Honor suffix byte ranges larger than the response body

CHANGES.txt

@@ -134,6 +134,10 @@ Experimental Features
 Bugfix
 ~~~~~~
 
+- Suffix byte ranges larger than a non-empty selected representation now
+  return the entire representation, as required by RFC 9110 section 14.1.2,
+  instead of being treated as unsatisfiable and producing a 416 response.
+
 - ``webob.cookies.serialize_cookie_date()`` now converts a timezone-aware
   ``datetime`` to UTC before serializing it, instead of writing its local
   wall-clock time out as GMT. Naive datetimes continue to be treated as UTC.

src/webob/byterange.py

@@ -28,7 +28,7 @@ def range_for_length(self, length):
         if end is None:
             end = length
             if start < 0:
-                start += length
+                start = max(0, start + length)
         if _is_content_range_valid(start, end, length):
             stop = min(end, length)
             return (start, stop)

tests/test_byterange.py

@@ -8,12 +8,17 @@
 
 
 def test_not_satisfiable():
-    range = Range.parse("bytes=-100")
-    assert range.range_for_length(50) is None
     range = Range.parse("bytes=100-")
     assert range.range_for_length(50) is None
 
 
+@pytest.mark.parametrize("suffix", [50, 51, 100])
+def test_suffix_range_covering_entire_representation(suffix):
+    range = Range.parse(f"bytes=-{suffix}")
+    assert range.range_for_length(50) == (0, 50)
+    assert tuple(range.content_range(50)) == (0, 50, 50)
+
+
 def test_range_parse():
     assert isinstance(Range.parse("bytes=0-99"), Range)
     assert isinstance(Range.parse("BYTES=0-99"), Range)

tests/test_response.py

@@ -360,6 +360,17 @@ def test_conditional_response_range_not_satisfiable_response():
     assert resp.body == b"Requested range not satisfiable: bytes=100-200"
 
 
+def test_conditional_response_suffix_range_larger_than_body():
+    req = Request.blank("/", range="bytes=-100")
+    resp = req.get_response(Response(b"foo\n", conditional_response=True))
+    assert resp.status_code == 206
+    assert resp.content_range.start == 0
+    assert resp.content_range.stop == 4
+    assert resp.content_range.length == 4
+    assert resp.content_length == 4
+    assert resp.body == b"foo\n"
+
+
 def test_HEAD_conditional_response_range_not_satisfiable_response():
     req = Request.blank("/", method="HEAD", range="bytes=100-200")
     resp = Response(app_iter=["foo\n"], content_length=4, conditional_response=True)