Implement a consistent realpath function in certbot.compat.filesystem (#7242)

Fixes #7115 

This PR creates a `realpath` method in `filesystem`, whose goal is to replace any call to `os.path.realpath` in Certbot. The reason is that `os.path.realpath` is broken on some versions of Python for Windows. See https://bugs.python.org/issue9949. The function created here works consistently across Linux and Windows.

As for the other forbidden functions in `os` module, our `certbot.compat.os` will raise an exception if its `path.realpath` function is invoked, and using the `os` module from Python is forbidden from the pylint check implemented in our CI.

Every call to `os.path.realpath` is corrected in `certbot` and `certbot-apache` modules.

* Forbid os.path.realpath

* Finish implementation

* Use filesystem.realpath

* Control symlink loops also for Linux

* Add a test for forbidden method

* Import a new object from os.path module

* Use same approach of wrapping than certbot.compat.os

* Correct errors

* Fix dependencies

* Make path module internal

certbot/tests/compat/filesystem_test.py

@@ -48,18 +48,6 @@ class WindowsChmodTests(TempDirTestCase):
         self.assertFalse(filesystem._compare_dacls(ref_dacl_probe, cur_dacl_probe))  # pylint: disable=protected-access
         self.assertTrue(filesystem._compare_dacls(ref_dacl_link, cur_dacl_link))  # pylint: disable=protected-access
 
-    def test_symlink_loop_mitigation(self):
-        link1_path = os.path.join(self.tempdir, 'link1')
-        link2_path = os.path.join(self.tempdir, 'link2')
-        link3_path = os.path.join(self.tempdir, 'link3')
-        os.symlink(link1_path, link2_path)
-        os.symlink(link2_path, link3_path)
-        os.symlink(link3_path, link1_path)
-
-        with self.assertRaises(RuntimeError) as error:
-            filesystem.chmod(link1_path, 0o755)
-        self.assertTrue('link1 is a loop!' in str(error.exception))
-
     def test_world_permission(self):
         everybody = win32security.ConvertStringSidToSid(EVERYBODY_SID)
 
@@ -320,7 +308,6 @@ class CopyOwnershipTest(test_util.TempDirTestCase):
 
 class OsReplaceTest(test_util.TempDirTestCase):
     """Test to ensure consistent behavior of rename method"""
-
     def test_os_replace_to_existing_file(self):
         """Ensure that replace will effectively rename src into dst for all platforms."""
         src = os.path.join(self.tempdir, 'src')
@@ -335,6 +322,46 @@ class OsReplaceTest(test_util.TempDirTestCase):
         self.assertTrue(os.path.exists(dst))
 
 
+class RealpathTest(test_util.TempDirTestCase):
+    """Tests for realpath method"""
+    def setUp(self):
+        super(RealpathTest, self).setUp()
+        self.probe_path = _create_probe(self.tempdir)
+
+    def test_symlink_resolution(self):
+        # Absolute resolution
+        link_path = os.path.join(self.tempdir, 'link_abs')
+        os.symlink(self.probe_path, link_path)
+
+        self.assertEqual(self.probe_path, filesystem.realpath(self.probe_path))
+        self.assertEqual(self.probe_path, filesystem.realpath(link_path))
+
+        # Relative resolution
+        curdir = os.getcwd()
+        link_path = os.path.join(self.tempdir, 'link_rel')
+        probe_name = os.path.basename(self.probe_path)
+        try:
+            os.chdir(os.path.dirname(self.probe_path))
+            os.symlink(probe_name, link_path)
+
+            self.assertEqual(self.probe_path, filesystem.realpath(probe_name))
+            self.assertEqual(self.probe_path, filesystem.realpath(link_path))
+        finally:
+            os.chdir(curdir)
+
+    def test_symlink_loop_mitigation(self):
+        link1_path = os.path.join(self.tempdir, 'link1')
+        link2_path = os.path.join(self.tempdir, 'link2')
+        link3_path = os.path.join(self.tempdir, 'link3')
+        os.symlink(link1_path, link2_path)
+        os.symlink(link2_path, link3_path)
+        os.symlink(link3_path, link1_path)
+
+        with self.assertRaises(RuntimeError) as error:
+            filesystem.realpath(link1_path)
+        self.assertTrue('link1 is a loop!' in str(error.exception))
+
+
 def _get_security_dacl(target):
     return win32security.GetFileSecurity(target, win32security.DACL_SECURITY_INFORMATION)
 

certbot/tests/compat/os_test.py

@@ -7,8 +7,12 @@ from certbot.compat import os
 class OsTest(unittest.TestCase):
     """Unit tests for os module."""
     def test_forbidden_methods(self):
+        # Checks for os module
         for method in ['chmod', 'chown', 'open', 'mkdir', 'makedirs', 'rename', 'replace']:
             self.assertRaises(RuntimeError, getattr(os, method))
+        # Checks for os.path module
+        for method in ['realpath']:
+            self.assertRaises(RuntimeError, getattr(os.path, method))
 
 
 if __name__ == "__main__":