diagnosing an emacs bug

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  • Like all emacs users, I have shaped my emacs environment over the years until it fits me like a second skin. Recently an integral part of that environment broke. After painstaking (painful?) diagnosis, I determined that the cause was an emacs bug and came up with a workaround. Lastly, I filed my first emacs bug report!



    My Shell Switcher mode Customize settings stopped persisting across emacs sessions. I’ve used Shell Switcher for a long time without any problems, then *poof*, the Customize settings stopped working. The Shell Switcher related key bindings

    (global-set-key (kbd "C-'") 'shell-switcher-switch-buffer)
    (global-set-key (kbd "C-x 4 '") 'shell-switcher-switch-buffer-other-window)

    I set outside of Customize in .emacs persisted across sessions, but the Shell Switcher settings

    '(shell-switcher-mode t)
    '(shell-switcher-new-shell-function (quote shell-switcher-make-shell))

    set through Customize did not. Once I used the Customize interface to set the settings during a session they would stay set, however, when I quit emacs and restarted later, the settings were no longer effective.

    The Shell Switcher entries in Customize’s custom-set-variables in .emacs were still present (and had not changed from when they had been effective), they just did not control Shell Switcher’s behavior.

    My install: GNU Emacs (i386-apple-darwin11.4.0, NS apple-appkit-1138.47) of 2012-05-15.


    In a new emacs setting, C-' (switch buffer) triggered Shell Switcher:

    No active shell buffer, create new one? (y or n)

    So far so good, that’s one of the key bindings I set in .emacs. But when I responded y, I could see Shell Switcher loading (it should already have done that when I started emacs).

    Far more blatant, though, was that the buffer it launched was running the wrong shell: it came up in emacs shell.

    Unlike custom-set-variables in .emacs, Customize Apropos shell-switcher displayed the values for the two settings that matched their behavior in a new emacs session:

    setting value
    shell-switcher-mode nil
    shell-switcher-new-shell-function shell-switcher-make-eshell

    Stranger still, both settings had State: CHANGED outside Customize

    If I used Customize to set the values they would take and stay set for that session. When next I launched emacs, however, they were gone.

    hunt and gather

    skip this section if you aren’t into software journaling

    When this happened I was mystified. I discovered something was wrong when hitting C-' one day produced a buffer running the wrong shell. I was deep in the midst of coding and wasn’t going to drop everything in order to diagnose a different problem: I reset the setting, made a note of what had happened and got back to work.

    Several days later, when I got my first chance to look at what was wrong, I could not identify any change in my emacs configuration that might have caused the problem. I had not made any recent changes except that I had started using the built-in irc client, erc. Nothing unusual showed up in the Messages buffer. Running emacs with the debug switch didn’t shed any light.

    The CHANGED outside Customize thing really threw me. Was one of the other packages, perhaps one that was a major mode, meddling with Shell Switcher, which is, after all, a minor mode? I dug around the emacs world for insight (emacswiki and friends), looked on stack, consulted the great oracle of Google. I couldn’t find anything (usually I can at least find something that shakes my thinking loose so I can solve the problem).

    In the end I wrote my first question on stack. When I didn’t get any answers to my question after a day or so, I followed their advice and posted a link to it to my Facebook feed. The discussion that followed was helpful in that it validated that the solution was not obvious, lol.

    I had projects that took priority. Time passed. I earned my fourth stack badge: tumbleweed.



    When I finally made a few hours to tackle the problem, I used an internal fault model.

    I developed an initial question and sketched out a plan based on possible outcomes. The ideal question:

    • is testable without undue work;
    • directly addresses the problem;
    • has seperable, easy to detect, concrete outcomes;
    • and moves the solution forwards regardless of outcome.

    I don’t require questions to have yes or no answers; it’s more valuable to have a question that reflects the underlying information architecture. The sketch of the plan addresses any difficulties with not having yes/no answers, as it clearly identifies what each of the outcomes mean. The sketch is a sketch because it makes no sense to have some abstract, methodological requirement that an engineer expend resources fleshing out many branches when only one will be realized. That’s what judgment is for, and methodology should support judgment, not hamper it.

    A second realm in which the engineer’s judgment is critical comes in weighing the relative importance of the listed criteria in context. As we don’t function in an ideal world, we often have to trade-off one criteria with other(s). Note that context is wildly situational, as an engineer makes progress diagnosing a problem the relative importance of the criteria may change, even in the same problem. Perhaps the easiest case to see is when, upon initial exploration, what appeared to be a minor annoyance turns out to be a symptom of a deep-seated problem. At that point, the definition of ‘undue’ shifts.


    Do Shell Switcher mode customizations persist across emacs sessions if there are no changes made outside custom-set-variables?

    plan (sketch)

    Possible outcomes and associated next steps:

    • Neither customization persists.

      This outcome would indicate:

      • that whatever was causing the setting to be rewritten was less to be a result of some other package interfering with the Shell Switcher mode settings
      • a bug in customize (the CHANGED outside Customize flag setting) As I was sketching out the plan, I remembered an answer I had seen on stack to a question concerning in Customize; the answer had identified a bug in Customize associated with the same flag. I went digging for the question, while it did not shed light directly on my problem, it certainly helped with the fault model!

      In the case of this outcome, I could see two routes to follow:

      1. Comment out all entries in custom-set-variables except for the Shell Switcher entries to check for packages affecting Shell Switcher programmatically, after all, if this path proved true I was also looking at two bugs.
      2. Move all the Shell Switcher configuration outside of Customize in my .emacs and see if it persisted across sessions. If so, at the least I would have a viable workaround. Less diagnostic value than the previous step, though.
    • Both customizations persist.

      The inverse case; this outcome would suggest that something in my .emacs customizations was causing the problem.

      In this case, I would look for candidates to reintroduce that might be responsible. If all else failed, I could binary search my way through the list to find the culprit.

    • The intermediate cases: Shell Switcher mode stays enabled but the buffer is launched in the wrong shell, or the buffer launches in the right shell but Shell Switcher mode is disabled.

      Neither case seemed either remotely likely. With not one, but two cases that fit my fault model, I decided not to put resources towards researching potential causes and paths for the intermediate cases.


    As I organized my .emacs into separate files by topic long ago, I could turn off customizations outside of custom-set-variables by commenting out a few load statements:

    ; (load "~/.emacs.d/hilz_config/tramp.el")
    ; (load "~/.emacs.d/hilz_config/ruby_extensions.el")
    ; (load "~/.emacs.d/hilz_config/shell_switcher.el")
    ; (load "~/.emacs.d/hilz_config/personalize.el")

    Then I used the Customize interface to

    • enable Shell Switcher mode
    • set shell-switcher-new-shell-function to shell-switcher-make-shell

    and hit Apply and Save. I opened .emacs and verified the presence of

    '(shell-switcher-mode t)
    '(shell-switcher-new-shell-function (quote shell-switcher-make-shell))

    towards the end of the custom-set-variables entry. I restarted emacs.


    No, the customizations were lost.

    As I no longer was defining the key bindings, I now had to use M-x shell-switcher-new-shell to test Shell Switcher.1 When I did, the behavior I observed was the same as before: Shell Switcher loaded when I used the command and the buffer came up in the wrong shell.

    I looked in .emacs. The entries were still present in custom-set-variables. I invoked Customize and saw what I’ve been seeing all along:

    • the values for the Shell Switcher variables did not match the values set in the custom-set-variables entry
    • the Shell Switcher variables were shown as being in state CHANGED outside Customize.

    No change.

    drill down

    As the question appeared to me to have more diagnostic value, I started with


    Do Shell Switcher mode customizations persist across emacs sessions if no other customizations are made in .emacs?


    Whichever outcome I got, I intended to move on to my second question from my original plan next, as I needed a workaround and had spent quite a bit of time on the issue that day by then (I kept notes, which I later expanded to this post). If the Shell Switcher mode customizations persisted across emacs sessions when all the other customizations were commented out, then I would return at a later date and tackle the issue as I would have had one of my own customizations proven responsible.


    I commented out everything under custom-set-variables other than the Shell Switcher mode settings. Even the basic options: inhibiting the startup screen, the initial scratch message and the like.

    This is the first time I have encountered a project that might justify setting up a full development environment for emacs. Were I not in the middle of job interviews, I would have been tempted to take the time to build one so that I could approach this issue as I would a ruby project. I’ve looked at the tools for emacs before, but lack the community to work with that is such a critical part of accelerating learning. Gah.



    Ok. Is this a bug in Customize that runs deeper than a cosmetic issue (displaying the wrong flag setting)? Or is there something wrong with Shell Switcher? Or, of course, an interaction between the two…

    On to the next step of my plan, and hopefully a workaround.


    Do Shell Switcher mode settings set outside of Customize persist across sessions?

    By that I mean do I encounter the same problem if I use

    (setq shell-switcher-mode t)
    (setq shell-switcher-new-shell-function 'shell-switcher-make-shell)

    instead of the Customize interface. Given the results I had seen so far, I rather expected the settings to persist.


    If the settings did not persist, it would suggest a problem with Shell Switcher mode. (My first step when the settings stopped persisting had been to read the code for the mode, but nothing had jumped out at me.)

    If, on the other hand, the settings did not persist, it would suggest that the problem rested with customize. Shell Switcher is a minor mode that behaves somewhat differently from most minor modes, it isn’t really surprising that it might break more easily.



    • re-enabled my other customizations in custom-set-variables so that emacs was not ghastly to work in
    • took the Shell Switcher lines out of custom-set-variables
    • added the setq lines listed in the previous section to .emacs after custom-set-variables
    • restarted emacs.


    Yes. Shell Switcher settings persist.

    Ahah! A workaround! And lots of info for the customize folks. Seems awfully likley that this problem is another symptom of an underlying defect that caused the issue discussed in the stack question I had noticed, despite the differences between the two. Or perhaps not: the other user’s settings did persist.

    bug report

    I’ve never done this before. Ah well, nothing ventured…

    M-x report-emacs-bug
    Bug Subject: Customize is borked!

    Hmmmm…. maybe not.

    Bug Subject: Shell Switcher mode customization not persisted across sessions

    Phew! I’m in a buffer with instructions and useful information.

    Please describe exactly what actions triggered the bug, and the
    precise symptoms of the bug.

    Somehow I think I’ve got that one covered. I’m not sure if it’s kosher to point them at a blog post, but all I can do is ask. If they want me to cut-n-paste, I can resubmit, I suppose.

    Information about running emacs in debug mode, nice to know I started out the way that makes sense. A lot of interesting new stuff too! Ah, pointer to the manual section on how to write an emacs bug report. Must. Read.

    I navigate to the Known Problems file; despite decades of using emacs as my daily dev environment, I have never seen this before. I convert the file to outline mode (C-c C-t), while I don’t find anything remotely similar to my problem, I can’t help but laugh as I skim by problems I’ve had with various installations of emacs. I wonder if those were known problems at the time?

    As I read through the entry on bug reporting, I can see how my approach is on target as well as ways I can improve it. After a bit of internal grumbling, I sort out a minimal path to reproduce the bug. I stash a copy of .emacs and delete .emacs.

    I realize that one thing I have never made explicit is that M-x shell has worked perfectly all along.

    I launch vim, lol, so I can edit this post and record every step and keystroke I make. Yes, emacs is recording, too, but nothing beats making notes by hand for realizing what you’ve missed.

    Steps to cause the bug:

    1. M-x customize-apropos<RET>shell-switcher
    2. set shell-switcher-new-shell-function to shell-switcher-make-shell
    3. set shell-switcher-mode to on
    4. click on Apply and Save

      above steps create .emacs containing exactly

      (custom-set-variables ;; custom-set-variables was added by Custom. ;; If you edit it by hand, you could mess it up, so be careful. ;; Your init file should contain only one such instance. ;; If there is more than one, they won’t work right. ‘(shell-switcher-mode t) ‘(shell-switcher-new-shell-function (quote shell-switcher-make-shell)) )

      (custom-set-faces ;; custom-set-faces was added by Custom. ;; If you edit it by hand, you could mess it up, so be careful. ;; Your init file should contain only one such instance. ;; If there is more than one, they won’t work right. )

    5. quit emacs
    6. launch emacs: in this case /Applications/Emacs.app/Contents/MacOS/Emacs
    7. M-x shell-switcher-new-shell (launches buffer in eshell)
    8. M-x customize-apropos<RET>shell-switcher (CHANGED outside Customize)
    9. C-x C-f ~/.emacs (conflicting customize settings)

    now M-x report-emacs-bug

    Bug Subject: customize settings not persisting across sessions +
      CHANGED outside Customize flag issue

    Geez, I hate to use that long a subject but they really emphasize ‘just-the-facts-mam’ so I’m trying to avoid anything that comes close to guessing causes. Given that one goes at software testing using internal fault models, that’s a really tough discipline to follow.

    Fill in the steps. Come back here, run generate and deploy on this post so I can refer to it in the bug report, and submit.


    The turn around from the emacs maintainer list was very fast. Seems it was one bug, local to Shell Switcher, and the bug had already been fixed in the current version on github (last updated nine days ago, substantially after I started down this path, lol.) The fellows who responded gave some helpful examples of how these sorts of bugs trigger the CHANGED outside Customize flag.

    I find it interesting that I didn’t think to check the most recent version on github, given that it would normally be the first thing I would check in, say, a ruby build. Humbling, that. A really good reminder that we all have problems generalizing.

    I also couldn’t find anyway to directly source a package from a github repo, akin to how one can in a Gemfile, making the current solution not particularly scalable. Although, ah, that’s what Chef is for. Sweet!

    1. M-x describe-mode has only listed Shell Switcher after the first time I invoke it in the time I’ve been using it, rendering the command useless as a diagnostic tool for this problem.