Notmuch is (currently?) not thread safe

[kevinboulain]

FYI.

Quoting the readme:

So, as far as I am concerned, it is not thread safe. Hence, all pointers are internally tracked with Rcs.

Sadly, that's not enough and I'm not sure what's the best course of action for notmuch-rs (introducing locking in all functions seems a bit overkill) but I've asked on the mailing list (with an example fix) if Notmuch is supposed to be thread safe.

Here is a repro:

#[cfg(test)]
mod tests {
    fn setup() {
        let directory = tempfile::tempdir().unwrap();
        let database = notmuch::Database::create(directory.path()).unwrap();
    }
    #[test]
    fn thread1() {
        setup()
    }
    #[test]
    fn thread2() {
        setup()
    }
}

while cargo test; do :; done

Resulting in:

running 2 tests
error: test failed, to rerun pass `--bin repro`

Caused by:
  process didn't exit successfully: `.../repro/target/debug/deps/repro-fe1206eee7835a6c` (signal: 11, SIGSEGV: invalid memory reference)