Don't use udev for joystick enumeration if running in a container

If we are running in a container, like Flatpak[1] or pressure-vessel[2],
it's likely that we are using user namespaces,
therefore udev event notification via netlink won't work reliably.
Use their filesystem API to detect them and automatically fallback to
the inotify-based enumeration.

[1] <https://flatpak.org/>
[2]
<https://gitlab.steamos.cloud/steamrt/steam-runtime-tools/-/tree/master/pressure-vessel>

Signed-off-by: Ludovico de Nittis <ludovico.denittis@collabora.com>

src/joystick/hidapi/SDL_hidapijoystick.c

@@ -740,6 +740,14 @@ HIDAPI_JoystickInit(void)
             SDL_LogDebug(SDL_LOG_CATEGORY_INPUT,
                          "udev disabled by SDL_HIDAPI_JOYSTICK_DISABLE_UDEV");
             linux_enumeration_method = ENUMERATION_FALLBACK;
+        } else if (access("/.flatpak-info", F_OK) == 0
+                   || access("/run/host/container-manager", F_OK) == 0) {
+            /* Explicitly check `/.flatpak-info` because, for old versions of
+             * Flatpak, this was the only available way to tell if we were in
+             * a Flatpak container. */
+            SDL_LogDebug(SDL_LOG_CATEGORY_INPUT,
+                         "Container detected, disabling HIDAPI udev integration");
+            linux_enumeration_method = ENUMERATION_FALLBACK;
         } else {
             SDL_LogDebug(SDL_LOG_CATEGORY_INPUT,
                          "Using udev for HIDAPI joystick device discovery");

src/joystick/linux/SDL_sysjoystick.c

@@ -658,7 +658,10 @@ LINUX_JoystickInit(void)
             enumeration_method = ENUMERATION_FALLBACK;
         }
         else if (access("/.flatpak-info", F_OK) == 0
-                 || access("/run/pressure-vessel", F_OK) == 0) {
+                 || access("/run/host/container-manager", F_OK) == 0) {
+            /* Explicitly check `/.flatpak-info` because, for old versions of
+             * Flatpak, this was the only available way to tell if we were in
+             * a Flatpak container. */
             SDL_LogDebug(SDL_LOG_CATEGORY_INPUT,
                          "Container detected, disabling udev integration");
             enumeration_method = ENUMERATION_FALLBACK;