Add new root option in Path.which method

This commit adds to Path utility a couple of methods to manipulate
paths. One to rebase given paths to a new root and another one to
trim the given root path. In addition a new option in Path.which
is added to allow searches into chroot evironments.

Fixes #1276

kiwi/bootloader/config/grub2.py

@@ -810,7 +810,7 @@ class BootLoaderConfigGrub2(BootLoaderConfigBase):
 
     def _get_grub2_mkconfig_tool(self):
         for grub_mkconfig_tool in ['grub2-mkconfig', 'grub-mkconfig']:
-            if Path.which(grub_mkconfig_tool):
+            if Path.which(grub_mkconfig_tool, root_dir=self.root_dir):
                 return grub_mkconfig_tool
 
     def _get_grub2_boot_path(self):

kiwi/path.py

@@ -159,10 +159,49 @@ class Path:
                     ['rmdir', '--ignore-fail-on-non-empty', sub_path]
                 )
 
+    @staticmethod
+    def move_to_root(root, elements):
+        """
+        Change the given path elements to a new root directory
+
+        :param str root: the root path to trim
+        :param list elements: list of path names
+
+        :return: changed elements
+
+        :rtype: list
+        """
+        result = []
+        for element in elements:
+            normalized_element = os.path.normpath(element)
+            result.append(
+                normalized_element.replace(
+                    os.path.normpath(root), os.sep
+                ).replace('{0}{0}'.format(os.sep), os.sep)
+            )
+        return result
+
+    @staticmethod
+    def rebase_to_root(root, elements):
+        """
+        Include the root prefix for the given paths elements
+
+        :param str root: the new root path
+        :param list elements: list of path names
+
+        :return: changed elements
+
+        :rtype: list
+        """
+        result = []
+        for element in elements:
+            result.append(os.path.normpath(os.sep.join([root, element])))
+        return result
+
     @staticmethod
     def which(
         filename, alternative_lookup_paths=None,
-        custom_env=None, access_mode=None
+        custom_env=None, access_mode=None, root_dir=None
     ):
         """
         Lookup file name in PATH
@@ -173,6 +212,7 @@ class Path:
         :param int access_mode: one of the os access modes or a combination of
          them (os.R_OK, os.W_OK and os.X_OK). If the provided access mode
          does not match the file is considered not existing
+        :param str root_dir: the root path to look at
 
         :return: absolute path to file or None
 
@@ -189,6 +229,8 @@ class Path:
             lookup_paths = system_path.split(os.pathsep)
         if alternative_lookup_paths:
             lookup_paths += alternative_lookup_paths
+        if root_dir:
+            lookup_paths = Path.rebase_to_root(root_dir, lookup_paths)
         multipart_message[0] += 'in paths "%s"' % ':'.join(lookup_paths)
         for path in lookup_paths:
             location = os.path.join(path, filename)

test/unit/path_test.py

@@ -63,6 +63,16 @@ class TestPath:
             assert 'remove_hierarchy: path /my_root/tmp is protected' in \
                 self._caplog.text
 
+    def test_move_to_root(self):
+        assert [
+            '/usr/bin', '/sbin'
+        ] == Path.move_to_root('/root_dir', ['/root_dir/usr/bin', '/sbin'])
+
+    def test_rebase_to_root(self):
+        assert [
+            '/root_dir/usr/bin', '/root_dir/sbin'
+        ] == Path.rebase_to_root('/root_dir', ['usr/bin', '/sbin'])
+
     @patch('os.access')
     @patch('os.environ.get')
     @patch('os.path.exists')
@@ -84,6 +94,11 @@ class TestPath:
             '/usr/local/bin/some-file'
         assert Path.which('some-file', custom_env={'PATH': 'custom_path'}) == \
             'custom_path/some-file'
+        assert Path.which(
+            'some-file',
+            custom_env={'PATH': 'custom_path'},
+            root_dir='/root_dir'
+        ) == '/root_dir/custom_path/some-file'
 
     @patch('os.access')
     @patch('os.environ.get')