diff -r 328781ae35d2 Modules/posixmodule.c
--- a/Modules/posixmodule.c	Fri Jul 05 01:41:30 2013 +0200
+++ b/Modules/posixmodule.c	Sat Jul 06 15:00:06 2013 +0200
@@ -4054,6 +4054,38 @@
     gid_t* alt_grouplist = grouplist;
     int n;
 
+    /* On MacOSX starting from 10.8 getgroups(2) doesn't return EINVAL,
+     * when there are more than MAX_GROUPS groups. Instead it returns
+     * an array with MAX_GROUPS entries.
+     *
+     * To work it around getgroups(2) can be called with 0 as the first
+     * argument so that it returns the actual number of groups and then
+     * subsequent call could be done to fetch the group ids.
+     */
+#ifdef __APPLE__
+    n = getgroups(0, NULL);
+    if (n < 0) {
+        return posix_error();
+    } else if (n <= MAX_GROUPS) {
+        /* Avoid malloc(0) */
+        alt_grouplist = grouplist;
+    } else {
+        alt_grouplist = PyMem_Malloc(n * sizeof(gid_t));
+        if (alt_grouplist == NULL) {
+            errno = EINVAL;
+            return posix_error();
+        }
+    }
+
+    n = getgroups(n, alt_grouplist);
+    if (n == -1) {
+        if (alt_grouplist != grouplist) {
+            PyMem_Free(alt_grouplist);
+        }
+
+        return posix_error();
+    }
+#else
     n = getgroups(MAX_GROUPS, grouplist);
     if (n < 0) {
         if (errno == EINVAL) {
@@ -4080,6 +4112,8 @@
             return posix_error();
         }
     }
+#endif
+
     result = PyList_New(n);
     if (result != NULL) {
         int i;