Test IncompleteRead is raised and add comments

src/urllib3/response.py

@@ -1027,10 +1027,15 @@ def stream(
                 if data:
                     yield data
                 else:
+                    # http.client.HTTPResponse will often close this for us and
+                    # _raw_read handles a few more cases, but not all of them.
+                    # e.g., when using read1 with a HEAD request this won't
+                    # be closed automatically
                     if self._fp:
                         self._fp.close()
                     break
-        if self.length_remaining:
+        # _raw_read does not raise when amt is None, do this now
+        if self.length_remaining and self.enforce_content_length:
             raise IncompleteRead(self._fp_bytes_read, self.length_remaining)
 
     # Overrides from io.IOBase

test/test_response.py

@@ -1047,6 +1047,18 @@ def close(self) -> None:
         assert list(resp.stream(None)) == chunks
         assert fp.closed
 
+    def test_incomplete_stream_via_read1(self) -> None:
+        headers = {
+            "content-length": str(2),
+        }
+        fp = BytesIO(b"a")
+        resp = HTTPResponse(fp, headers=headers, preload_content=False)
+        chunks: list[bytes] = []
+        with pytest.raises(IncompleteRead):
+            chunks.extend(resp.stream(None))
+        assert chunks == [b"a"]
+        assert fp.closed
+
     def test_mock_transfer_encoding_chunked(self) -> None:
         stream = [b"fo", b"o", b"bar"]
         fp = MockChunkedEncodingResponse(stream)