Re: lmdb: is read operation permitted inside the write transaction?

[Hallvard Breien Furuseth <h.b.furuseth@usit.uio.no>]

On 12/05/15 19:36, Howard Chu wrote:
> Dmytro Milinevskyy wrote:
> > Basically I need to have an opportunity to call mdb_get while in write
> > transaction.
> > The sequence is smth like:
> >   - txn = mdb_txn_begin(flags=0)
> >   for i in 0..x:
> >     - v = mdb_get(txn, i)
> >     - mdb_put(txn, x+i, v)
> >     - mdb_put(txn, i, v+1)
> >   - mdb_txn_commit(txn)
> >
> > Will it be always valid data?
>
> Yes.

...No.  I expect he means v = the MDB_val returned by mdb_get().
mdb_put() can modify the data it points at.

See the MDB_val documentation in ldmb.h:
 * Values returned from the database are valid only until a subsequent
 * update operation, or the end of the transaction. Do not modify or
 * free them, they commonly point into the database itself.