bug#21965: 24.5; Emacs freezes when canceling at open file

[Maneesh Yadav <maneeshkyadav@gmail.com>]

I was experiencing frequent 'freezing' after pressing 'Ctrl-g-g' (I
think I have defaulted to pressing twice to make sure I've cancelled out
of partial command, seeing "quit" makes me feel happier than "C-x C-g is
undefined").

This doesn't seem completely repeatable (but I did just do it on my
first try just now): I start 'emacs -Q' and Ctrl-x Ctrl-f to see a
prompt in the minibuffer for a file and then Ctrl-g Ctrl-g then emacs
freezes (does not respond to Ctrl-g or any keystrokes...I don't think
I'm naively invoking an process control key binding (checked with stty-a).




Ugh, I am behind on my debug skills, I couldn't find out how to
compile my macports version with debug symbols.  After I kill  with
"pkill -USR2 emacs", I get this output (is this useful?):

0   emacs                               0x000000010009ebe9
emacs_backtrace + 87
1   emacs                               0x0000000100084ffa
terminate_due_to_signal + 97
2   emacs                               0x000000010009d77b
init_baud_rate + 0
3   emacs                               0x000000010008e5d3
handle_interrupt + 590
4   emacs                               0x000000010009e808
deliver_process_signal + 53
5   libsystem_platform.dylib            0x00007fff8c44df1a _sigtramp +
26
6   ???                                 0x0646666406466664 0x0 +
452161392385353316
7   libglib-2.0.0.dylib                 0x0000000100888ba1 g_mutex_lock
+ 26
8   libglib-2.0.0.dylib                 0x000000010084c284
g_main_context_acquire + 42
9   emacs                               0x00000001001447cc xg_select +
135
10  emacs                               0x000000010012dbe2
wait_reading_process_output + 2074
11  emacs                               0x00000001000075b5 sit_for + 260
12  emacs                               0x000000010008be56 read_char +
5024
13  emacs                               0x000000010008918c
read_key_sequence + 1526
14  emacs                               0x0000000100088973
command_loop_1 + 3983
15  emacs                               0x00000001000f251e
internal_condition_case + 251
16  emacs                               0x00000001000966c9
command_loop_2 + 53
17  emacs                               0x00000001000f1f48
internal_catch + 243
18  emacs                               0x00000001000871ea
recursive_edit_1 + 206
19  emacs                               0x000000010008738d
Frecursive_edit + 236
20  emacs                               0x000000010008648e main + 4658
21  libdyld.dylib                       0x00007fff8707a5c9 start + 1
22  ???                                 0x0000000000000001 0x0 + 1



In GNU Emacs 24.5.1 (x86_64-apple-darwin14.4.0)
 of 2015-08-25 on Maneeshs-MacBook-Air.local
Configured using:
 `configure --prefix=/opt/local --without-x --without-dbus
 --without-gconf --without-libotf --without-m17n-flt --without-gpm
 --without-gnutls --with-xml2 --infodir /opt/local/share/info/emacs
 'CFLAGS=-pipe -Os -arch x86_64' CPPFLAGS=-I/opt/local/include
 'LDFLAGS=-L/opt/local/lib -Wl,-headerpad_max_install_names -Wl,-no_pie
 -arch x86_64''

Important settings:
  value of $LC_ALL: en_US.UTF-8
  value of $LC_CTYPE: en_US.UTF-8
  value of $LANG: us
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  electric-indent-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
  line-number-mode: t
  transient-mark-mode: t

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Making completion list... [2 times]

Load-path shadows:
None found.

Features:
(shadow sort gnus-util mail-extr emacsbug message format-spec rfc822 mml
mml-sec mm-decode mm-bodies mm-encode mail-parse rfc2231 mailabbrev
gmm-utils mailheader sendmail regexp-opt rfc2047 rfc2045 ietf-drums
mm-util help-fns mail-prsvr mail-utils help-mode easymenu xterm
time-date tooltip electric uniquify ediff-hook vc-hooks lisp-float-type
tabulated-list newcomment lisp-mode prog-mode register page menu-bar
rfn-eshadow timer select mouse jit-lock font-lock syntax facemenu
font-core frame cham georgian utf-8-lang misc-lang vietnamese tibetan
thai tai-viet lao korean japanese hebrew greek romanian slovak czech
european ethiopic indian cyrillic chinese case-table epa-hook
jka-cmpr-hook help simple abbrev minibuffer nadvice loaddefs button
faces cus-face macroexp files text-properties overlay sha1 md5 base64
format env code-pages mule custom widget hashtable-print-readable
backquote make-network-process gfilenotify multi-tty emacs)

Memory information:
((conses 16 74606 6954)
 (symbols 48 16694 0)
 (miscs 40 32 141)
 (strings 32 9404 4608)
 (string-bytes 1 257827)
 (vectors 16 7133)
 (vector-slots 8 335922 32343)
 (floats 8 52 663)
 (intervals 56 176 13)
 (buffers 960 12))