The code for check_GetFinalPathNameByHandle() goes like this:


static int has_GetFinalPathNameByHandle = 0;
[...]

static int
check_GetFinalPathNameByHandle()
{
    HINSTANCE hKernel32;
    /* only recheck */
    if (!has_GetFinalPathNameByHandle)
    {
        [...]
        has_GetFinalPathNameByHandle = Py_GetFinalPathNameByHandleA &&
                                       Py_GetFinalPathNameByHandleW;
    }
    return has_GetFinalPathNameByHandle;
}


It means that if the computed value is 0 (system doesn't have GetFinalPathNameByHandle*), the value will be re-computed each time the function is called.  has_GetFinalPathNameByHandle should probably be tri-state instead (0, 1, unknown).

Is this related to some other issue? The fix seems trivial, however I am curious as to how you stumbled upon this?

Is there more to this issue than just performance?

Yes, it should be trivial to fix.
I've stumbled on this simply because I needed to write similar code for another patch, so studied how it was done in posixmodule.c.

I looked at providing a patch for this issue as an introductory step (this would be my first patch).

But when following the code I discovered that the case that this issue is trying to optimize is never executed in current CPython. In that case, there isn't much value in optimizing it.

More precisely, check_GetFinalPathNameByHandle is called by posix__getfinalpathname which is nt._getfinalpathname in Python code. If the check fails, posix__getfinalpathname throws NotImplmenentedError. But nt._getfinalpathname is only used by ntpath.py which checks the Windows version and only calls nt._getfinalpathname for Vista and higher where the check won't fail.

To me it would make more sense that the nt module has a _getfinalpathname attribute only if it supports the feature instead of always having one that throws NotImplementedError. In that case, ntpath.py would not check the Windows version but the presence of _getfinalpathname in the nt module. Does this seem like a better approach to you, at least theoretically? And if so, is it worth implementing it?

Thanks for any advice.

Patch to cache result of check_GetFinalPathNameByHandle().

Thank you, patch looks fine to me. It will have to wait for after the 3.3 release, though, since we are now in feature freeze.

New changeset 8afa3ce5ff3e by Antoine Pitrou in branch 'default':
Issue #12034: Fix bogus caching of result in check_GetFinalPathNameByHandle.
http://hg.python.org/cpython/rev/8afa3ce5ff3e

Patch committed, thank you!