Hallvard Breien Furuseth:

Wietse Venema writes:

...

(...) what happens when the two above activities happen in different processes? That is, process A (opens the database with MDB_NOLOCK. opens a cursor, and walks the database without making any changes to it), while process B opens the same database with MDB_NOLOCK and performs the delete transactions.

Will the process A cursor read transaction trigger an assertion due to the process B delete transactions? If not, why not?

Process A may see garbage data. This can result in an assert(), a segfault or other misbehavior. Just like if you had process A == B. With MDB_NOLOCK, after a write transaction commits you need to end existing read-only transactions before starting a new writer. (Well, it's a bit looser than that, but that's a simple rule.)

Thanks for confirming my suspicion. I think that the correct option then is to use only short-lived cursor read transactions within fcntl()-protected blocks and to save/restore the previous cursor key where applicable.

Wietse