bug#18940: 24.4; vc-hg does not disable pager, leading to hangs (at least with tramp)

[Daniel Pittman]

C-x C-f /sshx:dpittman@remotehost.local:/path/to/file/in/hg/repo.txt

…and Emacs hangs.

I had problems with the Hg backend hanging via tramp; checking showed that it
was hung waiting on `less`, which was sitting there telling me that the
terminal wasn't fully featured and could it please, kindly, have some human
input to let it know that it was OK to continue.

Fixing this wasn't too dire: we can just ask hg to not use a pager, which
solves the problem.  It might be a good idea to ask it to be entirely
non-interactive, but I didn't test that, and don't require it for this to work
on my system.

After these changes are applied, the problem is resolved:

(require 'vc-hg)
(defun dp/vc-hg-state-interactive-pager-workaround (args)
  "Potentially modify process-file arguments to work around a vc-hg bug"
  (if (and (equal (first args) vc-hg-program)
           (not (cl-search vc-hg-global-switches args :test 'equal)))
      (concatenate 'list args vc-hg-global-switches)
    args))
(advice-add 'process-file :filter-args 
'dp/vc-hg-state-interactive-pager-workaround)
(setq vc-hg-global-switches '("--pager" "never"))


The `vc-hg-global-switches` setting *should* be enough, but unfortunately the
`vc-hg-state` function doesn't *use* that setting, which means that it out and
out needs awful hacks to make it work.

(I could have redefined the function, but this felt better, and should not
cause trouble if we actually fix the bug.)


Anyway, options that probably make sense to set to make this smoother:

1. `HGPLAIN` exists in the environment.

This disables things that might change output, and is recommended for
non-interactive calls to try and discourage random breakage.

2. `-y` or `--noninteractive`

These cause it to pick the first choice if a prompt would have been displayed,
instead of asking interactively.  Probably good on any of the read-only
operations, at the very least.

3. --color never

...because if you have a tty, and less/hg think it is an interactive enough
call to invoke the pager and complain to a human, you might well get color
displayed as well.

4. --pager never

...because you just don't want to page the output for non-interactive use.





In GNU Emacs 24.4.1 (x86_64-apple-darwin14.0.0, NS apple-appkit-1343.14)
 of 2014-10-22 on dpittman-mbp.local
Windowing system distributor `Apple', version 10.3.1343
Configured using:
 `configure --prefix=/opt/local --with-ns --without-x --without-dbus
 'CFLAGS=-pipe -Os -arch x86_64' CPPFLAGS=-I/opt/local/include
 'LDFLAGS=-L/opt/local/lib -Wl,-headerpad_max_install_names -arch x86_64''

Important settings:
  locale-coding-system: utf-8-unix

Major mode: Emacs-Lisp

Minor modes in effect:
  eldoc-mode: t
  shell-dirtrack-mode: t
  flyspell-mode: t
  show-paren-mode: t
  global-auto-revert-mode: t
  display-battery-mode: t
  desktop-save-mode: t
  auto-insert-mode: t
  tooltip-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  size-indication-mode: t
  column-number-mode: t
  line-number-mode: t
  auto-fill-function: do-auto-fill
  transient-mark-mode: t

Recent input:
<elided for privacy>

Recent messages:
Tramp: Decoding local file 
`/var/folders/29/dht2zzt17xnff164_5vrs41r95p38b/T/tramp.43368aIF.cinc' with 
`base64-decode-region'...done
Tramp: Inserting 
`/sshx:dpittman.sb.facebook.com:/home/dpittman/elided/for/privacy.txt'...done
privacy.txt has auto save data; consider M-x recover-this-file
Can't guess python-indent-offset, using defaults: 4

Tramp: Checking `vc-registered' for 
/sshx:dpittman.sb.facebook.com:/home/dpittman/elided/for/privacy.txt...done
Can't guess python-indent-offset, using defaults: 4
Running hg --pager never parent --template {rev} privacy.txt in foreground...
Running hg --pager never parent --template {rev} privacy.txt...OK = 0
Mark set

Load-path shadows:
~/.emacs.d/fb-emacs/git hides /opt/local/share/emacs/site-lisp/git
~/.emacs.d/fb-emacs/git-blame hides /opt/local/share/emacs/site-lisp/git-blame

Features:
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mm-encode mail-parse rfc2231 rfc2047 rfc2045 ietf-drums mailabbrev gmm-utils
mailheader gnus-win footnote skeleton vc-hg tramp-sh tramp tramp-compat
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