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 [Stale] Compare identical environments by using a same_file::Handle #179 (comment)
• First opening
• 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 Env open with Flags::MdbNoSubDir flag requires target file to exists #145 (comment)
• [Stale] Compare identical environments by using a same_file::Handle #179
• LMDB assumptions: Make transaction opening more safe #20 (comment)

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.