python
diff --git a/Include/internal/pycore_emscripten_trampoline.h b/Include/internal/pycore_emscripten_trampoline.h
@@ -0,0 +1,81 @@
+#ifndef Py_EMSCRIPTEN_TRAMPOLINE_H
+#define Py_EMSCRIPTEN_TRAMPOLINE_H
+
+#include "pycore_runtime.h"  // _PyRuntimeState
+
+/**
+ * C function call trampolines to mitigate bad function pointer casts.
+ *
+ * Section 6.3.2.3, paragraph 8 reads:
+ *
+ *      A pointer to a function of one type may be converted to a pointer to a
+ *      function of another type and back again; the result shall compare equal to
+ *      the original pointer. If a converted pointer is used to call a function
+ *      whose type is not compatible with the pointed-to type, the behavior is
+ *      undefined.
+ *
+ * Typical native ABIs ignore additional arguments or fill in missing values
+ * with 0/NULL in function pointer cast. Compilers do not show warnings when a
+ * function pointer is explicitly casted to an incompatible type.
+ *
+ * Bad fpcasts are an issue in WebAssembly. WASM's indirect_call has strict
+ * function signature checks. Argument count, types, and return type must match.
+ *
+ * Third party code unintentionally rely on problematic fpcasts. The call
+ * trampoline mitigates common occurrences of bad fpcasts on Emscripten.
+ */
+
+#if defined(__EMSCRIPTEN__) && defined(PY_CALL_TRAMPOLINE)
+
+void _Py_EmscriptenTrampoline_Init(_PyRuntimeState *runtime);
+
+PyObject*
+_PyEM_TrampolineCall_JavaScript(PyCFunctionWithKeywords func,
+                                PyObject* self,
+                                PyObject* args,
+                                PyObject* kw);
+
+PyObject*
+_PyEM_TrampolineCall_Reflection(PyCFunctionWithKeywords func,
+                                PyObject* self,
+                                PyObject* args,
+                                PyObject* kw);
+
+#define _PyEM_TrampolineCall(meth, self, args, kw) \
+    ((_PyRuntime.wasm_type_reflection_available) ? \
+        (_PyEM_TrampolineCall_Reflection((PyCFunctionWithKeywords)(meth), (self), (args), (kw))) : \
+        (_PyEM_TrampolineCall_JavaScript((PyCFunctionWithKeywords)(meth), (self), (args), (kw))))
+
+#define _PyCFunction_TrampolineCall(meth, self, args) \
+    _PyEM_TrampolineCall( \
+        (*(PyCFunctionWithKeywords)(void(*)(void))(meth)), (self), (args), NULL)
+
+#define _PyCFunctionWithKeywords_TrampolineCall(meth, self, args, kw) \
+    _PyEM_TrampolineCall((meth), (self), (args), (kw))
+
+#define descr_set_trampoline_call(set, obj, value, closure) \
+    ((int)_PyEM_TrampolineCall((PyCFunctionWithKeywords)(set), (obj), (value), (PyObject*)(closure)))
+
+#define descr_get_trampoline_call(get, obj, closure) \
+    _PyEM_TrampolineCall((PyCFunctionWithKeywords)(get), (obj), (PyObject*)(closure), NULL)
+
+
+#else // defined(__EMSCRIPTEN__) && defined(PY_CALL_TRAMPOLINE)
+
+#define _Py_EmscriptenTrampoline_Init(runtime)
+
+#define _PyCFunction_TrampolineCall(meth, self, args) \
+    (meth)((self), (args))
+
+#define _PyCFunctionWithKeywords_TrampolineCall(meth, self, args, kw) \
+    (meth)((self), (args), (kw))
+
+#define descr_set_trampoline_call(set, obj, value, closure) \
+    (set)((obj), (value), (closure))
+
+#define descr_get_trampoline_call(get, obj, closure) \
+    (get)((obj), (closure))
+
+#endif // defined(__EMSCRIPTEN__) && defined(PY_CALL_TRAMPOLINE)
+
+#endif // ndef Py_EMSCRIPTEN_SIGNAL_H
diff --git a/Include/internal/pycore_object.h b/Include/internal/pycore_object.h
@@ -10,6 +10,7 @@ extern "C" {
 
 #include <stdbool.h>
 #include "pycore_gc.h"            // _PyObject_GC_IS_TRACKED()
+#include "pycore_emscripten_trampoline.h" // _PyCFunction_TrampolineCall()
 #include "pycore_interp.h"        // PyInterpreterState.gc
 #include "pycore_pystate.h"       // _PyInterpreterState_GET()
 

@@ -267,6 +267,13 @@ typedef struct pyruntimestate {
 
     /* PyInterpreterState.interpreters.main */
     PyInterpreterState _main_interpreter;
+
+#if defined(__EMSCRIPTEN__) && defined(PY_CALL_TRAMPOLINE)
+    // Used in "Python/emscripten_trampoline.c" to choose between type
+    // reflection trampoline and EM_JS trampoline.
+    bool wasm_type_reflection_available;
+#endif
+
 } _PyRuntimeState;
 
 

diff --git a/Misc/NEWS.d/next/Core and Builtins/2023-06-29-09-42-56.gh-issue-106213.TCUgzM.rst b/Misc/NEWS.d/next/Core and Builtins/2023-06-29-09-42-56.gh-issue-106213.TCUgzM.rst
@@ -0,0 +1,2 @@
+Changed the way that Emscripten call trampolines work for compatibility with
+Wasm/JS Promise integration.
diff --git a/Objects/descrobject.c b/Objects/descrobject.c
@@ -4,6 +4,7 @@
 #include "pycore_abstract.h"      // _PyObject_RealIsSubclass()
 #include "pycore_call.h"          // _PyStack_AsDict()
 #include "pycore_ceval.h"         // _Py_EnterRecursiveCallTstate()
+#include "pycore_emscripten_trampoline.h" // descr_set_trampoline_call(), descr_get_trampoline_call()
 #include "pycore_descrobject.h"   // _PyMethodWrapper_Type
 #include "pycore_object.h"        // _PyObject_GC_UNTRACK()
 #include "pycore_pystate.h"       // _PyThreadState_GET()
@@ -16,25 +17,6 @@ class property "propertyobject *" "&PyProperty_Type"
 [clinic start generated code]*/
 /*[clinic end generated code: output=da39a3ee5e6b4b0d input=556352653fd4c02e]*/
 
-// see pycore_object.h
-#if defined(__EMSCRIPTEN__) && defined(PY_CALL_TRAMPOLINE)
-#include <emscripten.h>
-EM_JS(int, descr_set_trampoline_call, (setter set, PyObject *obj, PyObject *value, void *closure), {
-    return wasmTable.get(set)(obj, value, closure);
-});
-
-EM_JS(PyObject*, descr_get_trampoline_call, (getter get, PyObject *obj, void *closure), {
-    return wasmTable.get(get)(obj, closure);
-});
-#else
-#define descr_set_trampoline_call(set, obj, value, closure) \
-    (set)((obj), (value), (closure))
-
-#define descr_get_trampoline_call(get, obj, closure) \
-    (get)((obj), (closure))
-
-#endif // __EMSCRIPTEN__ && PY_CALL_TRAMPOLINE
-
 static void
 descr_dealloc(PyDescrObject *descr)
 {

diff --git a/Objects/methodobject.c b/Objects/methodobject.c
@@ -553,10 +553,3 @@ cfunction_call(PyObject *func, PyObject *args, PyObject *kwargs)
     return _Py_CheckFunctionResult(tstate, func, result, NULL);
 }
 
-#if defined(__EMSCRIPTEN__) && defined(PY_CALL_TRAMPOLINE)
-#include <emscripten.h>
-
-EM_JS(PyObject*, _PyCFunctionWithKeywords_TrampolineCall, (PyCFunctionWithKeywords func, PyObject *self, PyObject *args, PyObject *kw), {
-    return wasmTable.get(func)(self, args, kw);
-});
-#endif
diff --git a/Python/emscripten_trampoline.c b/Python/emscripten_trampoline.c
@@ -0,0 +1,82 @@
+#if defined(PY_CALL_TRAMPOLINE)
+
+#include <emscripten.h>             // EM_JS
+#include <Python.h>
+#include "pycore_runtime.h"         // _PyRuntime
+
+
+/**
+ * This is the GoogleChromeLabs approved way to feature detect type-reflection:
+ * https://github.com/GoogleChromeLabs/wasm-feature-detect/blob/main/src/detectors/type-reflection/index.js
+ */
+EM_JS(int, _PyEM_detect_type_reflection, (), {
+    return "Function" in WebAssembly;
+});
+
+void
+_Py_EmscriptenTrampoline_Init(_PyRuntimeState *runtime)
+{
+    runtime->wasm_type_reflection_available = _PyEM_detect_type_reflection();
+}
+
+/**
+ * Backwards compatible trampoline works with all JS runtimes
+ */
+EM_JS(PyObject*,
+_PyEM_TrampolineCall_JavaScript, (PyCFunctionWithKeywords func,
+                                  PyObject *arg1,
+                                  PyObject *arg2,
+                                  PyObject *arg3),
+{
+    return wasmTable.get(func)(arg1, arg2, arg3);
+}
+);
+
+/**
+ * In runtimes with WebAssembly type reflection, count the number of parameters
+ * and cast to the appropriate signature
+ */
+EM_JS(int, _PyEM_CountFuncParams, (PyCFunctionWithKeywords func),
+{
+    let n = _PyEM_CountFuncParams.cache.get(func);
+
+    if (n !== undefined) {
+        return n;
+    }
+    n = WebAssembly.Function.type(wasmTable.get(func)).parameters.length;
+    _PyEM_CountFuncParams.cache.set(func, n);
+    return n;
+}
+_PyEM_CountFuncParams.cache = new Map();
+)
+
+
+typedef PyObject* (*zero_arg)(void);
+typedef PyObject* (*one_arg)(PyObject*);
+typedef PyObject* (*two_arg)(PyObject*, PyObject*);
+typedef PyObject* (*three_arg)(PyObject*, PyObject*, PyObject*);
+
+
+PyObject*
+_PyEM_TrampolineCall_Reflection(PyCFunctionWithKeywords func,
+                                PyObject* self,
+                                PyObject* args,
+                                PyObject* kw)
+{
+    switch (_PyEM_CountFuncParams(func)) {
+        case 0:
+            return ((zero_arg)func)();
+        case 1:
+            return ((one_arg)func)(self);
+        case 2:
+            return ((two_arg)func)(self, args);
+        case 3:
+            return ((three_arg)func)(self, args, kw);
+        default:
+            PyErr_SetString(PyExc_SystemError,
+                            "Handler takes too many arguments");
+            return NULL;
+    }
+}
+
+#endif
@@ -5,6 +5,7 @@
 #include "pycore_ceval.h"
 #include "pycore_code.h"          // stats
 #include "pycore_dtoa.h"          // _dtoa_state_INIT()
+#include "pycore_emscripten_trampoline.h"  // _Py_EmscriptenTrampoline_Init()
 #include "pycore_frame.h"
 #include "pycore_initconfig.h"    // _PyStatus_OK()
 #include "pycore_object.h"        // _PyType_InitCache()
@@ -449,6 +450,10 @@ init_runtime(_PyRuntimeState *runtime,
 
     runtime->unicode_state.ids.next_index = unicode_next_index;
 
+#if defined(__EMSCRIPTEN__) && defined(PY_CALL_TRAMPOLINE)
+    _Py_EmscriptenTrampoline_Init(runtime);
+#endif
+
     runtime->_initialized = 1;
 }
 

@@ -4593,8 +4593,8 @@ PLATFORM_OBJS=
 
 AS_CASE([$ac_sys_system],
   [Emscripten], [
-    AS_VAR_APPEND([PLATFORM_OBJS], [' Python/emscripten_signal.o'])
-    AS_VAR_APPEND([PLATFORM_HEADERS], [' $(srcdir)/Include/internal/pycore_emscripten_signal.h'])
+    AS_VAR_APPEND([PLATFORM_OBJS], [' Python/emscripten_signal.o Python/emscripten_trampoline.o'])
+    AS_VAR_APPEND([PLATFORM_HEADERS], [' $(srcdir)/Include/internal/pycore_emscripten_signal.h $(srcdir)/Include/internal/pycore_emscripten_trampoline.h'])
   ],
 )
 AC_SUBST([PLATFORM_HEADERS])
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		Changed the way that Emscripten call trampolines work for compatibility with
		Wasm/JS Promise integration.