`inspect.getsource` from a function defined in a string? `s="def f(): return 5"`

Given a function defined inline, how do I get getsource to provide the output? - This is for a test, here's the kind of thing I'm trying:

from importlib.util import module_from_spec, spec_from_loader

_locals = module_from_spec(
    spec_from_loader("helper", loader=None, origin="str")  # loader=MemoryInspectLoader
)
exec(
    'def f(): return "foo"',
    _locals.__dict__,
)
f = getattr(_locals, "f")
setattr(f, "__loader__", MemoryInspectLoader)

With my attempt, as it looks like a linecache issue:

from importlib.abc import Loader

class MemoryInspectLoader(Loader):
    def get_code(self): raise NotImplementedError()

But the error is never raised. From getsource(f), I just get:

In [2]: import inspect
   ...: inspect.getsource(f)
---------------------------------------------------------------------------
OSError                                   Traceback (most recent call last)
<ipython-input-3-1348c7a45f75> in <module>
----> 1 inspect.getsource(f)

/usr/lib/python3.8/inspect.py in getsource(object)
    983     or code object.  The source code is returned as a single string.  An
    984     OSError is raised if the source code cannot be retrieved."""
--> 985     lines, lnum = getsourcelines(object)
    986     return ''.join(lines)
    987 

/usr/lib/python3.8/inspect.py in getsourcelines(object)
    965     raised if the source code cannot be retrieved."""
    966     object = unwrap(object)
--> 967     lines, lnum = findsource(object)
    968 
    969     if istraceback(object):

/usr/lib/python3.8/inspect.py in findsource(object)
    796         lines = linecache.getlines(file)
    797     if not lines:
--> 798         raise OSError('could not get source code')
    799 
    800     if ismodule(object):

OSError: could not get source code

How do I make getsource work with an inline-defined function in Python 3.6+?

from `inspect.getsource` from a function defined in a string? `s="def f(): return 5"`