treq.auth: Don't mutate request headers

Instead, copy and update.

Closes twisted#314.

changelog.d/314.bugfix.rst

@@ -0,0 +1 @@
+treq request APIs no longer mutates a :class:`http_headers.Headers <twisted.web.http_headers.Headers>` passed as the *headers* parameter when the *auth* parameter is also passed.

src/treq/auth.py

@@ -35,14 +35,14 @@ def __init__(self, agent, request_headers):
 
     def request(self, method, uri, headers=None, bodyProducer=None):
         if headers is None:
-            headers = self._request_headers
+            new = self._request_headers
         else:
-            # FIXME: This mutates *headers*
+            new = headers.copy()
             for header, values in self._request_headers.getAllRawHeaders():
-                headers.setRawHeaders(header, values)
+                new.setRawHeaders(header, values)
 
         return self._agent.request(
-            method, uri, headers=headers, bodyProducer=bodyProducer)
+            method, uri, headers=new, bodyProducer=bodyProducer)
 
 
 def add_basic_auth(agent, username, password):

src/treq/test/test_auth.py

@@ -39,6 +39,22 @@ def test_overrides_per_request_headers(self):
             Headers({b'X-Test-Header': [b'Test-Header-Value']}),
         )
 
+    def test_no_mutation(self):
+        """
+        The agent never mutates the headers passed to its request method.
+
+        This reproduces https://github.com/twisted/treq/issues/314
+        """
+        requestHeaders = Headers({})
+        agent = _RequestHeaderSettingAgent(
+            self.agent,
+            Headers({b'Added': [b'1']}),
+        )
+
+        agent.request(b'method', b'uri', headers=requestHeaders)
+
+        self.assertEqual(requestHeaders, Headers({}))
+
 
 class AddAuthTests(SynchronousTestCase):
     def test_add_basic_auth(self):