From 5b37b822d9bb14b5ff7853f552bc7ee0420eff57 Mon Sep 17 00:00:00 2001
From: Pavel Koneski <pavel.koneski@gmail.com>
Date: Wed, 21 Oct 2026 14:53:41 -0700
Subject: [PATCH] Implement umask on POSIX

---
 src/core/IronPython.Modules/nt.cs             |  4 ++++
 src/core/IronPython.Modules/posix.cs          |  6 ++++++
 tests/suite/modules/system_related/test_os.py | 18 ++++++++++++++++++
 3 files changed, 28 insertions(+)

diff --git a/src/core/IronPython.Modules/nt.cs b/src/core/IronPython.Modules/nt.cs
index 225fc2109..f5aa8aca5 100644
--- a/src/core/IronPython.Modules/nt.cs
+++ b/src/core/IronPython.Modules/nt.cs
@@ -1898,6 +1898,10 @@ public static object urandom(object? n)
         private static readonly object _umaskKey = new object();
 
         public static int umask(CodeContext/*!*/ context, int mask) {
+            if (RuntimeInformation.IsOSPlatform(OSPlatform.Linux) || RuntimeInformation.IsOSPlatform(OSPlatform.OSX)) {
+                return umaskUnix(mask & 0x1FF);
+            }
+
             mask &= 0x180;
             object oldMask = context.LanguageContext.GetSetModuleState(_umaskKey, mask);
             if (oldMask == null) {
diff --git a/src/core/IronPython.Modules/posix.cs b/src/core/IronPython.Modules/posix.cs
index fe802fcfa..c59b33d36 100644
--- a/src/core/IronPython.Modules/posix.cs
+++ b/src/core/IronPython.Modules/posix.cs
@@ -123,6 +123,12 @@ private static void chmodUnix(string path, int mode) {
         }
 
 
+        [SupportedOSPlatform("linux")]
+        [SupportedOSPlatform("macos")]
+        private static int umaskUnix(int mask)
+            => (int)NativeConvert.FromFilePermissions(Syscall.umask(NativeConvert.ToFilePermissions((uint)mask)));
+
+
         [SupportedOSPlatform("linux")]
         [SupportedOSPlatform("macos")]
         private static void linkUnix(string src, string dst) {
diff --git a/tests/suite/modules/system_related/test_os.py b/tests/suite/modules/system_related/test_os.py
index 1f77237d6..18e18b81a 100644
--- a/tests/suite/modules/system_related/test_os.py
+++ b/tests/suite/modules/system_related/test_os.py
@@ -3,6 +3,7 @@
 # See the LICENSE file in the project root for more information.
 
 import os
+import unittest
 
 from iptest import IronPythonTestCase, is_osx, is_linux, is_windows, run_test
 
@@ -55,4 +56,21 @@ def test_open_abplus(self):
         finally:
             os.close(fd)
 
+    @unittest.skipIf(is_windows, "umask is emulated on Windows")
+    def test_umask(self):
+        old = os.umask(0o022)
+        try:
+            self.assertEqual(os.umask(0o077), 0o022)
+            self.assertEqual(os.umask(0o022), 0o077)
+
+            os.umask(0o027)
+            try:
+                fd = os.open(self.temp_file, os.O_CREAT | os.O_WRONLY, 0o666)
+                os.close(fd)
+                self.assertEqual(os.stat(self.temp_file).st_mode & 0o777, 0o640)
+            finally:
+                self.delete_files(self.temp_file)
+        finally:
+            os.umask(old)
+
 run_test(__name__)