Prevent emacsclient from blocking other instances

Question

I use emacs in the terminal, typically with many emacs instances running at once in different tabs. This is a perfect use case for emacsclient, but it has always had various glitches that have stopped me from using it. In particular, sometimes one client will "block" the other, preventing it from doing anything. It looks like things work better in emacs 24.4, but it still happens.

For instance, if I start two clients, and run M-g M-g in one, to get the "Goto line" entry, the other client will be completely locked down until I quit in the first one.

I am using emacs 24.4 compiled without windowed support in Mac OS X 10.10 (in iTerm 2). I can reproduce it by starting emacs --daemon -Q. If you want the exact emacs binary I am using, download Miniconda for OS X and run conda install -c asmeurer emacs.

Answer 1

No, you can't "prevent emacsclient from blocking other [emacsclient] instances", because that's not what's happening.

emacsclient isn't running any elisp at all, so it's not the cause of the blockage. The Emacs server is running the elisp code, and Emacs is single-threaded -- so if the server thread is blocked then none of the clients will be receiving any output. There's nothing that a client can do about that.

You can, as mentioned in the comments, run multiple independent servers side by side. The simplest way to do this is via the --daemon=NAME command line argument:

emacs --daemon=foo
emacs --daemon=bar
emacsclient --socket-name=foo
emacsclient --socket-name=bar

or short-cut it to:

emacsclient -a "" --socket-name=foo
emacsclient -a "" --socket-name=bar

Answer 2

Got it! Good news is it works perfectly to have the minibuffer active in one window and type in another*. Bad news is it requires recompiling emacs, as it's the C-implementation of read_minibuf which forces single-kboard. It's probably possible to reimplement the minibuffer largely in elisp, intercepting most calls to it, but that would be somewhat complex...

Anyway, with emacs-26, the line to comment is src/minibuf.c:480,

temporarily_switch_to_single_kboard (XFRAME (mini_frame));

*I've tested it some, no promises it won't break things in subtle ways. Also note that the minibuffer uses a single, global input state, so opening the mini buffer closes any mini buffers opened in other frames. If you enable recursive-minibuffers, you do get your partial command back when the other person is done, but there's still likely to be some odd behaviour. Also, things like find-file use switch_to_single_kboard too.

I'll probably follow up on the mailing list to see if this can get turned into a proper customizable variable, possibly disabling all switch_to_single_kboard calls, as it's a must for sharing an emacs instance on a joint project.

If you want to disable single-keyboard mode everywhere, insert return; on a new line at src/keyboard.c:883.

As requested, a diff...

*** src/minibuf.c       2020-02-20 17:27:13.472345220 -0800
--- src.orig/minibuf.c  2020-02-20 17:26:20.164343599 -0800
***************
*** 478,482 ****
      Fraise_frame (mini_frame);

<   //temporarily_switch_to_single_kboard (XFRAME (mini_frame));

    /* We have to do this after saving the window configuration
--- 478,482 ----
      Fraise_frame (mini_frame);

>   temporarily_switch_to_single_kboard (XFRAME (mini_frame));

        /* We have to do this after saving the window configuration

And the other diff

*** src/keyboard.c      2020-02-20 17:27:01.452344855 -0800
--- src.orig/keyboard.c 2020-02-20 17:26:15.040343443 -0800
***************
*** 881,885 ****
  temporarily_switch_to_single_kboard (struct frame *f)
  {
<   return false;
    bool was_locked = single_kboard;
--- 881,884 ----

Answer 3

I frequently run into the problem of emacsclient instances being blocked, sometimes accompanied by an error message like "*ERROR*: Terminal 2 is locked, cannot read from it". A solution for unblocking it is to run:

emacsclient -e '(top-level)'

The problem and solution are discussed in more detail in https://github.com/emacs-mirror/emacs/blob/master/admin/notes/multi-tty and have to do with a session being stuck in a minibuffer prompt or recursive editing mode.