I noticed this when investigating an aerc bug report [1].

When the notmuch config contains a different location for
database.mail_root (e.g. ~/.mail/user) and database.path (unset,
should default to ~/.local/share/notmuch/default), the filepath returned
by "notmuch_message_get_filename" points to the database path instead of
the mail_root path.
Here is a simple reproduction in python [2], that uses the problematic
"notmuch_message_get_filename".
"notmuch show" on the other hand does have the correct filename/path.


[1]: https://todo.sr.ht/~rjarry/aerc/106
[2]: https://paste.xenrox.net/~xenrox/d7048c7b5392df940586b42673aeba97c176d489