Fixed #19508 -- Implemented uri_to_iri as per RFC.

Thanks Loic Bistuer for helping in shaping the patch and Claude Paroz
for the review.

django/utils/encoding.py

@@ -1,3 +1,4 @@
+# -*- encoding: utf-8 -*-
 from __future__ import unicode_literals
 
 import codecs
@@ -7,7 +8,9 @@ import locale
 
 from django.utils.functional import Promise
 from django.utils import six
-from django.utils.six.moves.urllib.parse import quote
+from django.utils.six.moves.urllib.parse import quote, unquote
+if six.PY3:
+    from urllib.parse import unquote_to_bytes
 
 
 class DjangoUnicodeDecodeError(UnicodeDecodeError):
@@ -185,7 +188,9 @@ def iri_to_uri(iri):
     assuming input is either UTF-8 or unicode already, we can simplify things a
     little from the full method.
 
-    Returns an ASCII string containing the encoded result.
+    Takes an IRI in UTF-8 bytes (e.g. '/I \xe2\x99\xa5 Django/') or unicode
+    (e.g. '/I ♥ Django/') and returns ASCII bytes containing the encoded result
+    (e.g. '/I%20%E2%99%A5%20Django/').
     """
     # The list of safe characters here is constructed from the "reserved" and
     # "unreserved" characters specified in sections 2.2 and 2.3 of RFC 3986:
@@ -204,6 +209,38 @@ def iri_to_uri(iri):
     return quote(force_bytes(iri), safe=b"/#%[]=:;$&()+,!?*@'~")
 
 
+def uri_to_iri(uri):
+    """
+    Converts a Uniform Resource Identifier(URI) into an Internationalized
+    Resource Identifier(IRI).
+
+    This is the algorithm from section 3.2 of RFC 3987.
+
+    Takes an URI in ASCII bytes (e.g. '/I%20%E2%99%A5%20Django/') and returns
+    unicode containing the encoded result (e.g. '/I \xe2\x99\xa5 Django/').
+    """
+    if uri is None:
+        return uri
+    uri = force_bytes(uri)
+    iri = unquote_to_bytes(uri) if six.PY3 else unquote(uri)
+    return repercent_broken_unicode(iri).decode('utf-8')
+
+
+def repercent_broken_unicode(path):
+    """
+    As per section 3.2 of RFC 3987, step three of converting a URI into an IRI,
+    we need to re-percent-encode any octet produced that is not part of a
+    strictly legal UTF-8 octet sequence.
+    """
+    try:
+        path.decode('utf-8')
+    except UnicodeDecodeError as e:
+        repercent = quote(path[e.start:e.end], safe=b"/#%[]=:;$&()+,!?*@'~")
+        path = repercent_broken_unicode(
+            path[:e.start] + force_bytes(repercent) + path[e.end:])
+    return path
+
+
 def filepath_to_uri(path):
     """Convert a file system path to a URI portion that is suitable for
     inclusion in a URL.