gh-120754: _io Ensure stat cache is cleared on fd change (#125166)

Performed an audit of `fileio.c` and `_pyio` and made sure anytime the
fd changes the stat result, if set, is also cleared/changed.

There's one case where it's not cleared, if code would clear it in
__init__, keep the memory allocated and just do another fstat with the
existing memory.

Lib/_pyio.py

@@ -1480,6 +1480,7 @@ def __init__(self, file, mode='r', closefd=True, opener=None):
         """
         if self._fd >= 0:
             # Have to close the existing file first.
+            self._stat_atopen = None
             try:
                 if self._closefd:
                     os.close(self._fd)
@@ -1583,6 +1584,7 @@ def __init__(self, file, mode='r', closefd=True, opener=None):
                     if e.errno != errno.ESPIPE:
                         raise
         except:
+            self._stat_atopen = None
             if owned_fd is not None:
                 os.close(owned_fd)
             raise
@@ -1756,6 +1758,7 @@ def close(self):
         called more than once without error.
         """
         if not self.closed:
+            self._stat_atopen = None
             try:
                 if self._closefd:
                     os.close(self._fd)

Modules/_io/fileio.c

@@ -131,6 +131,8 @@ internal_close(fileio *self)
         _Py_END_SUPPRESS_IPH
         Py_END_ALLOW_THREADS
     }
+    PyMem_Free(self->stat_atopen);
+    self->stat_atopen = NULL;
     if (err < 0) {
         errno = save_errno;
         PyErr_SetFromErrno(PyExc_OSError);
@@ -268,8 +270,9 @@ _io_FileIO___init___impl(fileio *self, PyObject *nameobj, const char *mode,
     if (self->fd >= 0) {
         if (self->closefd) {
             /* Have to close the existing file first. */
-            if (internal_close(self) < 0)
+            if (internal_close(self) < 0) {
                 return -1;
+            }
         }
         else
             self->fd = -1;
@@ -523,10 +526,8 @@ _io_FileIO___init___impl(fileio *self, PyObject *nameobj, const char *mode,
         internal_close(self);
         _PyErr_ChainExceptions1(exc);
     }
-    if (self->stat_atopen != NULL) {
-        PyMem_Free(self->stat_atopen);
-        self->stat_atopen = NULL;
-    }
+    PyMem_Free(self->stat_atopen);
+    self->stat_atopen = NULL;
 
  done:
 #ifdef MS_WINDOWS