race in closing btree handles

        if (F_ISSET(btree, WT_BTREE_OPEN))

                WT_STAT_DECR(conn->stats, file_open);

        /* Remove from the connection's list. */

        __wt_spin_lock(session, &conn->spinlock);

        inuse = (--btree->refcnt > 0);

        if (!inuse) {

                TAILQ_REMOVE(&conn->btqh, btree, q);

                --conn->btqcnt;

        }

        __wt_spin_unlock(session, &conn->spinlock);

        if (inuse)

                return (0);

        ret = __wt_btree_close(session);

        __wt_free(session, btree->name);

        __wt_free(session, btree->filename);

        __wt_free(session, btree->config);

Thread WT-1:

acquire conn->spinlock

find btree, refcnt == 0

remove btree from queue

release conn->spinlock

Thread WT-2:

acquire conn->spinlock

don't find btree entry

add btree to queue

acquire btree->rwlock

release conn->spinlock

open btree (reading the underlying root page and free list)

release btree->rwlock

Thread WT-1:

call __wt_btree_close (writing dirty pages and the underlying free list)