[avar]

No, you wouldn't index from two processes concurrently, but for Emacs to read your mail in mu4e and for you to concurrently issue "mu index" on the command-line two processes ("mu server" and "mu index") would need to poen the same Xapian database read-write at the same time, and just like SQLite Xapian doesn't support that.

[fusiongyro]

... I still don't see why I would do that. Emacs runs offlineimap and then Emacs runs mu index. Why would I do it again myself from the command line?

[ams6110]

Yeah I use notmuch with Emacs and it's the same. If you try to search, refresh search results, or read email while the index process is running you get an error. It's not a big deal for me, because I download and index my mail from the command line, so I'll never be reading or sending email at the same time.

[donio]

This used to be true but notmuch has added support for Xapian's DB_RETRY_LOCK feature a couple of years ago so if you have non-ancient notmuch (0.23+) and Xapian (1.3.2+) then the second process will wait for the lock instead of erroring out. The feature can be disabled at compile time if you don't want it for some reason.