Fix/8418 permission denied hg

docs/html/cli/pip_freeze.rst

@@ -72,3 +72,21 @@ Examples
 
          env1\bin\python -m pip freeze > requirements.txt
          env2\bin\python -m pip install -r requirements.txt
+
+
+Fixing "Permission denied:" errors
+==================================
+
+The purpose of this section of documentation is to provide practical
+suggestions to users seeing a `"Permission denied" error <https://github.com/pypa/pip/issues/8418>`__ on ``pip freeze``.
+
+This error occurs, for instance, when the command is installed only for another
+user, and the current user doesn't have the permission to execute the other
+user's command.
+
+To solve that issue, you can try one of the followings:
+
+- Install the command for yourself (e.g. in your home directory).
+- Ask the system admin to allow this command for all users.
+- Check and correct the PATH variable of your own environment.
+- Check the `ACL (Access-Control List) <https://en.wikipedia.org/wiki/Access-control_list>`_ for this command.

news/8418.bugfix.rst

@@ -0,0 +1 @@
+Fix ``pip freeze`` permission denied error in order to display an understandable error message and offer solutions.

news/8418.doc.rst

@@ -0,0 +1 @@
+Add a section in the documentation to suggest solutions to the ``pip freeze`` permission denied issue.

src/pip/_internal/vcs/versioncontrol.py

@@ -684,6 +684,18 @@ def run_command(
             raise BadCommand(
                 f'Cannot find command {cls.name!r} - do you have '
                 f'{cls.name!r} installed and in your PATH?')
+        except PermissionError:
+            # errno.EACCES = Permission denied
+            # This error occurs, for instance, when the command is installed
+            # only for another user. So, the current user don't have
+            # permission to call the other user command.
+            raise BadCommand(
+                f"No permission to execute {cls.name!r} - install it "
+                f"locally, globally (ask admin), or check your PATH. "
+                f"See possible solutions at "
+                f"https://pip.pypa.io/en/latest/reference/pip_freeze/"
+                f"#fixing-permission-denied."
+            )
 
     @classmethod
     def is_repository_directory(cls, path):

tests/unit/test_vcs.py

@@ -303,6 +303,27 @@ def test_version_control__get_url_rev_and_auth__no_revision(url):
     assert 'an empty revision (after @)' in str(excinfo.value)
 
 
+@pytest.mark.parametrize("vcs_cls", [Bazaar, Git, Mercurial, Subversion])
+@pytest.mark.parametrize(
+    "exc_cls, msg_re",
+    [
+        (FileNotFoundError, r"Cannot find command '{name}'"),
+        (PermissionError, r"No permission to execute '{name}'"),
+    ],
+    ids=["FileNotFoundError", "PermissionError"],
+)
+def test_version_control__run_command__fails(vcs_cls, exc_cls, msg_re):
+    """
+    Test that ``VersionControl.run_command()`` raises ``BadCommand``
+    when the command is not found or when the user have no permission
+    to execute it. The error message must contains the command name.
+    """
+    with patch("pip._internal.vcs.versioncontrol.call_subprocess") as call:
+        call.side_effect = exc_cls
+        with pytest.raises(BadCommand, match=msg_re.format(name=vcs_cls.name)):
+            vcs_cls.run_command([])
+
+
 @pytest.mark.parametrize('url, expected', [
     # Test http.
     ('bzr+http://bzr.myproject.org/MyProject/trunk/#egg=MyProject',