Fixed #26402 -- Added relative path support in include/extends template tags.

django/template/loader_tags.py

@@ -1,4 +1,5 @@
 import logging
+import posixpath
 from collections import defaultdict
 
 from django.utils import six
@@ -249,6 +250,36 @@ def do_block(parser, token):
     return BlockNode(block_name, nodelist)
 
 
+def construct_relative_path(current_template_name, relative_name):
+    """
+    Convert a relative path (starting with './' or '../') to the full template
+    name based on the current_template_name.
+    """
+    if not any(relative_name.startswith(x) for x in ["'./", "'../", '"./', '"../']):
+        # relative_name is a variable or a literal that doesn't contain a
+        # relative path.
+        return relative_name
+
+    new_name = posixpath.normpath(
+        posixpath.join(
+            posixpath.dirname(current_template_name.lstrip('/')),
+            relative_name.strip('\'"')
+        )
+    )
+    if new_name.startswith('../'):
+        raise TemplateSyntaxError(
+            "The relative path '%s' points outside the file hierarchy that "
+            "template '%s' is in." % (relative_name, current_template_name)
+        )
+    if current_template_name.lstrip('/') == new_name:
+        raise TemplateSyntaxError(
+            "The relative path '%s' was translated to template name '%s', the "
+            "same template in which the tag appears."
+            % (relative_name, current_template_name)
+        )
+    return '"%s"' % new_name
+
+
 @register.tag('extends')
 def do_extends(parser, token):
     """
@@ -263,6 +294,7 @@ def do_extends(parser, token):
     bits = token.split_contents()
     if len(bits) != 2:
         raise TemplateSyntaxError("'%s' takes one argument" % bits[0])
+    bits[1] = construct_relative_path(parser.origin.template_name, bits[1])
     parent_name = parser.compile_filter(bits[1])
     nodelist = parser.parse()
     if nodelist.get_nodes_by_type(ExtendsNode):
@@ -313,5 +345,6 @@ def do_include(parser, token):
         options[option] = value
     isolated_context = options.get('only', False)
     namemap = options.get('with', {})
+    bits[1] = construct_relative_path(parser.origin.template_name, bits[1])
     return IncludeNode(parser.compile_filter(bits[1]), extra_context=namemap,
                        isolated_context=isolated_context)