Checking for already opened LMDB Environments

[darnuria]

Heed strive to pursue opening LMDB safely, so a database environment must be opened only once inside a process address space.

Any subsequent opening shall return the corresponding already opened environment.

It was done with Path Canonization but it fails to tackle symlinks, hardlinks and renaming/move of the whole database environment (both locks and database).

In an ideal world the environment already open checking must prevail:

• [ ] The solution should avoid keeping the environment "open" aka holding a file descriptor elsewhere see https://github.com/meilisearch/heed/pull/179#issue-1795327221
• [x] First opening
• [x] Opening with the same path
• [ ] Access through symlink
• ~[ ] Access through hardlink~ Not possible since hard_link only apply to files and we track dir for envs.
• [ ] Renaming

Related pull requests/ Issue

• Idea based on https://github.com/meilisearch/heed/issues/145#issuecomment-1384539350
• #179
• LMDB assumptions: https://github.com/meilisearch/heed/issues/20#issuecomment-1378863562

Open question

Is Canonization still pertinant if we check already-openned by Unix: (device/inode) Windows: (Driver/fileID) through same_file?

How to do it compatible with windows without lefting file open.

Problems

Lmdb rely on Unixes on posix filelocking through fnctl

[darnuria]

@Kerollmops would suggest removing from 0.20.0 this issue the solution may cause more trouble than the initial issue.