Document st_birthtime fallback behavior

Author: jbosboom

Doc/library/os.rst

@@ -3225,7 +3225,8 @@ features:
       .. versionchanged:: 3.12
          ``st_birthtime`` is now available on Windows.
       .. versionchanged:: next
-         ``st_birthtime`` is now available on Linux.
+         ``st_birthtime`` is now present on Linux.  The value will be ``0.0``
+         on kernel versions < 4.11 or if not supported by the filesystem.
 
    .. attribute:: st_birthtime_ns
 
@@ -3235,7 +3236,8 @@ features:
 
       .. versionadded:: 3.12
       .. versionchanged:: next
-         ``st_birthtime_ns`` is now available on Linux.
+         ``st_birthtime_ns`` is now present on Linux.  The value will be ``0``
+         on kernel versions < 4.11 or if not supported by the filesystem.
 
    .. note::
 

Modules/posixmodule.c

@@ -2816,7 +2816,10 @@ _pystat_fromstructstat(PyObject *module, STRUCT_STAT *st)
 #elif defined(HAVE_STATX)
     /* We were built with statx support, so stat_result.st_birthtime[_ns]
        exists, but we fell back to stat because statx isn't available at
-       runtime.  User programs assume st_birthtime is not None. */
+       runtime.  structseq members are _Py_T_OBJECT (for which NULL means None,
+       not AttributeError), but user programs assume st_birthtime is not None.
+       When the statx syscall wrapper is available but the syscall itself is
+       not, we end up setting the birthtime to 0, so do that here too. */
     SET_ITEM(ST_BIRTHTIME_IDX, PyFloat_FromDouble(0.0));
     SET_ITEM(ST_BIRTHTIME_NS_IDX, _PyLong_GetZero());
 #elif defined(MS_WINDOWS)