MDEV-742: read-only, pure myisam, binlog-*, @@skip_log_bin corner casesbb-10.5-mdev_742

(Pushed to 10.5)
Are addressed along the following policies.
Prepared read-only, or on non-transactional engines, or
binlog-* filter in binlog, or skipped to binlog XA:s remains in the xid cache after disconnect.
But their consequent completion with Commit or Rollback differs.

1. The read-only at reconnect marks XID to fail for future
completion with ER_XA_RBROLLBACK.

2. `binlog-*` filtered XA when it changes engine data is regarded
as loggable even when nothing got cached for binlog.
An empty XA-prepare group is recorded. Consequent Commit-or-Rollback
succeeds in the Engine(s) as well as recorded into binlog.

3. The same applies to the non-transactional engine XA.

4. @@skip_log_bin=OFF does not record anything at XA-prepare (obviously),
but the completion event is recorded into binlog to admit
inconsistency with slave.

The following actions are taken by the patch.

At XA-prepare:
  when empty binlog cache - don't do anything to binlog if RO,
  otherwise write empty XA_prepare (assert(binlog-filter case)).
At Disconnect:
  when Prepared && RO (=> no binlogging was done)
    set Xid_cache_element::error := ER_XA_RBROLLBACK
    *keep* XID in the cache, and rollback the transaction.
At XA-"complete":
   Discover the error, if any don't binlog the "complete", return the error to the user.

RO patch review notes.

1 files changed, 23 insertions, 0 deletions

diff --git a/sql/handler.cc b/sql/handler.cc
index c5b6e05b448..5964f6cd7c5 100644
--- a/sql/handler.cc
+++ b/sql/handler.cc
@@ -1367,6 +1367,29 @@ int ha_prepare(THD *thd)
   DBUG_RETURN(error);
 }
 
+/*
+  Like ha_check_and_coalesce_trx_read_only to return counted number of
+  read-write transaction participants limited to two, but works in the 'all'
+  context.
+  Also returns the last found rw ha_info through the 2nd argument.
+*/
+uint ha_count_rw_all(THD *thd, Ha_trx_info **ptr_ha_info)
+{
+  unsigned rw_ha_count= 0;
+
+  for (auto ha_info= thd->transaction.all.ha_list; ha_info;
+       ha_info= ha_info->next())
+  {
+    if (ha_info->is_trx_read_write())
+    {
+      *ptr_ha_info= ha_info;
+      if (++rw_ha_count > 1)
+        break;
+    }
+  }
+  return rw_ha_count;
+}
+
 /**
   Check if we can skip the two-phase commit.