lmdb-zero

lmdb-zero is a near-zero-cost wrapper around LMDB designed to allow using the full range of features offered by LMDB while keeping it reasonably easy to write safe programs.

lmdb-zero is as much as possible a 1:1 mapping of the raw API, mainly providing RAII constructs and integration into Rust's borrow checker to ensure safety.

Documentation

Features

• Zero-copy API. Reads return references into the memory-mapped file. Using MDB_RESERVE to allocate space in the file and directly write to it is supported.

• Cursors directly map to the same operations provided by LMDB, but in a typesafe manner.

• Nested transactions.

• Full integration with the borrow checker. Read references are checked to not outlive their transaction or overlap with a write in the same transaction.

• Cursors and read transactions can be reset and reused.

Status

The API is complete and reasonably stable and is believed to be sound insofar as Rust's unsafety rules are actually defined.

This crate has not been thoroughly tested on architectures with strong alignment constraints, though the tests pass on ARM7. While the conversion API checks for correct alignment by default, issues such as #27060 could come up, and it is of course possible there are bugs in handling alignment here.

Changelog

0.4.4: New Environment and Database methods to enable interoperating with native C/C++ code. Database::dbi() renamed to as_raw(). The old name is still available but deprecated.

0.4.3: Fix panic on Cursor::get_multiple() if the current key has exactly one item. LmdbResultExt is now reexported from the crate root for better visibility.

0.4.2: Fix being unable to open databases in read-only environments. Fix future-incompatibility warning arising from Unaligned.

0.4.1: Tests updated to work on Rust 1.20. bitflags dependency updated. Neither of these are expected to have any impact on external code.

0.4.0: Minor breaking changes. ConstAccessor and WriteAccessor can now be dropped and re-obtained. Most types now support additional ownership/borrowing modes, which allows for dynamic lifetime management and other possibilities. Upgrade to liblmdb-sys 0.2.2 and bitflags 0.8.0. Fixes to documentation.

0.3.1: Metadata updates to reflect change of crate ownership. No software changes were made in this version.

0.3.0: Breaking Changes to the API, see section below. Migration is expected to be easy for most use-cases. Slight performance improvement due to additions of #[inline].

0.2.2: ResetTransaction is now actually public, making that part of the API more accessible. Add documentation for lifetimes.

0.2.1: Fix use-after-free when passing database name to mdb_dbi_open. Fix calling mdb_txn_abort after transaction commit fails. #1.

0.2.0: Switch from lmdb-sys to newer liblmdb-sys.

0.1.0: Initial release.

Breaking Changes in 0.4.0

A number of functions which formerly took an &SomeType parameter now take an Into<Supercow<SomeType>>. For the vast majority of existing code, this has no effect, but it could cause issues if older code was relying on an implicit Deref call (for example, via lazy_static!) to produce the correct reference type. If this causes issues, explicitly writing the dereferencing is required. For example, if your code originally had lmdb::Database::open(&ENV, ...) where ENV was declared via lazy_static!, it would need to be changed to lmdb::Database::open(&*ENV, ...).

ConstAccessor and WriteAccessor must now be strictly outlived by their transactions. No practical cases where this would be an issue are apparent, but if it comes up, code must be rearranged to ensure the accessor is dropped before the transaction. Note that now one can drop the accessor and later re-obtain it.

Breaking Changes in 0.3.0

lmdb::Error has been completely reworked. It is now an enum with the lmdb-zero errors cleanly separated from native LMDB errors. ValRejected now includes an error message.

FromLmdbBytes.from_lmdb_bytes() now returns a Result<&Self, String> instead of an Option. This is mainly to make alignment issues less subtle and point people directly to advice on how to fix the problem, but should be able to make other things clearer as well.

The mostly untested and somewhat questionable lax_alignment feature has been dropped. LmdbRaw now always enforces alignment requirements. Client code which wishes to operate on misaligned values which cannot use the Unaligned or #[repr(packed)] solutions will need to provide its own FromLmdbBytes implementations.

The primitive types which have alignment requirements (eg, i32, u64) are no longer LmdbRaw, as this made it too easy to write code depending on happenstance to align the values correctly. Client code now must wrap them in Unaligned to read them directly, or else provide its own unit structs if it has other needs. Note that these types and their arrays are still AsLmdbBytes.

Unfortunately, as a side-effect of the above, Wrapping<u8> and Wrapping<i8> are no longer LmdbRaw or LmdbOrdKey, but instead only LmdbRawIfUnaligned and LmdbOrdKeyIfUnaligned. Wrapping these in Unalinged will work in most cases without overhead.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.