How can svn_fs_abort_txn be implemented?

I was thinking about how svn_fs_abort_txn can be
implemented.

If I understand it correctly, svn_fs_abort_txn needs
svn_error_t *svn_fs__dag_destroy_tree (dag_node_t *root),
which walk thorough a tree and removes all mutable nodes
from nodes table by calling db->del(). In STACK,
svn_fs__dag_delete is mentioned. But it's for svn rm (or
whatever it's called) operation and it doesn't remove node
from nodes table, right?

There's an argument for making the nodes immutable when
close_file is called. You can't do it or you won't be able
to distinguish which nodes were created in that transaction.
Alternatively, you can mark a node immutable and put that
node into a table for later abort but I don't think it's
worth the trouble to have different table for this.

svn_fs_abort_txn will also need svn_fs__destroy_txn, which
will remove transaction entry from transactions table.

I thought about just implementing them but I might be
missing something obvious again. Am I?

-- 
Yoshiki Hayashi