Optimize flush tables with read lock (FTWRL) to not wait for select's

Part of MDEV-5336 Implement LOCK FOR BACKUP

The idea is that instead of waiting in close_cached_tables() for all
tables to be closed, we instead call flush_tables() that does:
- Flush not used objects in table cache to free memory
- Collect all tables that are open
- Call HA_EXTRA_FLUSH on the objects, to get them into "closed state"
- Added HA_EXTRA_FLUSH support to archive and CSV
- Added multi-user protection to HA_EXTRA_FLUSH in MyISAM and Aria

The benefit compared to old code is:
- FTWRL doesn't have to wait for long running read operations or
  open HANDLER's

1 files changed, 14 insertions, 0 deletions

diff --git a/storage/archive/ha_archive.cc b/storage/archive/ha_archive.cc
index bb15aa9297d..f97bc52b6a9 100644
--- a/storage/archive/ha_archive.cc
+++ b/storage/archive/ha_archive.cc
@@ -1737,6 +1737,20 @@ int ha_archive::info(uint flag)
 }
 
 
+int ha_archive::extra(enum ha_extra_function operation)
+{
+  switch (operation) {
+  case HA_EXTRA_FLUSH:
+    mysql_mutex_lock(&share->mutex);
+    share->close_archive_writer();
+    mysql_mutex_unlock(&share->mutex);
+    break;
+  default:
+    break;
+  }
+  return 0;
+}
+
 /*
   This method tells us that a bulk insert operation is about to occur. We set
   a flag which will keep write_row from saying that its data is dirty. This in