gh-156780: Emscripten: gate the suspending syscalls on the main() wrapper

Suspending in fd_read or poll needs a promising entry point, which
libpython no longer sets up itself. Check Module.Py_EmscriptenStackSwitching,
set by the interpreter's wrapper, instead of WebAssembly.promising alone.
An embedder with its own promising entry point sets the flag.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Author: clementperon

Modules/_testinternalcapi.c

@@ -2961,7 +2961,7 @@ EM_JS(int, emscripten_set_up_async_input_device_js, (void), {
         await sleep(5);
         return bufs[(idx ++) % 3];
     });
-    return !!WebAssembly.promising;
+    return !!Module.Py_EmscriptenStackSwitching;
 });
 
 static PyObject *

Programs/emscripten_beforemain.c

@@ -18,6 +18,7 @@ EM_JS(void, _PyEmscripten_BeforeMain_js, (void), {
     // promising() needs the raw export; _main may be a JS wrapper around it.
     const main = WebAssembly.promising(wasmExports.__main_argc_argv);
     _main = (...args) => {
+        Module.Py_EmscriptenStackSwitching = true;
         // Exit the way callMain() would have, once main() is actually done.
         main(...args).then((ret) => exitJS(ret, true)).catch(handleException);
         // Unwind to callMain() without letting it exit: main() is still

Python/emscripten_syscalls.c

@@ -124,13 +124,20 @@ _Static_assert(sizeof(__wasi_iovec_t) == IOVEC_T_SIZE,
 // If the stream has a readAsync handler, read to buffer defined in iovs, write
 // number of bytes read to *nread, and return a promise that resolves to the
 // errno. Otherwise, return null.
+//
+// Reading from an async input device and poll() suspend the wasm stack
+// instead of blocking when main() runs under WebAssembly.promising, which
+// Programs/emscripten_beforemain.c arranges for the interpreter. An embedder
+// running its own promising entry point opts in with
+//     Module.Py_EmscriptenStackSwitching = true;
+// Otherwise these calls keep their synchronous behavior.
 EM_JS_MACROS(__externref_t, __maybe_fd_read_async, (
     __wasi_fd_t fd,
     const __wasi_iovec_t *iovs,
     size_t iovcnt,
     __wasi_size_t *nread
 ), {
-    if (!WebAssembly.promising) {
+    if (!Module.Py_EmscriptenStackSwitching) {
         return null;
     }
     var stream;
@@ -214,7 +221,7 @@ _Static_assert(offsetof(struct pollfd, revents) == 6, "Unepxected pollfd struct
 _Static_assert(sizeof(struct pollfd) == 8, "Unepxected pollfd struct layout");
 
 EM_JS_MACROS(__externref_t, __maybe_poll_async, (intptr_t fds, int nfds, int timeout), {
-    if (!WebAssembly.promising) {
+    if (!Module.Py_EmscriptenStackSwitching) {
         return null;
     }
     return (async function() {