[WasmFS] Async proxied JS backend

system/lib/wasmfs/js_impl_backend.h

@@ -28,14 +28,16 @@
 // To write a new backend in JS, you basically do the following:
 //
 //  1. Add a declaration of the C function to create the backend in the
-//     "backend creation" section of emscripten/wasmfs.h.
+//     "backend creation" section of emscripten/wasmfs.h. (One line.)
 //  2. Add a cpp file for the new backend, and implement the C function from 1,
 //     which should create it on both the C++ (using JSImplBackend) and JS
 //     sides. (By convention, the C function should just call into C++ and JS
-//     which do the interesting work; the C is just a thin wrapper.)
+//     which do the interesting work; the C is just a thin wrapper.) (A few
+//     lines.)
 // 3. Write a new JS library, and add the implementation of the JS method just
 //    mentioned, which should set up the mapping from the C++ backend object's
-//    address to the JS code containing the hooks to read and write etc.
+//    address to the JS code containing the hooks to read and write etc. (99%
+//    of the work happens here.)
 //
 // For a simple example, see js_file_backend.cpp and library_wasmfs_js_file.js
 //

system/lib/wasmfs/proxied_async_js_impl_backend.h

@@ -18,6 +18,24 @@
 // Similar to JSImplBackend, but proxies to another thread where the JS can be
 // async.
 //
+// To write a new async backend in JS, you basically do the following:
+//
+//  1. Add a declaration of the C function to create the backend in the
+//     "backend creation" section of emscripten/wasmfs.h. (One line.)
+//  2. Add a cpp file for the new backend, and implement the C function from 1,
+//     which should create a ProxiedAsyncJSBackend while passing it code to
+//     set up the JS side. (A few lines.)
+// 3. Write a new JS library, and add the implementation of the JS method just
+//    mentioned, which should set up the mapping from the C++ backend object's
+//    address to the JS code containing the hooks to read and write etc. The
+//    hooks should each return a JS Promise. (99% of the work happens here.)
+//
+// This is basically the same as JSImplBackend, except that the code in step #3
+// is async (also, step #2 looks slightly different, but is similarly very
+// short).
+//
+// For a simple example, see fetch_backend.cpp and library_wasmfs_fetch.js.
+//
 
 using js_index_t = uint32_t;