heed & heed3

This release brings a new experimental functionality: the possibility to create multiple nested read transactions from an uncommitted write transaction. You can also examine another usage in the dedicated example file or in this Meilisearch pull request.

We are attempting to merge the necessary changes to upstream LMDB, but it will require additional work to ensure it produces correct C code. You can review the dedicated OpenLDAP tracking issue to see how it progresses.

// opening a write transaction
let mut wtxn = env.write_txn()?;

// [use the RwTxn to write into the env]

// opening multiple read-only transactions without committing beforehand
let rtxns = (0..1000).map(|_| env.nested_read_txn(&wtxn)).collect::<heed::Result<Vec<_>>>()?;

// [use the RoTxns and move them onto different threads]