Add `Database::get_or_put`: insert if value does not exist, otherwise return previous value

[nolanderc]

Pull Request

Related issue

N/A Based on #251

What does this PR do?

• Adds a new Database::get_or_put method which uses the MDB_NOOVERWRITE flag to simultaneously attempt to insert a key/value pair, or get the previous value if it already exists.

  MDB_NOOVERWRITE - enter the new key/data pair only if the key does not already appear in the database. The function will return MDB_KEYEXIST if the key already appears in the database, even if the database supports duplicates (MDB_DUPSORT). The data parameter will be set to point to the existing item.

• This is used as an optimization over the following pattern:

  if let Some(value) = db.get(&txn, key) {
  // ...
  } else {
  db.put(&mut txn, key, new_value);
  }

which can now be written as (with one fewer lookups):

if let Some(value) = db.get_or_put(&mut txn, key, new_value) {
    // ...
}

One drawback of the above is that it would serialize/allocate the new_value even if it would not end up being inserted. This can be avoided using the get_or_put_reserved* methods, mirroring the put_reserved* method.

PR checklist

Please check if your PR fulfills the following requirements:

• [x] Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
• [ ] Have you read the contributing guidelines?
• [x] Have you made sure that the title is accurate and descriptive of the changes?

Thank you so much for contributing to Meilisearch!

[nolanderc]

However, I am wondering if I don't want to write a system similar to the HashMap::entry method, at some point 🤔 What do you think?

This could probably be done using LMDB's cursor API. Looking at the source, there is an optimization in case a cursor is already positioned on the correct data item for an insert. Thus, an entry-API could efficiently be implemented as (pseudocode):

mdb_cursor_get(&mut cursor, &mut key, &mut data, MDB_SET);
if data.is_none() {
    // ... encode value into `data` ...
    mdb_cursor_put(&mut cursor, &mut key, &mut data);
}

[Kerollmops]

You are absolutely right, and I already worked on a cursor wrapper in the cursor.rs file, It can maybe be improved 🤔 Thank you for your changes 👍