h
index eebb8d136a9..93139971ca6 100644
--- a/sql/rpl_parallel.h
+++ b/sql/rpl_parallel.h
@@ -66,7 +66,8 @@ struct group_commit_orderer {
     This flag is set when this GCO has been installed into the next_gco pointer
     of the previous GCO.
   */
-  static const uint8 INSTALLED = 0x01;
+  bool installed;
+
   /*
     This flag is set for a GCO in which we have event groups with multiple
     different commit_id values from the master. This happens when we
@@ -77,13 +78,13 @@ struct group_commit_orderer {
     of current commit_id, as DDL is not safe to speculatively apply in parallel
     with prior event groups.
   */
-  static const uint8 MULTI_BATCH = 0x02;
+  static const uint8 MULTI_BATCH = 0x01;
   /*
     This flag is set for a GCO that contains DDL. If set, it forces a switch to
     a new GCO upon seeing a new commit_id, as DDL is not safe to speculatively
     replicate in parallel with subsequent transactions.
   */
-  static const uint8 FORCE_SWITCH = 0x04;
+  static const uint8 FORCE_SWITCH = 0x02;
   uint8 flags;
 };