bpo-41839: Fix error checking in sched_get_priority_ functions

Misc/NEWS.d/next/Library/2020-09-23-11-54-17.bpo-41839.kU5Ywl.rst

@@ -0,0 +1,2 @@
+Allow negative priority values from :func:`os.sched_get_priority_min` and
+:func:`os.sched_get_priority_max` functions.

Modules/posixmodule.c

@@ -8202,8 +8202,10 @@ os_sched_get_priority_max_impl(PyObject *module, int policy)
 {
     int max;
 
+    /* make sure that errno is cleared before the call */
+    errno = 0;
     max = sched_get_priority_max(policy);
-    if (max < 0)
+    if (max == -1 && errno)
         return posix_error();
     return PyLong_FromLong(max);
 }
@@ -8221,8 +8223,12 @@ static PyObject *
 os_sched_get_priority_min_impl(PyObject *module, int policy)
 /*[clinic end generated code: output=7595c1138cc47a6d input=21bc8fa0d70983bf]*/
 {
-    int min = sched_get_priority_min(policy);
-    if (min < 0)
+    int min;
+
+    /* make sure that errno is cleared before the call */
+    errno = 0;
+    min = sched_get_priority_min(policy);
+    if (min == -1 && errno)
         return posix_error();
     return PyLong_FromLong(min);
 }