gh-157548: Emscripten: take a signal from the signal buffer in one step

Lib/test/test_capi/test_emscripten.py

@@ -1,10 +1,14 @@
+import signal
 import unittest
 from test.support import is_emscripten
 
 if not is_emscripten:
     raise unittest.SkipTest("Emscripten-only test")
 
-from _testinternalcapi import emscripten_set_up_async_input_device
+from _testinternalcapi import (
+    emscripten_check_signal_buffer,
+    emscripten_set_up_async_input_device,
+)
 from pathlib import Path
 
 
@@ -23,3 +27,28 @@ def test_emscripten_async_input_device(self):
                 self.assertEqual(f.readline().strip(), "ab")
                 self.assertEqual(f.readline().strip(), "fi")
                 self.assertEqual(f.readline().strip(), "xy")
+
+
+class EmscriptenSignalBufferTest(unittest.TestCase):
+    def check_signal_buffer(self, shared):
+        received = []
+
+        def handler(signum, frame):
+            received.append(signum)
+
+        old_handler = signal.signal(signal.SIGUSR1, handler)
+        self.addCleanup(signal.signal, signal.SIGUSR1, old_handler)
+
+        left_in_buffer = emscripten_check_signal_buffer(signal.SIGUSR1, shared)
+        for _ in range(1000):
+            if received:
+                break
+
+        self.assertEqual(left_in_buffer, 0)
+        self.assertEqual(received, [signal.SIGUSR1])
+
+    def test_shared_buffer(self):
+        self.check_signal_buffer(shared=True)
+
+    def test_unshared_buffer(self):
+        self.check_signal_buffer(shared=False)

Misc/NEWS.d/next/Core_and_Builtins/2026-09-15-09-18-51.gh-issue-157548.cq9Iu2.rst

@@ -0,0 +1,2 @@
+On Emscripten, a signal written to the signal buffer while the interpreter
+was clearing it is no longer lost.

Modules/_testinternalcapi.c

@@ -2945,6 +2945,7 @@ incref_decref_delayed(PyObject *self, PyObject *op)
 
 #ifdef __EMSCRIPTEN__
 #include "emscripten.h"
+#include "pycore_emscripten_signal.h"  // Py_EMSCRIPTEN_SIGNAL_HANDLING
 
 EM_JS(int, emscripten_set_up_async_input_device_js, (void), {
     let idx = 0;
@@ -2972,6 +2973,36 @@ emscripten_set_up_async_input_device(PyObject *self, PyObject *Py_UNUSED(ignored
         Py_RETURN_FALSE;
     }
 }
+
+EM_JS(void, emscripten_set_signal_buffer_js, (int signum, int shared), {
+    Module._Py_saved_signal_buffer = Module.Py_EmscriptenSignalBuffer;
+    Module.Py_EmscriptenSignalBuffer = shared
+        ? new Uint8Array(new SharedArrayBuffer(1))
+        : new Uint8Array(1);
+    Module.Py_EmscriptenSignalBuffer[0] = signum;
+});
+
+EM_JS(int, emscripten_restore_signal_buffer_js, (void), {
+    const value = Module.Py_EmscriptenSignalBuffer[0];
+    Module.Py_EmscriptenSignalBuffer = Module._Py_saved_signal_buffer;
+    delete Module._Py_saved_signal_buffer;
+    return value;
+});
+
+static PyObject *
+emscripten_check_signal_buffer(PyObject *self, PyObject *args)
+{
+    int signum, shared;
+    if (!PyArg_ParseTuple(args, "ip", &signum, &shared)) {
+        return NULL;
+    }
+    int handling = Py_EMSCRIPTEN_SIGNAL_HANDLING;
+    emscripten_set_signal_buffer_js(signum, shared);
+    Py_EMSCRIPTEN_SIGNAL_HANDLING = 1;
+    _Py_CheckEmscriptenSignals();
+    Py_EMSCRIPTEN_SIGNAL_HANDLING = handling;
+    return PyLong_FromLong(emscripten_restore_signal_buffer_js());
+}
 #endif
 
 static PyObject *
@@ -3383,6 +3414,7 @@ static PyMethodDef module_functions[] = {
     GET_NEXT_DICT_KEYS_VERSION_METHODDEF
 #ifdef __EMSCRIPTEN__
     {"emscripten_set_up_async_input_device", emscripten_set_up_async_input_device, METH_NOARGS},
+    {"emscripten_check_signal_buffer", emscripten_check_signal_buffer, METH_VARARGS},
 #endif
     {"simple_pending_call", simple_pending_call, METH_O},
     {"set_vectorcall_nop", set_vectorcall_nop, METH_O},

Python/emscripten_signal.c

@@ -13,9 +13,7 @@ EM_JS(int, _Py_CheckEmscriptenSignals_Helper, (void), {
         return 0;
     }
     try {
-        let result = Module.Py_EmscriptenSignalBuffer[0];
-        Module.Py_EmscriptenSignalBuffer[0] = 0;
-        return result;
+        return Atomics.exchange(Module.Py_EmscriptenSignalBuffer, 0, 0);
     } catch(e) {
 #if !defined(NDEBUG)
         console.warn("Error occurred while trying to read signal buffer:", e);