Clamped WsgiToAsgi response body using Content-Length value

This makes it adhere correctly to this part of the WSGI spec. Fixes #195.

asgiref/wsgi.py

@@ -29,6 +29,7 @@ class WsgiToAsgiInstance:
     def __init__(self, wsgi_application):
         self.wsgi_application = wsgi_application
         self.response_started = False
+        self.response_content_length = None
 
     async def __call__(self, scope, receive, send):
         if scope["type"] != "http":
@@ -114,6 +115,11 @@ def start_response(self, status, response_headers, exc_info=None):
             (name.lower().encode("ascii"), value.encode("ascii"))
             for name, value in response_headers
         ]
+        # Extract content-length
+        self.response_content_length = None
+        for name, value in response_headers:
+            if name.lower() == "content-length":
+                self.response_content_length = int(value)
         # Build and send response start message.
         self.response_start = {
             "type": "http.response.start",
@@ -130,14 +136,25 @@ def run_wsgi_app(self, body):
         # Translate the scope and incoming request body into a WSGI environ
         environ = self.build_environ(self.scope, body)
         # Run the WSGI app
+        bytes_sent = 0
         for output in self.wsgi_application(environ, self.start_response):
             # If this is the first response, include the response headers
             if not self.response_started:
                 self.response_started = True
                 self.sync_send(self.response_start)
+            # If the application supplies a Content-Length header
+            if self.response_content_length is not None:
+                # The server should not transmit more bytes to the client than the header allows
+                bytes_allowed = self.response_content_length - bytes_sent
+                if len(output) > bytes_allowed:
+                    output = output[:bytes_allowed]
             self.sync_send(
                 {"type": "http.response.body", "body": output, "more_body": True}
             )
+            bytes_sent += len(output)
+            # The server should stop iterating over the response when enough data has been sent
+            if bytes_sent == self.response_content_length:
+                break
         # Close connection
         if not self.response_started:
             self.response_started = True

tests/test_wsgi.py

@@ -1,3 +1,5 @@
+import sys
+
 import pytest
 
 from asgiref.testing import ApplicationCommunicator
@@ -126,6 +128,130 @@ def wsgi_application(environ, start_response):
     assert (await instance.receive_output(1)) == {"type": "http.response.body"}
 
 
+@pytest.mark.asyncio
+async def test_wsgi_clamped_body():
+    """
+    Makes sure WsgiToAsgi clamps a body response longer than Content-Length
+    """
+
+    def wsgi_application(environ, start_response):
+        start_response("200 OK", [("Content-Length", "8")])
+        return [b"0123", b"45", b"6789"]
+
+    application = WsgiToAsgi(wsgi_application)
+    instance = ApplicationCommunicator(
+        application,
+        {
+            "type": "http",
+            "http_version": "1.0",
+            "method": "GET",
+            "path": "/",
+            "query_string": b"",
+            "headers": [],
+        },
+    )
+    await instance.send_input({"type": "http.request"})
+    assert (await instance.receive_output(1)) == {
+        "type": "http.response.start",
+        "status": 200,
+        "headers": [(b"content-length", b"8")],
+    }
+    assert (await instance.receive_output(1)) == {
+        "type": "http.response.body",
+        "body": b"0123",
+        "more_body": True,
+    }
+    assert (await instance.receive_output(1)) == {
+        "type": "http.response.body",
+        "body": b"45",
+        "more_body": True,
+    }
+    assert (await instance.receive_output(1)) == {
+        "type": "http.response.body",
+        "body": b"67",
+        "more_body": True,
+    }
+    assert (await instance.receive_output(1)) == {"type": "http.response.body"}
+
+
+@pytest.mark.asyncio
+async def test_wsgi_stops_iterating_after_content_length_bytes():
+    """
+    Makes sure WsgiToAsgi does not iterate after than Content-Length bytes
+    """
+
+    def wsgi_application(environ, start_response):
+        start_response("200 OK", [("Content-Length", "4")])
+        yield b"0123"
+        pytest.fail("WsgiToAsgi should not iterate after Content-Length bytes")
+        yield b"4567"
+
+    application = WsgiToAsgi(wsgi_application)
+    instance = ApplicationCommunicator(
+        application,
+        {
+            "type": "http",
+            "http_version": "1.0",
+            "method": "GET",
+            "path": "/",
+            "query_string": b"",
+            "headers": [],
+        },
+    )
+    await instance.send_input({"type": "http.request"})
+    assert (await instance.receive_output(1)) == {
+        "type": "http.response.start",
+        "status": 200,
+        "headers": [(b"content-length", b"4")],
+    }
+    assert (await instance.receive_output(1)) == {
+        "type": "http.response.body",
+        "body": b"0123",
+        "more_body": True,
+    }
+    assert (await instance.receive_output(1)) == {"type": "http.response.body"}
+
+
+@pytest.mark.asyncio
+async def test_wsgi_multiple_start_response():
+    """
+    Makes sure WsgiToAsgi only keep Content-Length from the last call to start_response
+    """
+
+    def wsgi_application(environ, start_response):
+        start_response("200 OK", [("Content-Length", "5")])
+        try:
+            raise ValueError("Application Error")
+        except ValueError:
+            start_response("500 Server Error", [], sys.exc_info())
+            return [b"Some long error message"]
+
+    application = WsgiToAsgi(wsgi_application)
+    instance = ApplicationCommunicator(
+        application,
+        {
+            "type": "http",
+            "http_version": "1.0",
+            "method": "GET",
+            "path": "/",
+            "query_string": b"",
+            "headers": [],
+        },
+    )
+    await instance.send_input({"type": "http.request"})
+    assert (await instance.receive_output(1)) == {
+        "type": "http.response.start",
+        "status": 500,
+        "headers": [],
+    }
+    assert (await instance.receive_output(1)) == {
+        "type": "http.response.body",
+        "body": b"Some long error message",
+        "more_body": True,
+    }
+    assert (await instance.receive_output(1)) == {"type": "http.response.body"}
+
+
 @pytest.mark.asyncio
 async def test_wsgi_multi_body():
     """