bug#36648: emacs signals under gdb - [Re: connecting to a lost server process

[Madhu]

[sorry if this is not the correct bug format, I'm replying to a message
on gnu.emacs.help]

* Eli Zaretskii <mailman.985.1562918079.2688.help-gnu-emacs@gnu.org> :
Wrote on Fri, 12 Jul 2019 10:54:28 +0300:

>> Date: 12 Jul 2019 06:20:57 -0000
>> From: Madhu <enometh@meer.net>
>>
>> Earlier it used to be possible to gdb attach to the emacs process and
>> to restart the server. Something like
>> Feval(Fcar(Fread_from_string(build_string("(server-start)"),Qnil,Qnil)),Qnil)
>> But for some time now that route hits a
>> terminate_due_to_signal. (Apparently make-network-process tries to
>> signal an error which calls emacs_abort which ends the show)
>
> Please report a bug with the details of that crash, it may or may not
> be a bug.

I noticed a couple of different failure modes but this problem seems to
be related to pselect.  I'm appending one backtrace. Note within the
transcript emacs is started with

args:  -Q --eval '(progn (load-library "custom") (load-library "server") (setq 
server-name "emacs-test"))'

After emacs is started by gdb I send it a TSTP to make it enter the
debugger.

Reading symbols from emacs-27-athena...
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 = dumb
Breakpoint 1 at 0x41657c: file 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/emacs.c, 
line 377.
Breakpoint 2 at 0x4c4b20: file 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/xterm.c, 
line 10041.

(gdb) set args  -Q --eval '(progn (load-library "custom") (load-library 
"server") (setq server-name "emacs-test"))'

(gdb) r
Starting program: /usr/bin/emacs-27-athena -Q --eval '(progn (load-library 
"custom") (load-library "server") (setq server-name "emacs-test"))'
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
desired fingerprint: 
4f725133cab4014b55eba47d01b62b95cf9f5c2332a7167947770875a0da328e
found fingerprint: 
87155434115e758f6db5abaf07c65b6b14264b02baec22545448e62abb35872f
[New Thread 0x7ffff4b5b700 (LWP 10762)]

Thread 1 "emacs-27-athena" received signal SIGTSTP, Stopped (user).
0x00007ffff6b47926 in __pselect (nfds=7, readfds=0x7fffffffcf50, 
    writefds=0x7fffffffcfd0, exceptfds=0x0, timeout=<optimized out>, 
    sigmask=<optimized out>) at ../sysdeps/unix/sysv/linux/pselect.c:69
69        result = CALL_PSELECT6 (nfds, readfds, writefds, exceptfds, timeout,

(gdb) p 
Feval(Fcar(Fread_from_string(build_string("(server-start)"),Qnil,Qnil)),Qnil)

Thread 1 "emacs-27-athena" hit Breakpoint 1, terminate_due_to_signal (
    sig=sig@entry=6, backtrace_limit=backtrace_limit@entry=40)
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/emacs.c:377
377     
The program being debugged stopped while in a function called from GDB.
Evaluation of the expression containing the function
(Feval) will be abandoned.
When the function is done executing, GDB will silently stop.
(gdb) back
#0  terminate_due_to_signal (sig=sig@entry=6, 
    backtrace_limit=backtrace_limit@entry=40)
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/emacs.c:377
#1  0x0000000000416a50 in emacs_abort ()
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/sysdep.c:2437
#2  0x0000000000418a90 in signal_or_quit (error_symbol=XIL(0x58e0), 
    data=XIL(0xdd5663), keyboard_quit=<optimized out>)
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/eval.c:1595
#3  0x0000000000418aa9 in Fsignal (error_symbol=<optimized out>, 
    data=data@entry=XIL(0xdd5663))
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/eval.c:1565
#4  0x0000000000417118 in xsignal (data=XIL(0xdd5663), 
    error_symbol=<optimized out>)
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/lisp.h:4101
#5  report_file_errno (string=<optimized out>, name=name@entry=XIL(0xdd55f3), 
    errorno=<optimized out>)
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/fileio.c:222
#6  0x000000000041a663 in connect_network_socket (proc=XIL(0xd7f415), 
    addrinfos=<optimized out>, use_external_socket_p=<optimized out>)
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/process.c:3633
#7  0x00000000005a2cc6 in Fmake_network_process (nargs=<optimized out>, 
    args=<optimized out>)
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/process.c:4246
#8  0x000000000055f533 in Ffuncall (nargs=11, args=args@entry=0x7fffffffc298)
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/lisp.h:2087
#9  0x0000000000599e60 in exec_byte_code (bytestr=<optimized out>, 
    vector=<optimized out>, maxdepth=<optimized out>, 
    args_template=<optimized out>, nargs=<optimized out>, 
    args=<optimized out>)
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/bytecode.c:633
#10 0x000000000055f4bf in Ffuncall (nargs=2, args=args@entry=0x7fffffffc688)
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/eval.c:2817
#11 0x0000000000599e60 in exec_byte_code (bytestr=<optimized out>, 
    vector=<optimized out>, maxdepth=<optimized out>, 
    args_template=<optimized out>, nargs=<optimized out>, 
    args=<optimized out>)
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/bytecode.c:633
#12 0x0000000000560ec1 in apply_lambda (fun=XIL(0xaee9f5), 
    args=<optimized out>, count=count@entry=6)
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/eval.c:2935
#13 0x00000000005611f3 in eval_sub (form=<optimized out>)
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/eval.c:2349
#14 0x0000000000562d89 in Feval (form=XIL(0xdd5543), lexical=<optimized out>)
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/eval.c:2103
#15 <function called from gdb>
#16 0x00007ffff6b47926 in __pselect (nfds=7, readfds=0x7fffffffcf50, 
    writefds=0x7fffffffcfd0, exceptfds=0x0, timeout=<optimized out>, 
    sigmask=<optimized out>) at ../sysdeps/unix/sysv/linux/pselect.c:69
#17 0x00000000005bead0 in really_call_select (arg=0x7fffffffce80)
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/thread.c:586
#18 0x00000000005bf3e7 in thread_select (func=<optimized out>, 
    max_fds=max_fds@entry=7, rfds=rfds@entry=0x7fffffffcf50, 
    wfds=wfds@entry=0x7fffffffcfd0, efds=efds@entry=0x0, 
    timeout=timeout@entry=0x7fffffffd550, sigmask=0x0)
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/thread.c:616
#19 0x00000000005dd722 in xg_select (fds_lim=7, 
    rfds=rfds@entry=0x7fffffffd660, wfds=wfds@entry=0x7fffffffd6e0, 
    efds=efds@entry=0x0, timeout=timeout@entry=0x7fffffffd550, 
    sigmask=sigmask@entry=0x0)
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/xgselect.c:117
#20 0x00000000005a3fa4 in wait_reading_process_output (
    time_limit=time_limit@entry=0, nsecs=nsecs@entry=0, 
    read_kbd=read_kbd@entry=-1, do_display=true, 
    wait_for_cell=wait_for_cell@entry=XIL(0), wait_proc=wait_proc@entry=0x0, 
    just_wait_proc=0)
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/process.c:5440
#21 0x00000000004f15c9 in kbd_buffer_get_event (end_time=0x0, 
    used_mouse_menu=0x7fffffffdebb, kbp=<synthetic pointer>)
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/lisp.h:1058
#22 read_event_from_main_queue (used_mouse_menu=0x7fffffffdebb, 
    local_getcjmp=0x7fffffffdb50, end_time=0x0)
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/keyboard.c:2138
#23 read_decoded_event_from_main_queue (used_mouse_menu=<optimized out>, 
    prev_event=<optimized out>, local_getcjmp=<optimized out>, 
    end_time=<optimized out>)
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/keyboard.c:2202
#24 read_char (commandflag=1, map=XIL(0xdb7d13), prev_event=XIL(0), 
    used_mouse_menu=0x7fffffffdebb, end_time=0x0)
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/keyboard.c:2810
#25 0x00000000004f3256 in read_key_sequence (keybuf=<optimized out>, 
    prompt=XIL(0), dont_downcase_last=<optimized out>, 
    can_return_switch_frame=true, fix_current_buffer=true, 
    prevent_redisplay=<optimized out>)
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/keyboard.c:9125
#26 0x00000000004f4b8e in command_loop_1 ()
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/lisp.h:1058
#27 0x000000000055e857 in internal_condition_case (
    bfun=bfun@entry=0x4f49b0 <command_loop_1>, 
    handlers=handlers@entry=XIL(0x50d0), hfun=hfun@entry=0x4ec060 <cmd_error>)
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/eval.c:1352
#28 0x00000000004e703c in command_loop_2 (ignore=ignore@entry=XIL(0))
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/lisp.h:1058
#29 0x000000000055e7b1 in internal_catch (tag=tag@entry=XIL(0xc9f0), 
    func=func@entry=0x4e7020 <command_loop_2>, arg=arg@entry=XIL(0))
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/eval.c:1113
#30 0x00000000004e6fe4 in command_loop ()
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/lisp.h:1058
#31 0x00000000004ebc76 in recursive_edit_1 ()
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/keyboard.c:714
#32 0x00000000004ebfa0 in Frecursive_edit ()
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/keyboard.c:786
#33 0x000000000041c2c3 in main (argc=4, argv=0x7fffffffe338)
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/emacs.c:2126

Lisp Backtrace:
"make-network-process" (0xffffc2a0)
"server-running-p" (0xffffc690)
"server-start" (0xffffcbc0)

(gdb) up 5
#5  report_file_errno (string=<optimized out>, name=name@entry=XIL(0xdd55f3), 
    errorno=<optimized out>)
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/fileio.c:222
222       xsignal (Fcar (data), Fcdr (data));

(gdb) pp data
(file-missing "make client process failed" "No such file or directory" :name 
"server-client-test" :family local :server nil :noquery t :service 
"/dev/shm/madhu/emacs/emacs-test")

(gdb) up
#6  0x000000000041a663 in connect_network_socket (proc=XIL(0xd7f415), 
    addrinfos=<optimized out>, use_external_socket_p=<optimized out>)
    at 
/var/tmp/portage/app-editors/emacs-27.0.50-r3/work/emacs-27.0.50/src/process.c:3633
3633              unbind_to (count, Qnil);
(gdb) 

Additional notes:
- make-network-process signals a (file-missing "make client process failed" "No 
such file or directory" :name "server-client-test" :family local :server nil 
:noquery t :service "/dev/shm/madhu/emacs/emacs-test")
  which is not supposed to happen since the file would be created by the
  system call to socket.  I suspect a wrong error is being reported
  because of some problem with pselect (which is interrupted when we
  enter gdb)

- load custom.el to try to avoid an unrelated emacs-abort which
  otherwise seems to happen from within the custom mechanism
- another unrelated emacs-bug. M-x gud-gdb -- emacs .. with the above
  arg line fails to pass the quoted arguments to gdb.