bug#47439: 27.2; In daemon mode, if after-init-hook errors out, the server does not start

[Gilles <gilles.usenet@gmail.com>]

1. Make an init file with the following line: (add-hook 'after-init-hook 'barf)
2. Start Emacs in daemon mode: HOME=$PWD emacs --no-site-file --daemon=foo

Expected behavior: Emacs starts normally (including starting the
server), with the error logged in *Messages* (and optionally on Emacs's
stderr), as happens for errors in the init file or in
emacs-startup-hook.

Actual behavior: the server does not start, so there is no way to
open a frame other than the initial daemon frame.

I did most of the investigation with a debug build of Emacs 27.2 on
Linux, but I observed the same symptoms with Emacs 27.1 on macOS,
with the official Ubuntu 20.04 build of Emacs 26.3, and with the
official Ubuntu 16.04 build of Emacs 24.5.1.

In GNU Emacs 27.2 (build 2, x86_64-pc-linux-gnu, X toolkit, Xaw scroll bars)
 of 2021-03-27 built on darkstar
Repository revision: deef5efafb70f4b171265b896505b92b6eef24e6
Repository branch: HEAD
Windowing system distributor 'The X.Org Foundation', version 11.0.12009000
System Description: Ubuntu 20.04.2 LTS

Configured using:
 'configure --with-x-toolkit=lucid --enable-checking=yes,glyphs
 --enable-check-lisp-object-type
 --prefix=/home/gilles/Packages/emacs-27.2-dbg 'CFLAGS=-g3 -O0'
 LDFLAGS=-g3'

Configured features:
XPM JPEG TIFF GIF PNG SOUND GSETTINGS GLIB NOTIFY INOTIFY LIBSELINUX
GNUTLS LIBXML2 FREETYPE HARFBUZZ XFT ZLIB TOOLKIT_SCROLL_BARS LUCID X11
XDBE XIM MODULES THREADS PDUMPER GMP

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

Backtrace of the daemon process:
[New LWP 1377142]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
0x00007f214d60f246 in __pselect (nfds=8, readfds=0x7ffeb73ecd70,
writefds=0x7ffeb73ecdf0, exceptfds=0x0, timeout=<optimized out>,
sigmask=<optimized out>) at ../sysdeps/unix/sysv/linux/pselect.c:48
48      ../sysdeps/unix/sysv/linux/pselect.c: No such file or directory.
SIGINT is used by the debugger.
Are you sure you want to change it? (y or n) [answered Y; input not
from terminal]
DISPLAY = :0
TERM = xterm
Breakpoint 1 at 0x562be7588dbd: file emacs.c, line 379.
Breakpoint 2 at 0x562be753a6f6: file xterm.c, line 10145.
#0  0x00007f214d60f246 in __pselect (nfds=8, readfds=0x7ffeb73ecd70,
writefds=0x7ffeb73ecdf0, exceptfds=0x0, timeout=<optimized out>,
sigmask=<optimized out>) at ../sysdeps/unix/sysv/linux/pselect.c:48
#1  0x0000562be7845fbe in really_call_select (arg=0x7ffeb73ecc60) at
thread.c:586
#2  0x0000562be76ac0ff in flush_stack_call_func (func=0x562be7845ef3
<really_call_select>, arg=0x7ffeb73ecc60) at alloc.c:4951
#3  0x0000562be78460ba in thread_select (func=0x7f214d60f180
<__pselect>, max_fds=8, rfds=0x7ffeb73ecd70, wfds=0x7ffeb73ecdf0,
efds=0x0, timeout=0x7ffeb73ed3a0, sigmask=0x0) at thread.c:616
#4  0x0000562be78ae770 in xg_select (fds_lim=8, rfds=0x7ffeb73ed410,
wfds=0x7ffeb73ed490, efds=0x0, timeout=0x7ffeb73ed3a0, sigmask=0x0) at
xgselect.c:117
#5  0x0000562be77b7c7c in wait_reading_process_output (time_limit=0,
nsecs=0, read_kbd=-1, do_display=true, wait_for_cell=XIL(0),
wait_proc=0x0, just_wait_proc=0) at process.c:5572
#6  0x0000562be759f260 in kbd_buffer_get_event (kbp=0x7ffeb73ed770,
used_mouse_menu=0x7ffeb73edd75, end_time=0x0) at keyboard.c:3866
#7  0x0000562be759989d in read_event_from_main_queue (end_time=0x0,
local_getcjmp=0x7ffeb73edb80, used_mouse_menu=0x7ffeb73edd75) at
keyboard.c:2156
#8  0x0000562be7599cb3 in read_decoded_event_from_main_queue
(end_time=0x0, local_getcjmp=0x7ffeb73edb80, prev_event=XIL(0),
used_mouse_menu=0x7ffeb73edd75) at keyboard.c:2220
#9  0x0000562be759c030 in read_char (commandflag=1,
map=XIL(0x562be8a95aa3), prev_event=XIL(0),
used_mouse_menu=0x7ffeb73edd75, end_time=0x0) at keyboard.c:2830
#10 0x0000562be75ae73b in read_key_sequence (keybuf=0x7ffeb73edf60,
prompt=XIL(0), dont_downcase_last=false, can_return_switch_frame=true,
fix_current_buffer=true, prevent_redisplay=false) at keyboard.c:9554
#11 0x0000562be7596f70 in command_loop_1 () at keyboard.c:1350
#12 0x0000562be770e339 in internal_condition_case (bfun=0x562be7596ad2
<command_loop_1>, handlers=XIL(0x90), hfun=0x562be7596082 <cmd_error>)
at eval.c:1356
#13 0x0000562be7596693 in command_loop_2 (ignore=XIL(0)) at keyboard.c:1091
#14 0x0000562be770d71c in internal_catch (tag=XIL(0xcc60),
func=0x562be7596662 <command_loop_2>, arg=XIL(0)) at eval.c:1117
#15 0x0000562be759662d in command_loop () at keyboard.c:1070
#16 0x0000562be7595b49 in recursive_edit_1 () at keyboard.c:714
#17 0x0000562be7595d49 in Frecursive_edit () at keyboard.c:786
#18 0x0000562be758b75d in main (argc=3, argv=0x7ffeb73ee3e8) at emacs.c:2067
[Inferior 1 (process 1377141) detached]