emacs/src/xgselect.c

/* Function for handling the GLib event loop.

Copyright (C) 2009-2024 Free Software Foundation, Inc.

This file is part of GNU Emacs.

GNU Emacs is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or (at
your option) any later version.

GNU Emacs is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */

#include <config.h>

#include "xgselect.h"

#ifdef HAVE_GLIB

#include <glib.h>
#include <errno.h>
#include "lisp.h"
#include "blockinput.h"
#include "systime.h"
#include "process.h"

static ptrdiff_t threads_holding_glib_lock;
static GMainContext *glib_main_context;

/* The depth of xg_select suppression.  */
static int xg_select_suppress_count;

void
release_select_lock (void)
{
#if GNUC_PREREQ (4, 7, 0)
  if (__atomic_sub_fetch (&threads_holding_glib_lock, 1, __ATOMIC_ACQ_REL) == 0)
    g_main_context_release (glib_main_context);
#else
  if (--threads_holding_glib_lock == 0)
    g_main_context_release (glib_main_context);
#endif
}

static void
acquire_select_lock (GMainContext *context)
{
#if GNUC_PREREQ (4, 7, 0)
  if (__atomic_fetch_add (&threads_holding_glib_lock, 1, __ATOMIC_ACQ_REL) == 0)
    {
      glib_main_context = context;
      while (!g_main_context_acquire (context))
	{
	  /* Spin. */
	}
    }
#else
  if (threads_holding_glib_lock++ == 0)
    {
      glib_main_context = context;
      while (!g_main_context_acquire (context))
	{
	  /* Spin. */
	}
    }
#endif
}

/* Call this to not use xg_select when using it would be a bad idea,
   i.e. during drag-and-drop.  */
void
suppress_xg_select (void)
{
  ++xg_select_suppress_count;
}

void
release_xg_select (void)
{
  if (!xg_select_suppress_count)
    emacs_abort ();

  --xg_select_suppress_count;
}

/* `xg_select' is a `pselect' replacement.  Why do we need a separate function?
   1. Timeouts.  Glib and Gtk rely on timer events.  If we did pselect
      with a greater timeout then the one scheduled by Glib, we would
      not allow Glib to process its timer events.  We want Glib to
      work smoothly, so we need to reduce our timeout to match Glib.
   2. Descriptors.  Glib may listen to more file descriptors than we do.
      So we add Glib descriptors to our pselect pool, but we don't change
      the value returned by the function.  The return value  matches only
      the descriptors passed as arguments, making it compatible with
      plain pselect.  */

int
xg_select (int fds_lim, fd_set *rfds, fd_set *wfds, fd_set *efds,
	   struct timespec *timeout, sigset_t *sigmask)
{
  fd_set all_rfds, all_wfds;
  struct timespec tmo;
  struct timespec *tmop = timeout;

  GMainContext *context;
  bool have_wfds = wfds != NULL;
  GPollFD gfds_buf[128];
  GPollFD *gfds = gfds_buf;
  int gfds_size = ARRAYELTS (gfds_buf);
  int n_gfds, retval = 0, our_fds = 0, max_fds = fds_lim - 1;
  int i, nfds, tmo_in_millisec, must_free = 0;
  bool need_to_dispatch;
#ifdef USE_GTK
  bool already_has_events;
#endif

  if (xg_select_suppress_count)
    return pselect (fds_lim, rfds, wfds, efds, timeout, sigmask);

  context = g_main_context_default ();
  acquire_select_lock (context);

#ifdef USE_GTK
  already_has_events = g_main_context_pending (context);
#ifndef HAVE_PGTK
  already_has_events = already_has_events && x_gtk_use_native_input;
#endif
#endif

  if (rfds) all_rfds = *rfds;
  else FD_ZERO (&all_rfds);
  if (wfds) all_wfds = *wfds;
  else FD_ZERO (&all_wfds);

  n_gfds = g_main_context_query (context, G_PRIORITY_LOW, &tmo_in_millisec,
				 gfds, gfds_size);

  if (gfds_size < n_gfds)
    {
      /* Avoid using SAFE_NALLOCA, as that implicitly refers to the
	 current thread.  Using xnmalloc avoids thread-switching
	 problems here.  */
      gfds = xnmalloc (n_gfds, sizeof *gfds);
      must_free = 1;
      gfds_size = n_gfds;
      n_gfds = g_main_context_query (context, G_PRIORITY_LOW, &tmo_in_millisec,
				     gfds, gfds_size);
    }

  for (i = 0; i < n_gfds; ++i)
    {
      if (gfds[i].events & G_IO_IN)
        {
          FD_SET (gfds[i].fd, &all_rfds);
          if (gfds[i].fd > max_fds) max_fds = gfds[i].fd;
        }
      if (gfds[i].events & G_IO_OUT)
        {
          FD_SET (gfds[i].fd, &all_wfds);
          if (gfds[i].fd > max_fds) max_fds = gfds[i].fd;
          have_wfds = true;
        }
    }

  if (must_free)
    xfree (gfds);

  if (n_gfds >= 0 && tmo_in_millisec >= 0)
    {
      tmo = make_timespec (tmo_in_millisec / 1000,
			   1000 * 1000 * (tmo_in_millisec % 1000));
      if (!timeout || timespec_cmp (tmo, *timeout) < 0)
	tmop = &tmo;
    }

#ifndef USE_GTK
  fds_lim = max_fds + 1;
  nfds = thread_select (pselect, fds_lim,
			&all_rfds, have_wfds ? &all_wfds : NULL, efds,
			tmop, sigmask);
#else
  /* On PGTK, when you type a key, the key press event are received,
     and one more key press event seems to be received internally.

     The same can happen with GTK native input, which makes input
     slow.

     The second event is not sent via the display connection, so the
     following is the case:

       - socket read buffer is empty
       - a key press event is pending

     In that case, we should not sleep in pselect, and dispatch the
     event immediately.  (Bug#52761) */
  if (!already_has_events)
    {
      fds_lim = max_fds + 1;
      nfds = thread_select (pselect, fds_lim,
			    &all_rfds, have_wfds ? &all_wfds : NULL, efds,
			    tmop, sigmask);
    }
  else
    {
      /* Emulate return values */
      nfds = 1;
      FD_ZERO (&all_rfds);
      if (have_wfds)
	FD_ZERO (&all_wfds);
      if (efds)
	FD_ZERO (efds);
      our_fds++;
    }
#endif

  if (nfds < 0)
    retval = nfds;
  else if (nfds > 0)
    {
      for (i = 0; i < fds_lim; ++i)
        {
          if (FD_ISSET (i, &all_rfds))
            {
              if (rfds && FD_ISSET (i, rfds)) ++retval;
              else ++our_fds;
            }
          else if (rfds)
            FD_CLR (i, rfds);

          if (have_wfds && FD_ISSET (i, &all_wfds))
            {
              if (wfds && FD_ISSET (i, wfds)) ++retval;
              else ++our_fds;
            }
          else if (wfds)
            FD_CLR (i, wfds);

          if (efds && FD_ISSET (i, efds))
            ++retval;
        }
    }

  /* If Gtk+ is in use eventually gtk_main_iteration will be called,
     unless retval is zero.  */
#ifdef USE_GTK
  need_to_dispatch = retval == 0;
#else
  need_to_dispatch = true;
#endif

  /* xwidgets make heavy use of GLib subprocesses, which add their own
     SIGCHLD handler at arbitrary locations.  That doesn't play well
     with Emacs's own handler, so once GLib does its thing with its
     subprocesses we restore our own SIGCHLD handler (which chains the
     GLib handler) here.

     There is an obvious race condition, but we can't really do
     anything about that, except hope a SIGCHLD arrives soon to clear
     up the situation.  */

#ifdef HAVE_XWIDGETS
  catch_child_signal ();
#endif

  if (need_to_dispatch)
    {
      acquire_select_lock (context);

      int pselect_errno = errno;
      /* Prevent g_main_dispatch recursion, that would occur without
         block_input wrapper, because event handlers call
         unblock_input.  Event loop recursion was causing Bug#15801.  */
      block_input ();
      while (g_main_context_pending (context))
        g_main_context_dispatch (context);
      unblock_input ();
      errno = pselect_errno;
      release_select_lock ();
    }

  /* To not have to recalculate timeout, return like this.  */
  if ((our_fds > 0 || (nfds == 0 && tmop == &tmo)) && (retval == 0))
    {
      retval = -1;
      errno = EINTR;
    }

  return retval;
}
#endif /* HAVE_GLIB */