Eagerly normalize the cache directory

Fixes pypa#7541

news/7541.bugfix

@@ -0,0 +1 @@
+Correctly handle relative cache directory provided via --cache-dir.

src/pip/_internal/cli/base_command.py

@@ -33,7 +33,7 @@
 from pip._internal.utils.deprecation import deprecated
 from pip._internal.utils.filesystem import check_path_owner
 from pip._internal.utils.logging import BrokenStdoutLoggingError, setup_logging
-from pip._internal.utils.misc import get_prog
+from pip._internal.utils.misc import get_prog, normalize_path
 from pip._internal.utils.temp_dir import global_tempdir_manager
 from pip._internal.utils.typing import MYPY_CHECK_RUNNING
 from pip._internal.utils.virtualenv import running_under_virtualenv
@@ -170,6 +170,7 @@ def _main(self, args):
                 sys.exit(VIRTUALENV_NOT_FOUND)
 
         if options.cache_dir:
+            options.cache_dir = normalize_path(options.cache_dir)
             if not check_path_owner(options.cache_dir):
                 logger.warning(
                     "The directory '%s' or its parent directory is not owned "

src/pip/_internal/utils/filesystem.py

@@ -33,6 +33,8 @@ def check_path_owner(path):
     if sys.platform == "win32" or not hasattr(os, "geteuid"):
         return True
 
+    assert os.path.isabs(path)
+
     previous = None
     while path != previous:
         if os.path.lexists(path):