MDEV-14843: Assertion `s_tx_list.size() == 0' failed in myrocks::Rdb_transaction::term_mutex

When the plugin is unloaded, walk the s_trx_list and delete the left over
Rdb_transaction objects.
It is responsibility of the SQL layer to make sure that the storage engine
has no open tables when the plugin is being unloaded.

storage/rocksdb/ha_rocksdb.cc

@@ -4416,6 +4416,49 @@ static int rocksdb_done_func(void *const p) {
     error = 1;
   }
 
+  /*
+    MariaDB: When the plugin is unloaded with UNINSTALL SONAME command, some
+    connections may still have Rdb_transaction objects.
+
+    These objects are not genuine transactions (as SQL layer makes sure that
+    a plugin that is being unloaded has no open tables), they are empty
+    Rdb_transaction objects that were left there to save on object
+    creation/deletion.
+
+    Go through the list and delete them.
+  */
+  {
+    class Rdb_trx_deleter: public Rdb_tx_list_walker {
+    public:
+      std::set<Rdb_transaction*> rdb_trxs;
+
+      void process_tran(const Rdb_transaction *const tx) override {
+        /*
+          Check if the transaction is really empty. We only check
+          non-WriteBatch-based transactions, because there is no easy way to
+          check WriteBatch-based transactions.
+        */
+        if (!tx->is_writebatch_trx()) {
+          const auto tx_impl = static_cast<const Rdb_transaction_impl *>(tx);
+          DBUG_ASSERT(tx_impl);
+          if (tx_impl->get_rdb_trx())
+            DBUG_ASSERT(0);
+        }
+        rdb_trxs.insert((Rdb_transaction*)tx);
+      };
+    } deleter;
+
+    Rdb_transaction::walk_tx_list(&deleter);
+
+    for (std::set<Rdb_transaction*>::iterator it= deleter.rdb_trxs.begin();
+         it != deleter.rdb_trxs.end();
+         ++it)
+    {
+      // When a transaction is deleted, it removes itself from s_tx_list.
+      delete *it;
+    }
+  }
+
   /*
     destructors for static objects can be called at _exit(),
     but we want to free the memory at dlclose()

storage/rocksdb/mysql-test/rocksdb/r/mariadb_plugin.result

+#
+# MDEV-14843: Assertion `s_tx_list.size() == 0' failed in myrocks::Rdb_transaction::term_mutex
+#
+INSTALL SONAME 'ha_rocksdb';
+CREATE TABLE t1 (i INT) ENGINE=RocksDB;
+insert into t1 values (1);
+connect  con1,localhost,root,,;
+connection con1;
+insert into test.t1 values (1);
+connection default;
+DROP TABLE t1;
+UNINSTALL SONAME 'ha_rocksdb';

storage/rocksdb/mysql-test/rocksdb/t/mariadb_plugin-master.opt

+--default-storage-engine=myisam --plugin-load='' --ignore-db-dirs=#rocksdb

storage/rocksdb/mysql-test/rocksdb/t/mariadb_plugin.test

+--source include/have_log_bin.inc
+--source include/have_binlog_format_row.inc
+
+--echo #
+--echo # MDEV-14843: Assertion `s_tx_list.size() == 0' failed in myrocks::Rdb_transaction::term_mutex
+--echo #
+
+INSTALL SONAME 'ha_rocksdb';
+
+CREATE TABLE t1 (i INT) ENGINE=RocksDB;
+insert into t1 values (1);
+
+connect (con1,localhost,root,,);
+connection con1;
+insert into test.t1 values (1);
+
+connection default;
+
+# Cleanup
+DROP TABLE t1;
+UNINSTALL SONAME 'ha_rocksdb';