On Sat, 2014-03-22 at 22:49 +0000, [email protected] wrote: > When you operate on a dbi using a cursor and accidentally close the dbi before > committing the transaction of the cursor, mdb_txn_commit will return > MDB_BAD_VALSIZE. This is quite misleading because the problem doesn't actually > have anything to do with what the error message suggests. ("Too big key/data, > key is empty, or wrong DUPFIXED size") > > I suggest to either add a new kind of error code for this situation or to > extend > the description of MDB_BAD_VALSIZE.
Let's see -- here are the options I can see (after an IRC chat): 1) Extending the MDB_BAD_VALSIZE description. Simple. However: This is documented as a user error, and it's one which lmdb won't always catch. If you also called mdb_dbi_open() and the DBI got reused, lmdb silently updates the wrong named database. So I'm a bit wary about both of these fixes: Documenting this case can be misleading if it makes users expect it to be caught. Unless people know better - it corresponds to EBADF for using a closed file descriptor, not caught if open() reused it. 2) Catch this case and instead return a generic "something is wrong" - MDB_BAD_TXN or MDB_INCOMPATIBLE. And maybe assert(did not happen) to teach users to avoid this. Needs to be done where md_name is used: Commit/drop(named DB), page_search(stale named DB), cursor_touch(clean named DB). Or pass F_SUBDATA inwards and tweak the "return MDB_BAD_VALSIZE" statements to detect this. After playing around a bit, I guess this is my favorite. 3) Leave the misleading message alone, which can send the user off to a wild goose chase (as described on IRC). Or we can add code to also catch close followed by open: 4) Put hash(DB name) in MDB_db.md_pad and verify it before overwriting a named DB. Costs execution time even when there's no error. Must be done in commit, drop and maybe cursor_touch. (I've pushed a demo patch for just commit.) 5) Add field MDB_dbx.md_dbiseq = DBI usage sequence number, incremented when dbi_open creates (not reuses) a DBI and in dbi_close. Copy it to a new malloced array txn->mt_dbiseqs[] in write txns, verify it in at least commit and drop. This means close()-open(same DB) will also be caught, which seems a good thing since with (4) it will work unreliably: Only if open() reuses the same DBI for the same DB.
