gh-95023: Added os.setns and os.unshare functions (#95046)

Added os.setns and os.unshare to easily switch between namespaces
on Linux.

Co-authored-by: Christian Heimes <christian@python.org>
Co-authored-by: CAM Gerlach <CAM.Gerlach@Gerlach.CAM>
Co-authored-by: Victor Stinner <vstinner@python.org>

Lib/test/test_posix.py

@@ -2191,6 +2191,53 @@ class TestPosixWeaklinking(unittest.TestCase):
                 os.utime("path", dir_fd=0)
 
 
+class NamespacesTests(unittest.TestCase):
+    """Tests for os.unshare() and os.setns()."""
+
+    @unittest.skipUnless(hasattr(os, 'unshare'), 'needs os.unshare()')
+    @unittest.skipUnless(hasattr(os, 'setns'), 'needs os.setns()')
+    @unittest.skipUnless(os.path.exists('/proc/self/ns/uts'), 'need /proc/self/ns/uts')
+    @support.requires_linux_version(3, 0, 0)
+    def test_unshare_setns(self):
+        code = """if 1:
+            import errno
+            import os
+            import sys
+            fd = os.open('/proc/self/ns/uts', os.O_RDONLY)
+            try:
+                original = os.readlink('/proc/self/ns/uts')
+                try:
+                    os.unshare(os.CLONE_NEWUTS)
+                except OSError as e:
+                    if e.errno == errno.ENOSPC:
+                        # skip test if limit is exceeded
+                        sys.exit()
+                    raise
+                new = os.readlink('/proc/self/ns/uts')
+                if original == new:
+                    raise Exception('os.unshare failed')
+                os.setns(fd, os.CLONE_NEWUTS)
+                restored = os.readlink('/proc/self/ns/uts')
+                if original != restored:
+                    raise Exception('os.setns failed')
+            except PermissionError:
+                # The calling process did not have the required privileges
+                # for this operation
+                pass
+            except OSError as e:
+                # Skip the test on these errors:
+                # - ENOSYS: syscall not available
+                # - EINVAL: kernel was not configured with the CONFIG_UTS_NS option
+                # - ENOMEM: not enough memory
+                if e.errno not in (errno.ENOSYS, errno.EINVAL, errno.ENOMEM):
+                    raise
+            finally:
+                os.close(fd)
+            """
+
+        assert_python_ok("-c", code)
+
+
 def tearDownModule():
     support.reap_children()