From 29a63a8b089ea2d89ce24fcf795f8a775c477e50 Mon Sep 17 00:00:00 2001
From: Helmut Grohne <helmut@subdivi.de>
Date: Thu, 4 Apr 2024 11:17:08 +0200
Subject: add method AtLocation.as_emptypath for cloning a location

---
 linuxnamespaces/atlocation.py | 17 +++++++++++++++++
 tests/test_atlocation.py      | 20 +++++++++++++++++++-
 2 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/linuxnamespaces/atlocation.py b/linuxnamespaces/atlocation.py
index 6aada55..8da5982 100644
--- a/linuxnamespaces/atlocation.py
+++ b/linuxnamespaces/atlocation.py
@@ -9,6 +9,7 @@ code for doing so.
 
 import enum
 import errno
+import fcntl
 import os
 import os.path
 import pathlib
@@ -102,6 +103,22 @@ class AtLocation:
             os.close(self.fd)
             self.fd = AT_FDCWD
 
+    def as_emptypath(self, inheritable: bool = True) -> "AtLocation":
+        """Return a new AtLocation with flag AT_EMPTY_PATH with a new file
+        descriptor. If self already is an empty path, its fd is duplicated. In
+        all cases, the caller is responsible for closing the result object.
+        """
+        if self.flags & AtFlags.AT_EMPTY_PATH:
+            newfd = fcntl.fcntl(
+                self.fd,
+                fcntl.F_DUPFD if inheritable else fcntl.F_DUPFD_CLOEXEC,
+                0,
+            )
+            return AtLocation(newfd, flags=self.flags)
+        return AtLocation(
+            self.open(flags=os.O_PATH | (0 if inheritable else os.O_CLOEXEC))
+        )
+
     def nosymfollow(self) -> "AtLocation":
         """Return a copy with the AT_SYMLINK_NOFOLLOW set."""
         return AtLocation(
diff --git a/tests/test_atlocation.py b/tests/test_atlocation.py
index 9836f3b..5d7286a 100644
--- a/tests/test_atlocation.py
+++ b/tests/test_atlocation.py
@@ -80,7 +80,7 @@ class AtLocationTest(unittest.TestCase):
     def create_all(
         self, skip: typing.Container[str] = ()
     ) -> typing.Iterator[tuple[str, typing.ContextManager[AtLocation]]]:
-        """Create various AtLocation objects referring to fils, directories
+        """Create various AtLocation objects referring to files, directories
         and other things in various ways.
         """
         for loctype in ("relative", "absolute", "emptypath", "withfd"):
@@ -146,6 +146,24 @@ class AtLocationTest(unittest.TestCase):
             self.assertEqual(atloc.access(os.R_OK), should_exist)
         self.assertEqual(atloc.exists(), should_exist)
 
+    def test_as_emptypath(self) -> None:
+        atloc = AtLocation(self.tempdir)
+        self.assertFalse(atloc.flags & linuxnamespaces.AtFlags.AT_EMPTY_PATH)
+        statres = atloc.stat()
+        atloc_ep = self.enterContext(atloc.as_emptypath())
+        self.assertTrue(atloc_ep.flags & linuxnamespaces.AtFlags.AT_EMPTY_PATH)
+        self.assertGreaterEqual(atloc_ep.fd, 0)
+        self.assertEqual(atloc_ep.location, "")
+        self.assertEqual(atloc_ep.stat().st_ino, statres.st_ino)
+        atloc_dup = self.enterContext(atloc_ep.as_emptypath())
+        self.assertTrue(
+            atloc_dup.flags & linuxnamespaces.AtFlags.AT_EMPTY_PATH
+        )
+        self.assertGreaterEqual(atloc_ep.fd, 0)
+        self.assertNotEqual(atloc_dup.fd, atloc_ep.fd)
+        self.assertEqual(atloc_dup.location, "")
+        self.assertEqual(atloc_dup.stat().st_ino, statres.st_ino)
+
     @atloc_subtest(skip=("absent", "file", "symlink"))
     def test_join_mkdir(self, _: str, atloc: AtLocation) -> None:
         subdir = atloc / "subdir"
-- 
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