Fix app crashing on self-restart in the AppImage release

src/core/Game.lua

@@ -391,9 +391,7 @@ end
 -- LÖVE process ensures scripts, registries, and assets are all rebuilt from
 -- the newly selected mod state.
 function Game:restartWithMods()
-  if love.event and love.event.quit then
-    love.event.quit("restart")
-  end
+  require("src.core.HostShell").restart()
 end
 
 function Game:keyreleased(key)

src/core/HostShell.lua

@@ -20,4 +20,32 @@ function HostShell.popen(command, mode)
   return pipe
 end
 
+-- Restart the whole app. The obvious love.event.quit("restart") re-runs LÖVE's
+-- boot in-process, which calls love.filesystem.init a second time -- and inside
+-- an AppImage physfs is already initialized, so that second init throws
+-- ("Failed to initialize filesystem: already initialized") and the relaunch
+-- crashes. So on an AppImage we relaunch the executable; the fresh process's
+-- Boot step mounts any downloaded update exactly as a manual relaunch would.
+-- On every other platform the in-process restart works, so keep it.
+function HostShell.restart()
+  if not (love and love.event and love.event.quit) then return end
+  local appimage = os.getenv("APPIMAGE")
+  if not appimage then
+    love.event.quit("restart")
+    return
+  end
+
+  -- We have to restart the process with this cursed execv call to prevent the
+  -- PID from changing, which might cause SteamOS and other Linux launchers to
+  -- think the app has crashed.
+  local ffi = require("ffi")
+  pcall(ffi.cdef, [[
+    int execv(const char *path, char *const argv[]);
+    int unsetenv(const char *name);
+  ]])
+  ffi.C.unsetenv("LD_LIBRARY_PATH")
+  local argv = ffi.new("const char *[2]", appimage, nil)
+  ffi.C.execv(appimage, ffi.cast("char *const *", argv))
+end
+
 return HostShell