Export FS objects on FS.filesystems

Fixes #3690

src/library_fs.js

@@ -29,6 +29,7 @@ mergeInto(LibraryManager.library, {
     },
     ErrnoError: null, // set during init
     genericErrors: {},
+    filesystems: null,
 
     handleFSError: function(e) {
       if (!(e instanceof FS.ErrnoError)) throw e + ' : ' + stackTrace();
@@ -1347,6 +1348,13 @@ mergeInto(LibraryManager.library, {
       FS.createDefaultDirectories();
       FS.createDefaultDevices();
       FS.createSpecialDirectories();
+
+      FS.filesystems = {
+        'MEMFS': MEMFS,
+        'IDBFS': IDBFS,
+        'NODEFS': NODEFS,
+        'WORKERFS': WORKERFS,
+      };
     },
     init: function(input, output, error) {
       assert(!FS.init.initialized, 'FS.init was previously called. If you want to initialize later with custom parameters, remove any earlier calls (note that one is automatically added to the generated code)');

tests/test_core.py

@@ -7508,6 +7508,18 @@ int main(int argc, char **argv) {
     self.emcc_args += ['--js-library', os.path.join(self.get_dir(), 'lib.js')]
     self.do_run(open(os.path.join(self.get_dir(), 'main.cpp'), 'r').read(), 'able to run memprof')
 
+  def test_fs_dict(self):
+      open(self.in_dir('pre.js'), 'w').write('''
+        var Module = {};
+        Module['preRun'] = function() {
+            console.log(typeof FS.filesystems['MEMFS']);
+            console.log(typeof FS.filesystems['IDBFS']);
+            console.log(typeof FS.filesystems['NODEFS']);
+        };
+      ''')
+      self.emcc_args += ['--pre-js', 'pre.js']
+      self.do_run('', 'object\nobject\nobject')
+
 # Generate tests for everything
 def make_run(fullname, name=-1, compiler=-1, embetter=0, quantum_size=0,
     typed_arrays=0, emcc_args=None, env=None):