bpo-47176: Interrupt handling for wasm32-emscripten builds without pthreads (GH-32209)

Co-authored-by: Christian Heimes <christian@python.org>
Co-authored-by: Brett Cannon <brett@python.org>

Python/emscripten_signal.c

@@ -0,0 +1,56 @@
+// To enable signal handling, the embedder should:
+// 1. set Module.Py_EmscriptenSignalBuffer = some_shared_array_buffer;
+// 2. set the Py_EMSCRIPTEN_SIGNAL_HANDLING flag to 1 as follows:
+//    Module.HEAP8[Module._Py_EMSCRIPTEN_SIGNAL_HANDLING] = 1
+//
+// The address &Py_EMSCRIPTEN_SIGNAL_HANDLING is exported as
+// Module._Py_EMSCRIPTEN_SIGNAL_HANDLING.
+#include <emscripten.h>
+#include "Python.h"
+
+EM_JS(int, _Py_CheckEmscriptenSignals_Helper, (void), {
+    if (!Module.Py_EmscriptenSignalBuffer) {
+        return 0;
+    }
+    try {
+        let result = Module.Py_EmscriptenSignalBuffer[0];
+        Module.Py_EmscriptenSignalBuffer[0] = 0;
+        return result;
+    } catch(e) {
+#if !defined(NDEBUG)
+        console.warn("Error occurred while trying to read signal buffer:", e);
+#endif
+        return 0;
+    }
+});
+
+EMSCRIPTEN_KEEPALIVE int Py_EMSCRIPTEN_SIGNAL_HANDLING = 0;
+
+void
+_Py_CheckEmscriptenSignals(void)
+{
+    if (!Py_EMSCRIPTEN_SIGNAL_HANDLING) {
+        return;
+    }
+    int signal = _Py_CheckEmscriptenSignals_Helper();
+    if (signal) {
+        PyErr_SetInterruptEx(signal);
+    }
+}
+
+
+#define PY_EMSCRIPTEN_SIGNAL_INTERVAL 50
+static int emscripten_signal_clock = PY_EMSCRIPTEN_SIGNAL_INTERVAL;
+
+void
+_Py_CheckEmscriptenSignalsPeriodically(void)
+{
+    if (!Py_EMSCRIPTEN_SIGNAL_HANDLING) {
+        return;
+    }
+    emscripten_signal_clock--;
+    if (emscripten_signal_clock == 0) {
+        emscripten_signal_clock = PY_EMSCRIPTEN_SIGNAL_INTERVAL;
+        _Py_CheckEmscriptenSignals();
+    }
+}