pythongh-126898 Emscripten support: Use es6 modules

We delete `node_pre.js` and `--pre-js`. Instead that logic moves to
`node_entry.mjs`. In order to access `FS` from outside of `python.mjs`, we need
to add `-sEXPORTED_RUNTIME_METHODS=FS`. Instead of invoking `python.js`
directly, we will invoke `node_entry.mjs`.

Special care has to be taken to ensure that we get `sys.executable` correct. It
should point to the original path that was invoked, without symlinks resolved. I
updated the generated `python.sh` to calculate it's own absolute path with
`realpath -s $0` and pass that in. Then `node_entry.mjs` sets `thisProgram` to
the path passed in. I manually validated this by creating a symlink to
`python.sh` and invoking it. `sys.executable` and `sys._base_exectable`
indeed point to the symlink, not to the build directory `python.sh` and not to
`node_entry.mjs`.

Tools/wasm/emscripten/__main__.py

@@ -4,20 +4,22 @@
 import contextlib
 import functools
 import os
-
-try:
-    from os import process_cpu_count as cpu_count
-except ImportError:
-    from os import cpu_count
-from pathlib import Path
 import shutil
 import subprocess
 import sys
 import sysconfig
 import tempfile
+from pathlib import Path
+from textwrap import dedent
+
+try:
+    from os import process_cpu_count as cpu_count
+except ImportError:
+    from os import cpu_count
 
-WASM_DIR = Path(__file__).parent.parent
-CHECKOUT = WASM_DIR.parent.parent
+
+EMSCRIPTEN_DIR = Path(__file__).parent
+CHECKOUT = EMSCRIPTEN_DIR.parent.parent.parent
 
 CROSS_BUILD_DIR = CHECKOUT / "cross-build"
 BUILD_DIR = CROSS_BUILD_DIR / "build"
@@ -72,7 +74,7 @@ def wrapper(context):
             print("⎯" * terminal_width)
             print("📁", working_dir)
             if clean_ok and getattr(context, "clean", False) and working_dir.exists():
-                print(f"🚮 Deleting directory (--clean)...")
+                print("🚮 Deleting directory (--clean)...")
                 shutil.rmtree(working_dir)
 
             working_dir.mkdir(parents=True, exist_ok=True)
@@ -207,9 +209,21 @@ def configure_emscripten_python(context, working_dir):
         quiet=context.quiet,
     )
 
-    python_js = working_dir / "python.js"
+    shutil.copy(EMSCRIPTEN_DIR / "node_entry.mjs", working_dir / "node_entry.mjs")
+
+    node_entry = working_dir / "node_entry.mjs"
     exec_script = working_dir / "python.sh"
-    exec_script.write_text(f'#!/bin/sh\nexec {host_runner} {python_js} "$@"\n')
+    exec_script.write_text(
+        dedent(
+            f"""\
+            #!/bin/sh
+
+            # We compute our own path, not following symlinks and pass it in so that
+            # node_entry.mjs can set sys.executable correctly.
+            exec {host_runner} {node_entry} "$(realpath -s $0)" "$@"
+            """
+        )
+    )
     exec_script.chmod(0o755)
     print(f"🏃‍♀️ Created {exec_script} ... ")
     sys.stdout.flush()

Tools/wasm/emscripten/node_entry.mjs

@@ -0,0 +1,30 @@
+import EmscriptenModule from "./python.mjs";
+import { dirname } from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+if (process?.versions?.node) {
+  const nodeVersion = Number(process.versions.node.split(".", 1)[0]);
+  if (nodeVersion < 18) {
+      process.stderr.write(
+          `Node version must be >= 18, got version ${process.version}\n`,
+      );
+      process.exit(1);
+  }
+}
+
+const settings = {
+  preRun(Module) {
+    const __dirname = dirname(fileURLToPath(import.meta.url));
+    Module.FS.mkdirTree("/lib/");
+    Module.FS.mount(Module.FS.filesystems.NODEFS, { root: __dirname + "/lib/" }, "/lib/");
+  },
+  // The first three arguments are: "node", path to this file, path to
+  // python.sh. After that come the arguments the user passed to python.sh.
+  arguments: process.argv.slice(3),
+  // Ensure that sys.executable, sys._base_executable, etc point to python.sh
+  // not to this file. To properly handle symlinks, python.sh needs to compute
+  // its own path.
+  thisProgram: process.argv[2],
+};
+
+await EmscriptenModule(settings);

configure.ac

@@ -1327,7 +1327,7 @@ AC_ARG_WITH([suffix],
   )
 ], [
   AS_CASE([$ac_sys_system],
-    [Emscripten], [EXEEXT=.js],
+    [Emscripten], [EXEEXT=.mjs],
     [WASI], [EXEEXT=.wasm],
     [EXEEXT=]
   )
@@ -2328,6 +2328,7 @@ AS_CASE([$ac_sys_system],
 
     dnl Include file system support
     AS_VAR_APPEND([LDFLAGS_NODIST], [" -sFORCE_FILESYSTEM -lidbfs.js -lnodefs.js -lproxyfs.js -lworkerfs.js"])
+    AS_VAR_APPEND([LDFLAGS_NODIST], [" -sEXPORTED_RUNTIME_METHODS=FS"])
 
     AS_VAR_IF([enable_wasm_dynamic_linking], [yes], [
       AS_VAR_APPEND([LINKFORSHARED], [" -sMAIN_MODULE"])
@@ -2341,7 +2342,6 @@ AS_CASE([$ac_sys_system],
     AS_VAR_APPEND([LDFLAGS_NODIST], [" -sALLOW_MEMORY_GROWTH"])
     dnl not completely sure whether or not we want -sEXIT_RUNTIME, keeping it for now.
     AS_VAR_APPEND([LDFLAGS_NODIST], [" -sEXIT_RUNTIME"])
-    AS_VAR_APPEND([LDFLAGS_NODIST], [" --pre-js=\$(srcdir)/Tools/wasm/emscripten/node_pre.js"])
     WASM_LINKFORSHARED_DEBUG="-gseparate-dwarf --emit-symbol-map"
 
     AS_VAR_IF([wasm_debug], [yes], [