Eric J <eric@deptj.eu> writes:

> However, if I do it twice, in different processes, at the same time, one
> file is added and tagged properly, the other is not (totally unfindable
> by notmuch search). Neither process reports any error, and they both log
> their actions normally. Actually a third simultaneous process also fails
> to leave any result in the database.

It should be impossible for more than one process to open a Xapian
database for writing at the same time. So if the processes are really
running in parallel, you should be getting error codes from the later
calls to notmuch_database_open{_verbose}. You claim that's not
happening, which is puzzling. Maybe you can try to duplicate your
problem with a tiny C program.