MDEV-24049 InnoDB: Failing assertion: node->is_open() in fil_space_t::flush_low

As part of MDEV-23855, we eliminated fil_system.LRU
and changed the way how InnoDB data files are opened.
We are also enforcing the innodb_open_files limit when new
data files are created.

The function fil_space_t::flush() would be invoked by
row_quiesce_table_start(). If the table was already in clean state,
it is possible that the data file is not open.

fil_space_t::flush_low(): If the data file is not open,
check with a debug assertion that there are no unflushed changes,
and carry on.

Reviewed by: Eugene Kosov and Thirunarayanan Balathandayuthapani

mysql-test/suite/innodb/r/flush.result

+SELECT @@GLOBAL.innodb_open_files;
+@@GLOBAL.innodb_open_files
+10
+CREATE TABLE t0 (a INT) ENGINE=InnoDB;
+FLUSH TABLE t0 WITH READ LOCK;
+UNLOCK TABLES;
+CREATE TABLE t1 (a INT) ENGINE=InnoDB;
+CREATE TABLE t2 (a INT) ENGINE=InnoDB;
+CREATE TABLE t3 (a INT) ENGINE=InnoDB;
+CREATE TABLE t4 (a INT) ENGINE=InnoDB;
+CREATE TABLE t5 (a INT) ENGINE=InnoDB;
+CREATE TABLE t6 (a INT) ENGINE=InnoDB;
+CREATE TABLE t7 (a INT) ENGINE=InnoDB;
+FLUSH TABLE t0 WITH READ LOCK;
+UNLOCK TABLES;
+DROP TABLE t0, t1, t2, t3, t4, t5, t6, t7;

mysql-test/suite/innodb/t/flush.opt

+--innodb-open-files=10

mysql-test/suite/innodb/t/flush.test

+--source include/have_innodb.inc
+
+SELECT @@GLOBAL.innodb_open_files;
+
+CREATE TABLE t0 (a INT) ENGINE=InnoDB;
+# Ensure that the created table t0 is clean.
+FLUSH TABLE t0 WITH READ LOCK;
+UNLOCK TABLES;
+CREATE TABLE t1 (a INT) ENGINE=InnoDB;
+CREATE TABLE t2 (a INT) ENGINE=InnoDB;
+CREATE TABLE t3 (a INT) ENGINE=InnoDB;
+CREATE TABLE t4 (a INT) ENGINE=InnoDB;
+CREATE TABLE t5 (a INT) ENGINE=InnoDB;
+CREATE TABLE t6 (a INT) ENGINE=InnoDB;
+CREATE TABLE t7 (a INT) ENGINE=InnoDB;
+# Now, the data file for t0 should not be open anymore.
+FLUSH TABLE t0 WITH READ LOCK;
+UNLOCK TABLES;
+DROP TABLE t0, t1, t2, t3, t4, t5, t6, t7;

storage/innobase/fil/fil0fil.cc

@@ -514,7 +514,11 @@ inline void fil_space_t::flush_low()
   for (fil_node_t *node= UT_LIST_GET_FIRST(chain); node;
        node= UT_LIST_GET_NEXT(chain, node))
   {
-    ut_a(node->is_open());
+    if (!node->is_open())
+    {
+      ut_ad(!is_in_unflushed_spaces);
+      continue;
+    }
     IF_WIN(if (node->is_raw_disk) continue,);
     os_file_flush(node->handle);
   }