fix twisted multi-value header handling

rolo/serving/twisted.py

@@ -221,8 +221,9 @@ def writeHeaders(
         else:
             # newer twisted versions instead pass the headers object
             for name, values in headers.getAllRawHeaders():
-                line = name + b": " + b",".join(values) + b"\r\n"
-                headerSequence.append(line)
+                for value in values:
+                    line = name + b": " + value + b"\r\n"
+                    headerSequence.append(line)
 
         headerSequence.append(b"\r\n")
         self.transport.writeSequence(headerSequence)

tests/gateway/test_headers.py

@@ -1,4 +1,6 @@
+import http.client
 import json
+from collections import defaultdict
 
 import pytest
 import requests
@@ -14,6 +16,8 @@ def handler(chain: HandlerChain, context: RequestContext, response: Response):
         response.mimetype = "application/json"
         response.headers["X-fOO_bar"] = "FooBar"
         response.headers["content-md5"] = "af5e58f9a7c4682e1b410f2e9392a539"
+        response.headers.add("multi-value", "value1")
+        response.headers.add("multi-value", "value2")
         return response
 
     gateway = Gateway(request_handlers=[handler])
@@ -39,3 +43,31 @@ def handler(chain: HandlerChain, context: RequestContext, response: Response):
     assert "X-fOO_bar" in response_headers
     # even though it's a standard header, it should be in the original case
     assert "content-md5" in response_headers
+    assert response_headers["multi-value"] == "value1, value2"
+
+
+@pytest.mark.parametrize("serve_gateway", ["asgi", "twisted"], indirect=True)
+def test_multivalue_header_handling(serve_gateway):
+    def handler(chain: HandlerChain, context: RequestContext, response: Response):
+        response.data = json.dumps({"headers": dict(context.request.headers)})
+        response.mimetype = "application/json"
+        response.headers.add("multi-value", "value1")
+        response.headers.add("multi-value", "value2")
+        return response
+
+    gateway = Gateway(request_handlers=[handler])
+
+    srv = serve_gateway(gateway)
+
+    # we need to use a low level HTTP client because `requests` does some header manipulation and concatenation which
+    # obscures the behavior
+    conn = http.client.HTTPConnection(host="127.0.0.1", port=srv.port)
+
+    conn.request("GET", url="/hello")
+    response = conn.getresponse()
+    response_headers = defaultdict(list)
+
+    for k, v in response.headers.items():
+        response_headers[k].append(v)
+
+    assert response_headers["multi-value"] == ["value1", "value2"]