mbsync v1.3.2 breaking change: SubFolders config required

As part of my ongoing quest to Emacs-all-the-things, I’m a long-time satisfied user of the mu email client. I even wrote the latest CI infrastructure setup for the project as a way to give back to Dirk-Jan and the rest of the awesome mu team.

Anyway, mu doesn’t care how you get your email onto your machine (i.e. into your ~/Maildir), and so for that task I use isync/mbsync to download my mail (via IMAP) from fastmail. My fastmail config is online if you want to see how I set it all up.

Everything was working swimmingly, until a recent patch version update to mbsync (1.3.1 -> 1.3.2) broke things. All of a sudden, I started getting errors saying Maildir error: found subfolder ‘INBOX/Sent Items’, but store ‘fastmail-local’ does not specify SubFolders style

There was no mention of a breaking change relating to SubFolders style in the v1.3.2 release notes, but a look at the manual with man mbsync revealed this info:

  • SubFolders Verbatim|Maildir++|Legacy

    The on-disk folder naming style used for hierarchical mailboxes. This option has no effect when Flatten is used. Suppose mailboxes with the canonical paths top/sub/subsub and INBOX/sub/subsub, the styles will yield the following on-disk paths:

    • Verbatim - Path/top/sub/subsub and Inbox/sub/subsub (this is the style you probably want to use)

    • Maildir++ - Inbox/.top.sub.subsub and Inbox/..sub.subsub (this style is compatible with Courier and Dovecot - but note that the mailbox metadata format is not compatible). Note that attempts to set Path are rejected in this mode.

    • Legacy - Path/top/.sub/.subsub and Inbox/.sub/.subsub (this is mbsync’s historical style)

    • Default: unset; will error out when sub-folders are encountered

By setting SubFolders Verbatim in my .mbsyncrc things started working again. Hooray!

Of course, it might be the case that this was all documented somewhere, or that I’m just doing it wrong. But I hope that this is helpful for anyone who runs into the same issue, because having your email break is super frustrating; email is such a crucial part of my (and everyone’s) job, so it just needs to work.

Bonus: pretty mu4e mbsync filter

If you are using the same mu & mbsync combo for email, then you might find the following elisp snippet handy. It does some cool tricks with the mu4e~get-mail-process-filter so that the “in-progress” output from mu, which looks like this:

C: 1/1  B: 0/0  M: +0/0 *0/0 #0/0  S: +0/0 *0/0 #0/0

gets pretty colours in your *mu4e-update* buffer. It’s purely cosmetic, but I care about that stuff, and you (maybe?) should too. Anyway, here’s the relevant elisp to put in your Emacs init file.

(defun mu4e-pretty-mbsync-process-filter (proc msg)
  (ignore-errors
    (with-current-buffer (process-buffer proc)
      (let ((inhibit-read-only t))
        (delete-region (point-min) (point-max))
        (insert (car (reverse (split-string msg "\r"))))
        (when (re-search-backward "\\(C:\\).*\\(B:\\).*\\(M:\\).*\\(S:\\)")
          (add-face-text-property
           (match-beginning 1) (match-end 1) 'font-lock-keyword-face)
          (add-face-text-property
           (match-beginning 2) (match-end 2) 'font-lock-function-name-face)
          (add-face-text-property
           (match-beginning 3) (match-end 3) 'font-lock-builtin-face)
          (add-face-text-property
           (match-beginning 4) (match-end 4) 'font-lock-type-face))))))

(advice-add
 'mu4e~get-mail-process-filter
 :override #'mu4e-pretty-mbsync-process-filter)
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