gh-94912: Added marker for non-standard coroutine function detection (#99247)

This introduces a new decorator `@inspect.markcoroutinefunction`,
which, applied to a sync function, makes it appear async to
`inspect.iscoroutinefunction()`.

Lib/inspect.py

@@ -125,6 +125,7 @@ __all__ = [
     "ismodule",
     "isroutine",
     "istraceback",
+    "markcoroutinefunction",
     "signature",
     "stack",
     "trace",
@@ -391,12 +392,33 @@ def isgeneratorfunction(obj):
     See help(isfunction) for a list of attributes."""
     return _has_code_flag(obj, CO_GENERATOR)
 
+# A marker for markcoroutinefunction and iscoroutinefunction.
+_is_coroutine_marker = object()
+
+def _has_coroutine_mark(f):
+    while ismethod(f):
+        f = f.__func__
+    f = functools._unwrap_partial(f)
+    if not (isfunction(f) or _signature_is_functionlike(f)):
+        return False
+    return getattr(f, "_is_coroutine_marker", None) is _is_coroutine_marker
+
+def markcoroutinefunction(func):
+    """
+    Decorator to ensure callable is recognised as a coroutine function.
+    """
+    if hasattr(func, '__func__'):
+        func = func.__func__
+    func._is_coroutine_marker = _is_coroutine_marker
+    return func
+
 def iscoroutinefunction(obj):
     """Return true if the object is a coroutine function.
 
-    Coroutine functions are defined with "async def" syntax.
+    Coroutine functions are normally defined with "async def" syntax, but may
+    be marked via markcoroutinefunction.
     """
-    return _has_code_flag(obj, CO_COROUTINE)
+    return _has_code_flag(obj, CO_COROUTINE) or _has_coroutine_mark(obj)
 
 def isasyncgenfunction(obj):
     """Return true if the object is an asynchronous generator function.