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Fixed bug 3328 - Race condition in Wayland_VideoInit

Robert Folland

When running this little test program with SDL2 on Wayland it often crashes in SDL_Init.

From a backtrace it is apparent that there is a race condition in creating a xkb_context_ref. Sometimes it is 0x0.

By moving the relevant lines higher up in Wayland_VideoInit (in SDL2-2.0.4/src/video/wayland/SDL_waylandvideo.c:302) this seems to get fixed.

I moved the call to WAYLAND_xkb_context_new() up to before the call to WAYLAND_wl_display_connect().

Here is the test program (just a loop of init and quit), and a backtrace from gdb:

#include <cstdio>
#include <stdlib.h>
#include <SDL2/SDL.h>
#include <unistd.h>
#include <iostream>

int main(int argc, char **argv)
{
    int count = atoi(argv[1]);

    for (int i = 0; i < count; i++) {
        std::cout << "Init " << i << std::endl;
        if (SDL_Init(SDL_INIT_VIDEO) < 0) {
            SDL_LogError(SDL_LOG_CATEGORY_APPLICATION,
                         "Couldn't initialize SDL: %s\n",
                         SDL_GetError());
            return 1;
        }
        std::cout << "Quit" << std::endl;
        SDL_Quit();
    }
    return 0;
}


Init 12
Quit
Init 13

Program received signal SIGSEGV, Segmentation fault.
xkb_context_ref (ctx=ctx@entry=0x0) at src/context.c:156
156         ctx->refcnt++;
(gdb) bt
#0  xkb_context_ref (ctx=ctx@entry=0x0) at src/context.c:156
#1  0x00007ffff5e1cd4c in xkb_keymap_new (ctx=0x0, format=XKB_KEYMAP_FORMAT_TEXT_V1, flags=flags@entry=XKB_KEYMAP_COMPILE_NO_FLAGS) at src/keymap-priv.c:65
#2  0x00007ffff5e1c6cc in xkb_keymap_new_from_buffer (ctx=<optimized out>,
    buffer=0x7ffff7fd5000 "xkb_keymap {\nxkb_keycodes \"(unnamed)\" {\n\tminimum = 8;\n\tmaximum = 255;\n\t<ESC>", ' ' <repeats 16 times>, "= 9;\n\t<AE01>", ' ' <re
peats 15 times>, "= 10;\n\t<AE02>", ' ' <repeats 15 times>, "= 11;\n\t<AE03>", ' ' <repeats 15 times>, "= 12;\n\t<AE04>", ' ' <repeats 12 times>..., length=48090,
    format=<optimized out>, flags=<optimized out>) at src/keymap.c:191
#3  0x00007ffff7b8ea4e in keyboard_handle_keymap (data=0x6169b0, keyboard=<optimized out>, format=<optimized out>, fd=5, size=48091)
    at /home/vlab/abs/sdl2/src/SDL2-2.0.4/src/video/wayland/SDL_waylandevents.c:269
#4  0x00007ffff64501f0 in ffi_call_unix64 () from /usr/lib/libffi.so.6
#5  0x00007ffff644fc58 in ffi_call () from /usr/lib/libffi.so.6
#6  0x00007ffff665be3e in wl_closure_invoke (closure=closure@entry=0x61f000, flags=flags@entry=1, target=<optimized out>, target@entry=0x616d20,
    opcode=opcode@entry=0, data=<optimized out>) at src/connection.c:949
#7  0x00007ffff6658be0 in dispatch_event (display=<optimized out>, queue=<optimized out>) at src/wayland-client.c:1274
#8  0x00007ffff6659db4 in dispatch_queue (queue=0x617398, display=0x6172d0) at src/wayland-client.c:1420
#9  wl_display_dispatch_queue_pending (display=0x6172d0, queue=0x617398) at src/wayland-client.c:1662
#10 0x00007ffff665a0cf in wl_display_roundtrip_queue (display=0x6172d0, queue=0x617398) at src/wayland-client.c:1085
#11 0x00007ffff7b8faa0 in Wayland_VideoInit (_this=<optimized out>) at /home/vlab/abs/sdl2/src/SDL2-2.0.4/src/video/wayland/SDL_waylandvideo.c:302
#12 0x00007ffff7b7aed6 in SDL_VideoInit_REAL (driver_name=<optimized out>, driver_name@entry=0x0) at /home/vlab/abs/sdl2/src/SDL2-2.0.4/src/video/SDL_video.c:513
#13 0x00007ffff7ae0ee7 in SDL_InitSubSystem_REAL (flags=16416) at /home/vlab/abs/sdl2/src/SDL2-2.0.4/src/SDL.c:173
#14 0x0000000000400b24 in main (argc=2, argv=0x7fffffffebb8) at vplay-init.cpp:13
(gdb)
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slouken committed Oct 13, 2016
1 parent f94bd05 commit 063f752e0de1362f47a81da299ffee9e283c8026
Showing with 5 additions and 5 deletions.
  1. +5 −5 src/video/wayland/SDL_waylandvideo.c
@@ -340,6 +340,11 @@ Wayland_VideoInit(_THIS)

_this->driverdata = data;

data->xkb_context = WAYLAND_xkb_context_new(0);
if (!data->xkb_context) {
return SDL_SetError("Failed to create XKB context");
}

data->display = WAYLAND_wl_display_connect(NULL);
if (data->display == NULL) {
return SDL_SetError("Failed to connect to a Wayland display");
@@ -358,11 +363,6 @@ Wayland_VideoInit(_THIS)
// Second roundtrip to receive all output events.
WAYLAND_wl_display_roundtrip(data->display);

data->xkb_context = WAYLAND_xkb_context_new(0);
if (!data->xkb_context) {
return SDL_SetError("Failed to create XKB context");
}

Wayland_InitMouse();

/* Get the surface class name, usually the name of the application */

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