Re: [PATCH] Interferences between xwidgets and async processes?

[Akira Kyle]

On Thu, Oct 29, 2020 at 08:23 AM, Eli Zaretskii <eliz@gnu.org> 
wrote:

> > Using "top", I can see that the status of the process is "Z".
>
> Which AFAIU means that the process indeed exited, but our 
> sentinel was
> not called, and we therefore didn't yet request the process's 
> exit
> status.
>
> Is it possible that xwidgets somehow block SIGCHLD, or cause us 
> to
> miss it?


Its seems like this is what is happening. The relevant glib 
function which sets up the SIGCHLD signal handler is 
ref_unix_signal_handler_unlocked. It doesn't respect existing 
signal handlers like Emacs does in catch_child_signal and call 
them after it's own handling hence the need for the hack in 
init_process_emacs (process.c) that forces glib to not catch 
SIGCHLD. Setting a breakpoint on ref_unix_signal_handler_unlocked 
the backtrace:

#0  0x0000fffff704b66c in ref_unix_signal_handler_unlocked () from 
/nix/store/y7i19b8kyif8nqq47xvjxcv2rrd2qca0-glib-2.64.5/lib/libglib-2.0.so.0
#1  0x0000fffff704b8f0 in g_child_watch_source_new () from 
/nix/store/y7i19b8kyif8nqq47xvjxcv2rrd2qca0-glib-2.64.5/lib/libglib-2.0.so.0
#2  0x0000fffff725a6b4 in initable_init () from 
/nix/store/y7i19b8kyif8nqq47xvjxcv2rrd2qca0-glib-2.64.5/lib/libgio-2.0.so.0
#3  0x0000fffff72596b0 in g_subprocess_launcher_spawnv () from 
/nix/store/y7i19b8kyif8nqq47xvjxcv2rrd2qca0-glib-2.64.5/lib/libgio-2.0.so.0
#4  0x0000ffffc906707c in WebKit::ProcessLauncher::launchProcess() 
() from 
/nix/store/g197syqpsd3n2qb460i37lif4qhw6a6k-webkitgtk-2.28.4/lib/libwebkit2gtk-4.0.so.37
#5  0x0000ffffc8eea1d0 in WebKit::AuxiliaryProcessProxy::connect() 
() from 
/nix/store/g197syqpsd3n2qb460i37lif4qhw6a6k-webkitgtk-2.28.4/lib/libwebkit2gtk-4.0.so.37
#6  0x0000ffffc8f45660 in 
WebKit::WebProcessProxy::create(WebKit::WebProcessPool&, 
WebKit::WebsiteDataStore*, WebKit::WebProcessProxy::IsPrewarmed, 
WebKit::WebProcessProxy::ShouldLaunchProcess) ()
  from 
  /nix/store/g197syqpsd3n2qb460i37lif4qhw6a6k-webkitgtk-2.28.4/lib/libwebkit2gtk-4.0.so.37
#7  0x0000ffffc8f64bb4 in 
WebKit::WebProcessPool::createNewWebProcess(WebKit::WebsiteDataStore*, 
WebKit::WebProcessProxy::IsPrewarmed) ()
  from 
  /nix/store/g197syqpsd3n2qb460i37lif4qhw6a6k-webkitgtk-2.28.4/lib/libwebkit2gtk-4.0.so.37
#8  0x0000ffffc8f6b740 in 
WebKit::WebProcessPool::processForRegistrableDomain(WebKit::WebsiteDataStore&, 
WebKit::WebPageProxy*, WebCore::RegistrableDomain const&) ()
  from 
  /nix/store/g197syqpsd3n2qb460i37lif4qhw6a6k-webkitgtk-2.28.4/lib/libwebkit2gtk-4.0.so.37
#9  0x0000ffffc8f6b844 in 
WebKit::WebPageProxy::launchProcess(WebCore::RegistrableDomain 
const&, WebKit::WebPageProxy::ProcessLaunchReason) ()
  from 
  /nix/store/g197syqpsd3n2qb460i37lif4qhw6a6k-webkitgtk-2.28.4/lib/libwebkit2gtk-4.0.so.37
#10 0x0000ffffc8f6de3c in 
WebKit::WebPageProxy::loadRequest(WebCore::ResourceRequest&&, 
WebCore::ShouldOpenExternalURLsPolicy, API::Object*) ()
  from 
  /nix/store/g197syqpsd3n2qb460i37lif4qhw6a6k-webkitgtk-2.28.4/lib/libwebkit2gtk-4.0.so.37
#11 0x0000ffffc9006c70 in webkit_web_view_load_uri () from 
/nix/store/g197syqpsd3n2qb460i37lif4qhw6a6k-webkitgtk-2.28.4/lib/libwebkit2gtk-4.0.so.37
#12 0x0000ffffd684c528 in ?? () from 
/run/user/1001/webkitgtk-module.soxgSF5U.so
#13 0x000000000056eadc in funcall_module (function=0x3c97cf5, 
nargs=2, arglist=0xffffffff3110) at emacs-module.c:1186
#14 0x0000000000545180 in funcall_lambda (fun=fun@entry=0x2a90, 
nargs=nargs@entry=2, arg_vector=0xffffffff20f0, 
arg_vector@entry=0xffffffff3110) at eval.c:3083
#15 0x0000000000544570 in apply_lambda (fun=0x2a90, 
fun@entry=0x3c97cf5, args=<optimized out>, count=count@entry=24) 
at eval.c:3021
#16 0x00000000005447cc in eval_sub (form=<optimized out>) at 
eval.c:2424
#17 0x0000000000544dbc in Fprogn (body=0x0) at eval.c:471
#18 0x0000000000545048 in funcall_lambda (fun=<optimized out>, 
nargs=nargs@entry=2, arg_vector=arg_vector@entry=0xffffffff33e0) 
at lisp.h:1426
#19 0x000000000054274c in Ffuncall (nargs=nargs@entry=3, 
args=args@entry=0xffffffff33d8) at eval.c:2888
#20 0x000000000053f248 in Ffuncall_interactively (nargs=3, 
args=0xffffffff33d8) at callint.c:253
#21 0x0000000000542870 in Ffuncall (nargs=nargs@entry=4, 
args=args@entry=0xffffffff33d0) at lisp.h:2091
#22 0x0000000000542b70 in Fapply (nargs=nargs@entry=3, 
args=0xffffffff3570, args@entry=0xffffffff3660) at eval.c:2502
#23 0x00000000005405e4 in Fcall_interactively 
(function=0xffffffff3730, record_flag=0xffffffff3630, 
keys=0x57d418 <exec_byte_code+736>) at lisp.h:1008
#24 0x0000fffff27262d0 in 
F636f6d6d616e642d65786563757465_command_execute_0 ()
  from 
  /home/akyle/git/emacs/src/../native-lisp/28.0.50-aarch64-unknown-linux-gnu-08c187d985df753847bb1095eac8e39c/simple-fab5b0cffee734040ac60e9bc557cd20-fad2d213fe4820237be2ea92815644a6.eln
#25 0x0000000000542870 in Ffuncall (nargs=<optimized out>, 
args=args@entry=0xffffffff36a0) at lisp.h:2091
#26 0x000000000057d418 in exec_byte_code (bytestr=<optimized out>, 
vector=<optimized out>, maxdepth=<optimized out>, 
args_template=0x406, nargs=nargs@entry=1, args=<optimized out>, 
args@entry=0xffffffff3868)
   at bytecode.c:632
#27 0x0000000000545130 in fetch_and_exec_byte_code 
(args=0xffffffff3868, nargs=1, syms_left=<optimized out>, 
fun=<optimized out>) at lisp.h:1838
#28 funcall_lambda (fun=<optimized out>, nargs=nargs@entry=1, 
arg_vector=arg_vector@entry=0xffffffff3868) at eval.c:3077
#29 0x000000000054274c in Ffuncall (nargs=2, 
args=args@entry=0xffffffff3860) at eval.c:2888
#30 0x000000000057d418 in exec_byte_code (bytestr=<optimized out>, 
vector=<optimized out>, maxdepth=<optimized out>, 
args_template=0x2, nargs=nargs@entry=0, args=<optimized out>, 
args@entry=0xffffffff3ad0)
   at bytecode.c:632
#31 0x0000000000545130 in fetch_and_exec_byte_code 
(args=0xffffffff3ad0, nargs=0, syms_left=<optimized out>, 
fun=<optimized out>) at lisp.h:1838
#32 funcall_lambda (fun=<optimized out>, nargs=nargs@entry=0, 
arg_vector=arg_vector@entry=0xffffffff3ad0) at eval.c:3077
#33 0x000000000054274c in Ffuncall (nargs=1, 
args=args@entry=0xffffffff3ac8) at eval.c:2888
#34 0x000000000057d418 in exec_byte_code (bytestr=<optimized out>, 
vector=<optimized out>, maxdepth=<optimized out>, 
args_template=0xa0a, nargs=nargs@entry=16, args=<optimized out>, 
args@entry=0xffffffff3fc8)
   at bytecode.c:632
#35 0x0000000000545130 in fetch_and_exec_byte_code 
(args=0xffffffff3fc8, nargs=16, syms_left=<optimized out>, 
fun=<optimized out>) at lisp.h:1838
#36 funcall_lambda (fun=<optimized out>, nargs=nargs@entry=16, 
arg_vector=arg_vector@entry=0xffffffff3fc8) at eval.c:3077
#37 0x000000000054274c in Ffuncall (nargs=nargs@entry=17, 
args=args@entry=0xffffffff3fc0) at eval.c:2888
#38 0x0000000000542b70 in Fapply (nargs=<optimized out>, 
args=0xffffffff4150) at eval.c:2502
#39 0x0000000000542870 in Ffuncall (nargs=<optimized out>, 
args=args@entry=0xffffffff4148) at lisp.h:2091
#40 0x000000000057d418 in exec_byte_code (bytestr=<optimized out>, 
vector=<optimized out>, maxdepth=<optimized out>, 
args_template=0x606, nargs=nargs@entry=17, args=<optimized out>, 
args@entry=0xffffffff4298)
   at bytecode.c:632
#41 0x0000000000545130 in fetch_and_exec_byte_code 
(args=0xffffffff4298, nargs=17, syms_left=<optimized out>, 
fun=<optimized out>) at lisp.h:1838
#42 funcall_lambda (fun=<optimized out>, nargs=nargs@entry=17, 
arg_vector=arg_vector@entry=0xffffffff4298) at eval.c:3077
#43 0x000000000054274c in Ffuncall (nargs=nargs@entry=18, 
args=args@entry=0xffffffff4290) at eval.c:2888
#44 0x0000000000542b70 in Fapply (nargs=<optimized out>, 
args=0xffffffff4418) at eval.c:2502
#45 0x0000000000542870 in Ffuncall (nargs=<optimized out>, 
args=args@entry=0xffffffff4410) at lisp.h:2091
#46 0x000000000057d418 in exec_byte_code (bytestr=<optimized out>, 
vector=<optimized out>, maxdepth=<optimized out>, 
args_template=0x202, nargs=nargs@entry=16, args=<optimized out>, 
args@entry=0xffffffff4570)
   at bytecode.c:632
#47 0x0000000000545130 in fetch_and_exec_byte_code 
(args=0xffffffff4570, nargs=16, syms_left=<optimized out>, 
fun=<optimized out>) at lisp.h:1838
#48 funcall_lambda (fun=<optimized out>, nargs=nargs@entry=16, 
arg_vector=arg_vector@entry=0xffffffff4570) at eval.c:3077
#49 0x000000000054274c in Ffuncall (nargs=17, 
args=args@entry=0xffffffff4568) at eval.c:2888
#50 0x000000000057d418 in exec_byte_code (bytestr=<optimized out>, 
vector=<optimized out>, maxdepth=<optimized out>, 
args_template=0x402, nargs=nargs@entry=0, args=<optimized out>, 
args@entry=0xffffffff4950)
   at bytecode.c:632
#51 0x0000000000545130 in fetch_and_exec_byte_code 
(args=0xffffffff4950, nargs=0, syms_left=<optimized out>, 
fun=<optimized out>) at lisp.h:1838
#52 funcall_lambda (fun=<optimized out>, nargs=nargs@entry=0, 
arg_vector=arg_vector@entry=0xffffffff4950) at eval.c:3077
#53 0x000000000054274c in Ffuncall (nargs=nargs@entry=1, 
args=args@entry=0xffffffff4948) at eval.c:2888
#54 0x000000000053f248 in Ffuncall_interactively (nargs=1, 
args=0xffffffff4948) at callint.c:253
#55 0x0000000000542870 in Ffuncall (nargs=nargs@entry=2, 
args=args@entry=0xffffffff4940) at lisp.h:2091
#56 0x0000000000542c1c in Fapply (nargs=nargs@entry=3, 
args=0xffffffff4940, args@entry=0xffffffff4a30) at eval.c:2455
#57 0x00000000005405e4 in Fcall_interactively 
(function=0xffffffff4a60, record_flag=0xffffffff4a00, 
keys=0x542950 <call1+56>) at lisp.h:1008
#58 0x0000fffff27262d0 in 
F636f6d6d616e642d65786563757465_command_execute_0 ()
  from 
  /home/akyle/git/emacs/src/../native-lisp/28.0.50-aarch64-unknown-linux-gnu-08c187d985df753847bb1095eac8e39c/simple-fab5b0cffee734040ac60e9bc557cd20-fad2d213fe4820237be2ea92815644a6.eln
#59 0x0000000000542870 in Ffuncall (nargs=nargs@entry=2, 
args=args@entry=0xffffffff4a88) at lisp.h:2091
#60 0x0000000000542950 in call1 (fn=fn@entry=0x42c0, 
arg1=<optimized out>) at eval.c:2732
#61 0x00000000004dc4b8 in command_loop_1 () at lisp.h:1008
#62 0x00000000005419b8 in internal_condition_case 
(bfun=bfun@entry=0x4dc100 <command_loop_1>, 
handlers=handlers@entry=0x90, hfun=hfun@entry=0x4d1518 
<cmd_error>) at eval.c:1365
#63 0x00000000004cded8 in command_loop_2 (ignore=ignore@entry=0x0) 
at lisp.h:1008
#64 0x000000000054191c in internal_catch (tag=tag@entry=0xe040, 
func=func@entry=0x4cdea8 <command_loop_2>, arg=arg@entry=0x0) at 
eval.c:1126
#65 0x00000000004cde74 in command_loop () at lisp.h:1008
#66 0x00000000004d108c in recursive_edit_1 () at keyboard.c:718
#67 0x00000000004d143c in Frecursive_edit () at keyboard.c:790
#68 0x000000000041f0c4 in main (argc=<optimized out>, 
argv=<optimized out>) at emacs.c:2095

So it seems webkitgtk's use of GSubprocess is what is causing glib 
to take back control of the SIGCHLD sigaction and thus Emacs never 
receives it. Specifically this is the troublesome section (line 
590 in glib/gio 2.64.5 gsubprocess.c)

/* Start attempting to reap the child immediately */
if (success)
{
   GMainContext *worker_context;
   GSource *source;

   worker_context = GLIB_PRIVATE_CALL (g_get_worker_context) ();
   source = g_child_watch_source_new (self->pid);
   g_source_set_callback (source, (GSourceFunc) 
   g_subprocess_exited, g_object_ref (self), g_object_unref);
   g_source_attach (source, worker_context);
   g_source_unref (source);
}

This ends up calling ref_unix_signal_handler_unlocked in gmain.c 
which installs its own signal handler and glib doesn't restore the 
previous signal handler when its done 
unref_unix_signal_handler_unlocked. This issue was brought up on 
the GNOME mailing lists but it doesn't look like anyone responded 
[1].

We can just handle restoring the signal handler ourselves with the 
attached patch.


[1] 
https://mail.gnome.org/archives/gtk-devel-list/2016-October/msg00015.html

0001-Fix-xwidget-s-webkitgtk-widget-overriding-of-Emacs-S.patch
Description: xwidget patch