Prevent opening datastore as read-write when it is already opened in read-only mode

[liss-h]

Currently, metall prevents to open the same datastore as read-write twice at the same time.
However, it allows to open a datastore as read-write even if it is already opened in read-only mode.

This causes issues/corruption when the read-writer actually writes. Metall should prevent this.

The desired behaviour would be, allow either:

• exactly one read-writer, no one else
• many readers, but no read-writer

Small snippet to reproduce:

{ // create datastore
    metall::manager mgr{metall::create_only, path};
}

metall::manager mgr_ro{metall::open_read_only, path};
metall::manager mgr_rw{metall::open_only, path}; // this should fail

[KIwabuchi]

That's a good point. I'll work on that.

[liss-h]

Thanks for merging my PR, would you mind creating a release?

[liss-h]

Nevermind I found a bug, I will create a PR to fix it

[liss-h]

#355

[liss-h]

Hi @KIwabuchi, would you mind creating a new release now that all PRs are merged?

[KIwabuchi]

@liss-h I'm working on some code clean up now. I'll try to release a new version today.