Fix #1112: download role creates duplicated copies

`:download:` role creates duplicated copies when the document contains
two or more the role for the same file, but in different form.  It
considers two paths are different when one contains relative path like
`path/to/../file.dat`.

Internally, `env.relfn2path()` does not normalize the given path in
relative form.  As a result, download role can't detect the same paths
are given.  This adds `os.path.normpath()` to `env.relfn2path()` to
normalize the path.

CHANGES

@@ -61,6 +61,8 @@ Bugs fixed
 * C, C++: in general fix intersphinx and role lookup types.
 * #8683: :confval:`html_last_updated_fmt` does not support UTC offset (%z)
 * #8683: :confval:`html_last_updated_fmt` generates wrong time zone for %Z
+* #1112: ``download`` role creates duplicated copies when relative path is
+  specified
 
 Testing
 --------

sphinx/environment/__init__.py

@@ -10,6 +10,7 @@
 
 import os
 import pickle
+import posixpath
 import warnings
 from collections import defaultdict
 from copy import copy
@@ -356,9 +357,9 @@ def relfn2path(self, filename: str, docname: str = None) -> Tuple[str, str]:
             docdir = path.dirname(self.doc2path(docname or self.docname,
                                                 base=None))
             rel_fn = path.join(docdir, filename)
-        # the path.abspath() might seem redundant, but otherwise artifacts
-        # such as ".." will remain in the path
-        return rel_fn, path.abspath(path.join(self.srcdir, rel_fn))
+
+        return (posixpath.normpath(rel_fn),
+                path.normpath(path.join(self.srcdir, rel_fn)))
 
     @property
     def found_docs(self) -> Set[str]:

tests/test_environment.py

@@ -138,6 +138,11 @@ def test_env_relfn2path(app):
     assert relfn == '../logo.jpg'
     assert absfn == app.srcdir.parent / 'logo.jpg'
 
+    # relative path traversal
+    relfn, absfn = app.env.relfn2path('subdir/../logo.jpg', 'index')
+    assert relfn == 'logo.jpg'
+    assert absfn == app.srcdir / 'logo.jpg'
+
     # omit docname (w/ current docname)
     app.env.temp_data['docname'] = 'subdir/document'
     relfn, absfn = app.env.relfn2path('images/logo.jpg')