WasmFS: Fix file packager use after startup (#19615)

During startup if we run file packager code then the createDataFile etc. calls
are turned into code that caches the info for later, as we can't call into
WasmFS during startup. Then when WasmFS is running, it reads the cached
info and creates the files. But we also need to handle the case where file
packager code is run after startup, which is easier to support even if it
is more rare - just create the file or directory.

Author: kripken

src/library_wasmfs.js

@@ -7,13 +7,20 @@
 mergeInto(LibraryManager.library, {
   $wasmFSPreloadedFiles: [],
   $wasmFSPreloadedDirs: [],
+  // We must note when preloading has been "flushed", that is, the time at which
+  // WasmFS has started up and read the preloaded data. After that time, no more
+  // data needs to be preloaded (and it would be invalid to do so, as any
+  // further additions to wasmFSPreloadedFiles|Dirs would be ignored).
+  $wasmFSPreloadingFlushed: false,
+
   $FS__postset: `
 FS.init();
 FS.createPreloadedFile = FS_createPreloadedFile;
 `,
   $FS__deps: [
     '$wasmFSPreloadedFiles',
     '$wasmFSPreloadedDirs',
+    '$wasmFSPreloadingFlushed',
     '$PATH',
     '$stringToUTF8OnStack',
     '$withStackSave',
@@ -55,10 +62,19 @@ FS.createPreloadedFile = FS_createPreloadedFile;
       FS.ErrnoError.prototype.constructor = FS.ErrnoError;
     },
     createDataFile: (parent, name, fileData, canRead, canWrite, canOwn) => {
-      // Data files must be cached until the file system itself has been initialized.
-      var mode = FS_getMode(canRead, canWrite);
       var pathName = name ? parent + '/' + name : parent;
-      wasmFSPreloadedFiles.push({pathName, fileData, mode});
+      var mode = FS_getMode(canRead, canWrite);
+
+      if (!wasmFSPreloadingFlushed) {
+        // WasmFS code in the wasm is not ready to be called yet. Cache the
+        // files we want to create here in JS, and WasmFS will read them
+        // later.
+        wasmFSPreloadedFiles.push({pathName, fileData, mode});
+      } else {
+        // WasmFS is already running, so create the file normally.
+        FS.create(pathName, mode);
+        FS.writeFile(pathName, fileData);
+      }
     },
     createPath: (parent, path, canRead, canWrite) => {
       // Cache file path directory names.
@@ -67,7 +83,11 @@ FS.createPreloadedFile = FS_createPreloadedFile;
         var part = parts.pop();
         if (!part) continue;
         var current = PATH.join2(parent, part);
-        wasmFSPreloadedDirs.push({parentPath: parent, childName: part});
+        if (!wasmFSPreloadingFlushed) {
+          wasmFSPreloadedDirs.push({parentPath: parent, childName: part});
+        } else {
+          FS.mkdir(current);
+        }
         parent = current;
       }
       return current;
@@ -322,8 +342,17 @@ FS.createPreloadedFile = FS_createPreloadedFile;
 
 #endif
   },
-  _wasmfs_get_num_preloaded_files__deps: ['$wasmFSPreloadedFiles'],
+
+  _wasmfs_get_num_preloaded_files__deps: [
+    '$wasmFSPreloadedFiles',
+    '$wasmFSPreloadingFlushed'],
   _wasmfs_get_num_preloaded_files: function() {
+    // When this method is called from WasmFS it means that we are about to
+    // flush all the preloaded data, so mark that. (There is no call that
+    // occurs at the end of that flushing, which would be more natural, but it
+    // is fine to mark the flushing here as during the flushing itself no user
+    // code can run, so nothing will check whether we have flushed or not.)
+    wasmFSPreloadingFlushed = true;
     return wasmFSPreloadedFiles.length;
   },
   _wasmfs_get_num_preloaded_dirs__deps: ['$wasmFSPreloadedDirs'],

test/test_browser.py

@@ -1884,6 +1884,7 @@ def test_clientside_vertex_arrays_es3(self):
   def test_emscripten_api(self):
     self.btest_exit('emscripten_api_browser.c', args=['-sEXPORTED_FUNCTIONS=_main,_third', '-lSDL'])
 
+  @also_with_wasmfs
   def test_emscripten_async_load_script(self):
     def setup():
       create_file('script1.js', '''