emscripten: add hack to prevent ERRNO_CODES from being optimized away

Makefile.emscripten

@@ -42,7 +42,8 @@ LIBS    := -s USE_ZLIB=1
 LDFLAGS := -L. --no-heap-copy -s USE_ZLIB=1 -s TOTAL_MEMORY=$(MEMORY) -s NO_EXIT_RUNTIME=0 -s FULL_ES2=1 \
            -s EXPORTED_FUNCTIONS="['_main', '_malloc', '_cmd_savefiles', '_cmd_save_state', '_cmd_load_state', '_cmd_take_screenshot']" \
            --js-library emscripten/library_rwebaudio.js \
-           --js-library emscripten/library_rwebcam.js
+           --js-library emscripten/library_rwebcam.js \
+           --js-library emscripten/library_errno_codes.js
 ifneq ($(PTHREAD), 0)
    LDFLAGS += -s USE_PTHREADS=$(PTHREAD) -s PTHREAD_POOL_SIZE=2
 endif

emscripten/library_errno_codes.js

@@ -0,0 +1,15 @@
+//"use strict";
+
+// HACK: This is a dummy library that forces ERRNO_CODES to be used, so it's not optimized away.
+// Needed for BrowserFS.
+
+var LibraryErrnoCodes = {
+   dummyErrnoCodes__deps: ['$ERRNO_CODES'],
+   dummyErrnoCodes: function() {
+      if (!ERRNO_CODES) {
+         console.error("ERRNO_CODES not imported!");
+      }
+   }
+};
+
+mergeInto(LibraryManager.library, LibraryErrnoCodes);

frontend/drivers/platform_emscripten.c

@@ -47,6 +47,7 @@
 #include "../../file_path_special.h"
 
 void RWebAudioRecalibrateTime(void);
+void dummyErrnoCodes(void);
 
 static unsigned emscripten_fullscreen_reinit;
 static unsigned emscripten_frame_count = 0;
@@ -226,6 +227,8 @@ int main(int argc, char *argv[])
 {
    EMSCRIPTEN_RESULT r;
 
+   dummyErrnoCodes();
+
    emscripten_set_canvas_element_size("#canvas", 800, 600);
    emscripten_set_element_css_size("#canvas", 800.0, 600.0);
    emscripten_set_main_loop(emscripten_mainloop, 0, 0);