Fix mounting WORKERFS images from the main thread using the JS API

NEWS.md

@@ -6,6 +6,10 @@
 
 * When explicitly creating a list using the `RList` constructor, nested JavaScript objects at a deeper level are also converted into R list objects. This does not affect the generic `RObject` constructor, as the default is for JavaScript objects to map to R `data.frame` objects using the `RDataFrame` constructor.
 
+## Bug Fixes
+
+* (Regression) Mounting filesystem images using the `WORKERFS` filesystem type now works again in the browser using the JavaScript API from the main thread (#488).
+
 # webR 0.4.2
 
 ## New features

src/webR/mount.ts

@@ -16,6 +16,31 @@ type WorkerFileSystemType = Emscripten.FileSystemType & {
     contents: ArrayBufferView, mtime?: Date) => FS.FSNode;
 };
 
+/**
+ * Hooked FS.mount() for using WORKERFS under Node.js or with `Blob` objects
+ * replaced with Uint8Array over the communication channel.
+ * @internal
+ */
+export function mountFS(type: Emscripten.FileSystemType, opts: FSMountOptions, mountpoint: string) {
+  // For non-WORKERFS filesystem types, just call the original FS.mount()
+  if (type !== Module.FS.filesystems.WORKERFS) {
+    return Module.FS._mount(type, opts, mountpoint) as void;
+  }
+
+  // Otherwise, handle `packages` using our own internal mountImageData()
+  if ('packages' in opts && opts.packages) {
+    opts.packages.forEach((pkg) => {
+      mountImageData(pkg.blob as ArrayBufferLike, pkg.metadata, mountpoint);
+    });
+  } else {
+    // TODO: Handle `blobs` and `files` keys.
+    throw new Error(
+      "Can't mount data. You must use the `packages` key when mounting with `WORKERFS` in webR."
+    );
+  }
+}
+
+
 /**
  * Download an Emscripten FS image and mount to the VFS
  * @internal
@@ -88,29 +113,6 @@ export function mountImagePath(path: string, mountpoint: string) {
   }
 }
 
-/**
- * An implementation of FS.mount() for WORKERFS under Node.js
- * @internal
- */
-export function mountFSNode(type: Emscripten.FileSystemType, opts: FSMountOptions, mountpoint: string) {
-  if (!IN_NODE || type !== Module.FS.filesystems.WORKERFS) {
-    return Module.FS._mount(type, opts, mountpoint) as void;
-  }
-
-  if ('packages' in opts && opts.packages) {
-    opts.packages.forEach((pkg) => {
-      // Main thread communication casts `Blob` to Uint8Array
-      // FIXME: Use a replacer + reviver to handle `Blob`s
-      mountImageData(pkg.blob as ArrayBufferLike, pkg.metadata, mountpoint);
-    });
-  } else {
-    throw new Error(
-      "Can't mount data under Node. " +
-      "Mounting with `WORKERFS` under Node must use the `packages` key."
-    );
-  }
-}
-
 // Mount the filesystem image `data` and `metadata` to the VFS at `mountpoint`
 function mountImageData(data: ArrayBufferLike | Buffer, metadata: FSMetaData, mountpoint: string) {
   if (IN_NODE) {
@@ -140,7 +142,9 @@ function mountImageData(data: ArrayBufferLike | Buffer, metadata: FSMetaData, mo
       WORKERFS.createNode(dirNode, file, WORKERFS.FILE_MODE, 0, contents);
     });
   } else {
-    Module.FS.mount(Module.FS.filesystems.WORKERFS, {
+    // Main thread communication casts `Blob` to Uint8Array
+    // FIXME: Use a replacer + reviver to handle `Blob`s
+    Module.FS._mount(Module.FS.filesystems.WORKERFS, {
       packages: [{
         blob: new Blob([data]),
         metadata,

src/webR/webr-worker.ts

@@ -10,7 +10,7 @@ import { WebRPayloadRaw, WebRPayloadPtr, WebRPayloadWorker, isWebRPayloadPtr } f
 import { RPtr, RType, RCtor, WebRData, WebRDataRaw } from './robj';
 import { protect, protectInc, unprotect, parseEvalBare, UnwindProtectException, safeEval } from './utils-r';
 import { generateUUID } from './chan/task-common';
-import { mountFSNode, mountImageUrl, mountImagePath } from './mount';
+import { mountFS, mountImageUrl, mountImagePath } from './mount';
 import type { parentPort } from 'worker_threads';
 
 import {
@@ -840,8 +840,6 @@ function init(config: Required<WebROptions>) {
 
   Module.preRun.push(() => {
     if (IN_NODE) {
-      Module.FS._mount = Module.FS.mount;
-      Module.FS.mount = mountFSNode;
       globalThis.FS = Module.FS;
       (globalThis as any).chan = chan;
     }
@@ -852,6 +850,10 @@ function init(config: Required<WebROptions>) {
     Module.ENV.HOME = _config.homedir;
     Module.FS.chdir(_config.homedir);
     Module.ENV = Object.assign(Module.ENV, env);
+
+    // Hook Emscripten's FS.mount() to handle ArrayBuffer data from the channel
+    Module.FS._mount = Module.FS.mount;
+    Module.FS.mount = mountFS;
   });
 
   chan?.setDispatchHandler(dispatch);