gh-108948: tarfile should handle sticky bit in FreeBSD

Doc/library/tarfile.rst

@@ -942,9 +942,9 @@ reused in custom filters:
    .. note::
 
       Although the filter preserves all permission bits unchanged, the sticky
-      bit (:const:`~stat.S_ISVTX`) will not be set on extracted files when the
-      system does not allow it. For compatibility reasons, no error is raised
-      in this case.
+      bit (:const:`~stat.S_ISVTX`) will not be set on an extracted file if the
+      file system does not allow it. For compatibility reasons, no exception is
+      raised in this case.
 
 .. function:: tar_filter(member, path)
 

Lib/test/support/os_helper.py

@@ -275,6 +275,39 @@ def skip_unless_working_chmod(test):
     return test if ok else unittest.skip(msg)(test)
 
 
+_can_chmod_set_sticky_bit = None
+
+
+def can_chmod_set_sticky_bit():
+    """Return True if chmod allows to set the sticky bit on a regular file.
+
+    Return False if chmod fails with an error, or it succeeds but does not set
+    the sticky bit.
+    """
+
+    def can_chmod_set_sticky_bit_inner():
+        if not can_chmod():
+            return False
+        filename = TESTFN
+        try:
+            with open(filename, "w") as f:
+                # assume it won't fail because can_chmod() returned True:
+                os.chmod(f.fileno(), 0o666)
+                try:
+                    os.chmod(f.fileno(), 0o666 | stat.S_ISVTX)
+                except OSError:
+                    return False
+                mode = os.stat(f.fileno()).st_mode
+                return bool(mode & stat.S_ISVTX)
+        finally:
+            unlink(filename)
+
+    global _can_chmod_set_sticky_bit
+    if _can_chmod_set_sticky_bit is None:
+        _can_chmod_set_sticky_bit = can_chmod_set_sticky_bit_inner()
+    return _can_chmod_set_sticky_bit
+
+
 # Check whether the current effective user has the capability to override
 # DAC (discretionary access control). Typically user root is able to
 # bypass file read, write, and execute permission checks. The capability

Lib/test/test_tarfile.py

@@ -3057,34 +3057,6 @@ def test_keyword_only(self, mock_geteuid):
                               tarfl.extract, filename_1, TEMPDIR, False, True)
 
 
-_can_chmod_set_sticky = None
-
-
-def can_chmod_set_sticky():
-    """Can chmod set the sticky bit on a file?"""
-
-    def can_chmod_set_sticky_inner():
-        if not os_helper.can_chmod():
-            return False
-        filename = os_helper.TESTFN + "-chmod-sticky"
-        try:
-            with open(filename, "w") as f:
-                os.chmod(f.fileno(), 0o666)
-                try:
-                    os.chmod(f.fileno(), 0o666 | stat.S_ISVTX)
-                except OSError:
-                    return False
-                mode = os.stat(f.fileno()).st_mode
-                return bool(mode & stat.S_ISVTX)
-        finally:
-            os.unlink(filename)
-
-    global _can_chmod_set_sticky
-    if _can_chmod_set_sticky is None:
-        _can_chmod_set_sticky = can_chmod_set_sticky_inner()
-    return _can_chmod_set_sticky
-
-
 @os_helper.skip_unless_working_chmod
 class ExtractStickyFileTest(unittest.TestCase):
 
@@ -3108,7 +3080,10 @@ def extract_sticky_file(self, file_name="sticky"):
     def test_extract_chmod_eftype_success(self):
         # Extracting a file as non-root should skip the sticky bit (gh-108948)
         # even on platforms where chmod fails with EFTYPE (i.e. FreeBSD)
-        expected_mode = self.sticky_mode if can_chmod_set_sticky() else self.non_sticky_mode
+        if os_helper.can_chmod_set_sticky_bit():
+            expected_mode = self.sticky_mode
+        else:
+            expected_mode = self.non_sticky_mode
         self.extract_sticky_file()
         got_mode = stat.filemode(os.stat(self.extracted_path).st_mode)
         self.assertEqual(got_mode, expected_mode)
@@ -3906,7 +3881,7 @@ def test_modes(self):
         with open(tmp_filename, 'w'):
             pass
         new_mode = os.stat(tmp_filename).st_mode | stat.S_ISGID | stat.S_ISUID
-        if can_chmod_set_sticky():
+        if os_helper.can_chmod_set_sticky_bit():
             new_mode |= stat.S_ISVTX
         os.chmod(tmp_filename, new_mode)
         got_mode = os.stat(tmp_filename).st_mode