From 092435e932dee1802784ec28f39454f50fdd879a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Toke=20H=C3=B8iland-J=C3=B8rgensen?= <toke@redhat.com>
Date: Mon, 16 Dec 2019 13:23:55 +0100
Subject: [PATCH] bpo-38811: Check for presence of os.link method in pathlib
 (GH-17225)
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Commit 6b5b013bcc22 ("bpo-26978: Implement pathlib.Path.link_to (Using
os.link) (GH-12990)") introduced a new link_to method in pathlib. However,
this makes pathlib crash when the 'os' module is missing a 'link' method.

Fix this by checking for the presence of the 'link' method on pathlib
module import, and if it's not present, turn it into a runtime error like
those emitted when there is no lchmod() or symlink().

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
 Lib/pathlib.py                                         |  7 ++++++-
 Lib/test/test_pathlib.py                               | 10 ++++++++++
 .../Library/2019-11-15-18-06-04.bpo-38811.AmdQ6M.rst   |  1 +
 3 files changed, 17 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2019-11-15-18-06-04.bpo-38811.AmdQ6M.rst

diff --git a/Lib/pathlib.py b/Lib/pathlib.py
index f0537cfea19..7d1d1150b0d 100644
--- a/Lib/pathlib.py
+++ b/Lib/pathlib.py
@@ -418,7 +418,12 @@ class _NormalAccessor(_Accessor):
 
     unlink = os.unlink
 
-    link_to = os.link
+    if hasattr(os, "link"):
+        link_to = os.link
+    else:
+        @staticmethod
+        def link_to(self, target):
+            raise NotImplementedError("os.link() not available on this system")
 
     rmdir = os.rmdir
 
diff --git a/Lib/test/test_pathlib.py b/Lib/test/test_pathlib.py
index b8e7fcc2e30..3232649f554 100644
--- a/Lib/test/test_pathlib.py
+++ b/Lib/test/test_pathlib.py
@@ -1759,6 +1759,7 @@ class _BasePathTest(object):
         self.assertFileNotFound(p.stat)
         self.assertFileNotFound(p.unlink)
 
+    @unittest.skipUnless(hasattr(os, "link"), "os.link() is not present")
     def test_link_to(self):
         P = self.cls(BASE)
         p = P / 'fileA'
@@ -1778,6 +1779,15 @@ class _BasePathTest(object):
         self.assertEqual(os.stat(r).st_size, size)
         self.assertTrue(q.stat)
 
+    @unittest.skipIf(hasattr(os, "link"), "os.link() is present")
+    def test_link_to_not_implemented(self):
+        P = self.cls(BASE)
+        p = P / 'fileA'
+        # linking to another path.
+        q = P / 'dirA' / 'fileAA'
+        with self.assertRaises(NotImplementedError):
+            p.link_to(q)
+
     def test_rename(self):
         P = self.cls(BASE)
         p = P / 'fileA'
diff --git a/Misc/NEWS.d/next/Library/2019-11-15-18-06-04.bpo-38811.AmdQ6M.rst b/Misc/NEWS.d/next/Library/2019-11-15-18-06-04.bpo-38811.AmdQ6M.rst
new file mode 100644
index 00000000000..0e4a7f5bdf5
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2019-11-15-18-06-04.bpo-38811.AmdQ6M.rst
@@ -0,0 +1 @@
+Fix an unhandled exception in :mod:`pathlib` when :meth:`os.link` is missing. Patch by Toke Høiland-Jørgensen.