gh-127111: Emscripten Make web example work again (#127113)

Moves the Emscripten web example into a standalone folder, and updates 
Makefile targets to build the web example. Instructions for usage have
also been added.

Makefile.pre.in

@@ -269,10 +269,6 @@ SRCDIRS= 	@SRCDIRS@
 # Other subdirectories
 SUBDIRSTOO=	Include Lib Misc
 
-# assets for Emscripten browser builds
-WASM_ASSETS_DIR=.$(prefix)
-WASM_STDLIB=$(WASM_ASSETS_DIR)/lib/python$(VERSION)/os.py
-
 # Files and directories to be distributed
 CONFIGFILES=	configure configure.ac acconfig.h pyconfig.h.in Makefile.pre.in
 DISTFILES=	README.rst ChangeLog $(CONFIGFILES)
@@ -737,6 +733,9 @@ build_all:	check-clean-src check-app-store-compliance $(BUILDPYTHON) platform sh
 build_wasm: check-clean-src $(BUILDPYTHON) platform sharedmods \
 		python-config checksharedmods
 
+.PHONY: build_emscripten
+build_emscripten: build_wasm web_example
+
 # Check that the source is clean when building out of source.
 .PHONY: check-clean-src
 check-clean-src:
@@ -1016,23 +1015,38 @@ $(DLLLIBRARY) libpython$(LDVERSION).dll.a: $(LIBRARY_OBJS)
 	else true; \
 	fi
 
-# wasm32-emscripten browser build
-# wasm assets directory is relative to current build dir, e.g. "./usr/local".
-# --preload-file turns a relative asset path into an absolute path.
+# wasm32-emscripten browser web example
+
+WEBEX_DIR=$(srcdir)/Tools/wasm/emscripten/web_example/
+web_example/python.html: $(WEBEX_DIR)/python.html
+	@mkdir -p web_example
+	@cp $< $@
+
+web_example/python.worker.mjs: $(WEBEX_DIR)/python.worker.mjs
+	@mkdir -p web_example
+	@cp $< $@
 
-.PHONY: wasm_stdlib
-wasm_stdlib: $(WASM_STDLIB)
-$(WASM_STDLIB): $(srcdir)/Lib/*.py $(srcdir)/Lib/*/*.py \
-	    $(srcdir)/Tools/wasm/wasm_assets.py \
+web_example/server.py: $(WEBEX_DIR)/server.py
+	@mkdir -p web_example
+	@cp $< $@
+
+WEB_STDLIB=web_example/python$(VERSION)$(ABI_THREAD).zip
+$(WEB_STDLIB): $(srcdir)/Lib/*.py $(srcdir)/Lib/*/*.py \
+	    $(WEBEX_DIR)/wasm_assets.py \
 	    Makefile pybuilddir.txt Modules/Setup.local
-	$(PYTHON_FOR_BUILD) $(srcdir)/Tools/wasm/wasm_assets.py \
-	    --buildroot . --prefix $(prefix)
+	$(PYTHON_FOR_BUILD) $(WEBEX_DIR)/wasm_assets.py \
+	    --buildroot . --prefix $(prefix) -o $@
 
-python.html: $(srcdir)/Tools/wasm/python.html python.worker.js
-	@cp $(srcdir)/Tools/wasm/python.html $@
+web_example/python.mjs web_example/python.wasm: $(BUILDPYTHON)
+	@if test $(HOST_GNU_TYPE) != 'wasm32-unknown-emscripten' ; then \
+		echo "Can only build web_example when target is Emscripten" ;\
+		exit 1 ;\
+	fi
+	cp python.mjs web_example/python.mjs
+	cp python.wasm web_example/python.wasm
 
-python.worker.js: $(srcdir)/Tools/wasm/python.worker.js
-	@cp $(srcdir)/Tools/wasm/python.worker.js $@
+.PHONY: web_example
+web_example: web_example/python.mjs web_example/python.worker.mjs web_example/python.html web_example/server.py $(WEB_STDLIB)
 
 ############################################################################
 # Header files
@@ -3053,8 +3067,7 @@ clean-retain-profile: pycremoval
 	find build -name '*.py[co]' -exec rm -f {} ';' || true
 	-rm -f pybuilddir.txt
 	-rm -f _bootstrap_python
-	-rm -f python.html python*.js python.data python*.symbols python*.map
-	-rm -f $(WASM_STDLIB)
+	-rm -rf web_example python.mjs python.wasm python*.symbols python*.map
 	-rm -f Programs/_testembed Programs/_freeze_module
 	-rm -rf Python/deepfreeze
 	-rm -f Python/frozen_modules/*.h

Misc/NEWS.d/next/Build/2024-11-30-16-36-09.gh-issue-127111.QI9mMZ.rst

@@ -0,0 +1,2 @@
+Updated the Emscripten web example to use ES6 modules and be built into a
+distinct ``web_example`` subfolder.

Tools/wasm/README.md

@@ -23,9 +23,9 @@ https://github.com/psf/webassembly for more information.
 
 To cross compile to the ``wasm32-emscripten`` platform you need
 [the Emscripten compiler toolchain](https://emscripten.org/), 
-a Python interpreter, and an installation of Node version 18 or newer. Emscripten
-version 3.1.42 or newer is recommended. All commands below are relative to a checkout
-of the Python repository.
+a Python interpreter, and an installation of Node version 18 or newer.
+Emscripten version 3.1.73 or newer is recommended. All commands below are
+relative to a checkout of the Python repository.
 
 #### Install [the Emscripten compiler toolchain](https://emscripten.org/docs/getting_started/downloads.html)
 
@@ -50,7 +50,7 @@ sourced. Otherwise the source script removes the environment variable.
 export EM_COMPILER_WRAPPER=ccache
 ```
 
-### Compile and build Python interpreter
+#### Compile and build Python interpreter
 
 You can use `python Tools/wasm/emscripten` to compile and build targetting
 Emscripten. You can do everything at once with:
@@ -70,6 +70,88 @@ instance, to do a debug build, you can use:
 python Tools/wasm/emscripten build --with-py-debug
 ```
 
+### Running from node
+
+If you want to run the normal Python CLI, you can use `python.sh`. It takes the
+same options as the normal Python CLI entrypoint, though the REPL does not
+function and will crash.
+
+`python.sh` invokes `node_entry.mjs` which imports the Emscripten module for the
+Python process and starts it up with the appropriate settings. If you wish to
+make a node application that "embeds" the interpreter instead of acting like the
+CLI you will need to write your own alternative to `node_entry.mjs`.
+
+
+### The Web Example
+
+When building for Emscripten, the web example will be built automatically. It is
+in the ``web_example`` directory. To run the web example, ``cd`` into the
+``web_example`` directory, then run ``python server.py``. This will start a web
+server; you can then visit ``http://localhost:8000/python.html`` in a browser to
+see a simple REPL example.
+
+The web example relies on a bug fix in Emscripten version 3.1.73 so if you build
+with earlier versions of Emscripten it may not work. The web example uses
+``SharedArrayBuffer``. For security reasons browsers only provide
+``SharedArrayBuffer`` in secure environments with cross-origin isolation. The
+webserver must send cross-origin headers and correct MIME types for the
+JavaScript and WebAssembly files. Otherwise the terminal will fail to load with
+an error message like ``ReferenceError: SharedArrayBuffer is not defined``. See
+more information here:
+https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer#security_requirements
+
+Note that ``SharedArrayBuffer`` is _not required_ to use Python itself, only the
+web example. If cross-origin isolation is not appropriate for your use case you
+may make your own application embedding `python.mjs` which does not use
+``SharedArrayBuffer`` and serve it without the cross-origin isolation headers.
+
+### Embedding Python in a custom JavaScript application
+
+You can look at `python.worker.mjs` and `node_entry.mjs` for inspiration. At a
+minimum you must import ``createEmscriptenModule`` and you need to call
+``createEmscriptenModule`` with an appropriate settings object. This settings
+object will need a prerun hook that installs the Python standard library into
+the Emscripten file system.
+
+#### NodeJs
+
+In Node, you can use the NodeFS to mount the standard library in your native
+file system into the Emscripten file system:
+```js
+import createEmscriptenModule from "./python.mjs";
+
+await createEmscriptenModule({
+    preRun(Module) {
+        Module.FS.mount(
+            Module.FS.filesystems.NODEFS,
+            { root: "/path/to/python/stdlib" },
+            "/lib/",
+        );
+    },
+});
+```
+
+#### Browser
+
+In the browser, the simplest approach is to put the standard library in a zip
+file it and install it. With Python 3.14 this could look like:
+```js
+import createEmscriptenModule from "./python.mjs";
+
+await createEmscriptenModule({
+  async preRun(Module) {
+    Module.FS.mkdirTree("/lib/python3.14/lib-dynload/");
+    Module.addRunDependency("install-stdlib");
+    const resp = await fetch("python3.14.zip");
+    const stdlibBuffer = await resp.arrayBuffer();
+    Module.FS.writeFile(`/lib/python314.zip`, new Uint8Array(stdlibBuffer), {
+      canOwn: true,
+    });
+    Module.removeRunDependency("install-stdlib");
+  },
+});
+```
+
 ### Limitations and issues
 
 #### Network stack
@@ -151,38 +233,6 @@ python Tools/wasm/emscripten build --with-py-debug
 - Test modules are disabled by default. Use ``--enable-test-modules`` build
   test modules like ``_testcapi``.
 
-### wasm32-emscripten in node
-
-Node builds use ``NODERAWFS``.
-
-- Node RawFS allows direct access to the host file system without need to
-  perform ``FS.mount()`` call.
-
-### Hosting Python WASM builds
-
-The simple REPL terminal uses SharedArrayBuffer. For security reasons
-browsers only provide the feature in secure environments with cross-origin
-isolation. The webserver must send cross-origin headers and correct MIME types
-for the JavaScript and WASM files. Otherwise the terminal will fail to load
-with an error message like ``Browsers disable shared array buffer``.
-
-#### Apache HTTP .htaccess
-
-Place a ``.htaccess`` file in the same directory as ``python.wasm``.
-
-```
-# .htaccess
-Header set Cross-Origin-Opener-Policy same-origin
-Header set Cross-Origin-Embedder-Policy require-corp
-
-AddType application/javascript js
-AddType application/wasm wasm
-
-<IfModule mod_deflate.c>
-    AddOutputFilterByType DEFLATE text/html application/javascript application/wasm
-</IfModule>
-```
-
 ## WASI (wasm32-wasi)
 
 See [the devguide on how to build and run for WASI](https://devguide.python.org/getting-started/setup-building/#wasi).

Tools/wasm/python.html → Tools/wasm/emscripten/web_example/python.html

@@ -47,7 +47,7 @@
 
     async initialiseWorker() {
         if (!this.worker) {
-            this.worker = new Worker(this.workerURL)
+            this.worker = new Worker(this.workerURL, {type: "module"})
             this.worker.addEventListener('message', this.handleMessageFromWorker)
         }
     }
@@ -347,7 +347,7 @@
         programRunning(false)
     }
 
-    const pythonWorkerManager = new WorkerManager('./python.worker.js', stdio, readyCallback, finishedCallback)
+    const pythonWorkerManager = new WorkerManager('./python.worker.mjs', stdio, readyCallback, finishedCallback)
 }
     </script>
 </head>

Tools/wasm/python.worker.js → Tools/wasm/emscripten/web_example/python.worker.mjs

@@ -1,3 +1,5 @@
+import createEmscriptenModule from "./python.mjs";
+
 class StdinBuffer {
     constructor() {
         this.sab = new SharedArrayBuffer(128 * Int32Array.BYTES_PER_ELEMENT)
@@ -59,29 +61,44 @@ const stderr = (charCode) => {
 
 const stdinBuffer = new StdinBuffer()
 
-var Module = {
+const emscriptenSettings = {
     noInitialRun: true,
     stdin: stdinBuffer.stdin,
     stdout: stdout,
     stderr: stderr,
     onRuntimeInitialized: () => {
         postMessage({type: 'ready', stdinBuffer: stdinBuffer.sab})
+    },
+    async preRun(Module) {
+        const versionHex = Module.HEAPU32[Module._Py_Version/4].toString(16);
+        const versionTuple = versionHex.padStart(8, "0").match(/.{1,2}/g).map((x) => parseInt(x, 16));
+        const [major, minor, ..._] = versionTuple;
+        // Prevent complaints about not finding exec-prefix by making a lib-dynload directory
+        Module.FS.mkdirTree(`/lib/python${major}.${minor}/lib-dynload/`);
+        Module.addRunDependency("install-stdlib");
+        const resp = await fetch(`python${major}.${minor}.zip`);
+        const stdlibBuffer = await resp.arrayBuffer();
+        Module.FS.writeFile(`/lib/python${major}${minor}.zip`, new Uint8Array(stdlibBuffer), { canOwn: true });
+        Module.removeRunDependency("install-stdlib");
     }
 }
 
-onmessage = (event) => {
+const modulePromise = createEmscriptenModule(emscriptenSettings);
+
+
+onmessage = async (event) => {
     if (event.data.type === 'run') {
+        const Module = await modulePromise;
         if (event.data.files) {
             for (const [filename, contents] of Object.entries(event.data.files)) {
                 Module.FS.writeFile(filename, contents)
             }
         }
-        const ret = callMain(event.data.args)
+        const ret = Module.callMain(event.data.args);
         postMessage({
             type: 'finished',
             returnCode: ret
         })
     }
 }
 
-importScripts('python.js')

Tools/wasm/wasm_webserver.py → Tools/wasm/emscripten/web_example/server.py

@@ -14,13 +14,6 @@
 
 
 class MyHTTPRequestHandler(server.SimpleHTTPRequestHandler):
-    extensions_map = server.SimpleHTTPRequestHandler.extensions_map.copy()
-    extensions_map.update(
-        {
-            ".wasm": "application/wasm",
-        }
-    )
-
     def end_headers(self) -> None:
         self.send_my_headers()
         super().end_headers()
@@ -42,5 +35,6 @@ def main() -> None:
         bind=args.bind,
     )
 
+
 if __name__ == "__main__":
     main()