LMDB: issue with mdb_cursor_del

[timur.kristof@gmail.com]

Hi,

I have an app that uses LMDB, and I've experienced an interesting
issue: when trying to delete a certain item with mdb_cursor_del, it
crashed with the following backtrace: https://pastebin.com/7p9wtkj9

It appears that it couldn't mark a page as dirty.
Here is the relevant assertion from mdb_page_dirty:
rc = insert(txn->mt_u.dirty_list, &mid);
mdb_tassert(txn, rc == 0); // assertion failed

What might I be doing wrong in my application that triggers this sort
of error?

Also interesting:
The database was about 60MB, and I now compacted it using mdb_copy -c.
Now it is only ~6MB, and running the app with the compacted database,
the above error also disappeared.

I'm not sure why the compacting fixed the problem, could somebody offer
an insight about this?

Thank you guys in advance for your answers!
Best regards,
Timur