Avoid double encode

.gitignore

@@ -6,4 +6,5 @@ dist
 *.egg-info
 .mypy_cache
 .pytest_cache
-.vscode
+.vscode
+build

README.md

@@ -73,6 +73,11 @@ app.add_middleware(
 - _(Optional)_ `minimum_size`: Only compress responses that are bigger than this value in bytes.
 - _(Optional)_ `gzip_fallback`: If `True`, uses gzip encoding if `br` is not in the Accept-Encoding header.
 
+**Notes**:
+
+- It won't apply Brotli compression on responses that already have a Content-Encoding set, to prevent them from being encoded twice.
+- If the GZip fallback is applied, and the response already had a Content-Encoding set, the double-encoding-prevention will only happen if you're using Starlette `>=0.22.0` (see [the PR](https://github.com/encode/starlette/pull/1901)).
+
 ## Performance
 
 To better understand the benefits of Brotli over GZip, see, [Gzip vs. Brotli: Comparing Compression Techniques](https://www.coralnodes.com/gzip-vs-brotli/), where detailed information and benchmarks are provided.

brotli_asgi/__init__.py

@@ -113,6 +113,7 @@ def __init__(
         self.send = unattached_send  # type: Send
         self.initial_message = {}  # type: Message
         self.started = False
+        self.content_encoding_set = False
         self.br_file = Compressor(
             quality=self.quality, mode=self.mode, lgwin=self.lgwin, lgblock=self.lgblock
         )
@@ -131,6 +132,13 @@ async def send_with_brotli(self, message: Message) -> NoReturn:
             # Don't send the initial message until we've determined how to
             # modify the outgoing headers correctly.
             self.initial_message = message
+            headers = Headers(raw=self.initial_message["headers"])
+            self.content_encoding_set = "content-encoding" in headers
+        elif message_type == "http.response.body" and self.content_encoding_set:
+            if not self.started:
+                self.started = True
+                await self.send(self.initial_message)
+            await self.send(message)
         elif message_type == "http.response.body" and not self.started:
             self.started = True
             body = message.get("body", b"")

requirements.txt

@@ -1,2 +1,2 @@
 Brotli==1.0.9
-starlette==0.21.0
+starlette==0.22.0

setup.py

@@ -1,14 +1,14 @@
 """A compression AGSI middleware using brotli.
 
-Built using starlette under the hood, it can be used as a drop in replacement to
+Built using starlette under the hood, it can be used as a drop-in replacement to
 GZipMiddleware for Starlette or FastAPI.
 """
 
 from setuptools import setup  # type: ignore
 
 extras = {
-    'test_brotli': ['requests==2.23.0', 'mypy==0.770'],
-    'test_brotlipy': ['requests==2.23.0', 'mypy==0.770', 'brotlipy==0.7.0']
+    'test_brotli': ['requests>=2.23.0', 'mypy>=0.770'],
+    'test_brotlipy': ['requests>=2.23.0', 'mypy>=0.770', 'brotlipy>=0.7.0']
 }
 
 setup(

tests.py

@@ -1,10 +1,13 @@
-"""Main test for brotli middleware.
+"""Main tests for brotli middleware.
 
-This tests are the same as the ones from starlette.tests.middleware.test_gzip but using
-brotli insted.
+Some of these tests are the same as the ones from starlette.tests.middleware.test_gzip
+but using brotli instead.
 """
+import gzip
+import io
+
 from starlette.applications import Starlette
-from starlette.responses import PlainTextResponse, StreamingResponse, JSONResponse
+from starlette.responses import PlainTextResponse, StreamingResponse, JSONResponse, Response
 from starlette.testclient import TestClient
 from starlette.middleware import Middleware
 
@@ -25,7 +28,6 @@ def homepage(request):
     assert response.status_code == 200
     assert response.text == "x" * 4000
     assert response.headers["Content-Encoding"] == "br"
-    print(len(response.headers["Content-Length"]))
     assert int(response.headers["Content-Length"]) < 4000
 
 
@@ -159,3 +161,27 @@ def homepage(request):
     assert response.text == "x" * 4000
     assert "Content-Encoding" not in response.headers
     assert int(response.headers["Content-Length"]) == 4000
+
+
+def test_brotli_avoids_double_encoding():
+    # See https://github.com/encode/starlette/pull/1901
+
+    app = Starlette()
+
+    app.add_middleware(BrotliMiddleware, minimum_size=1)
+
+    @app.route("/")
+    def homepage(request):
+        gzip_buffer = io.BytesIO()
+        gzip_file = gzip.GzipFile(mode="wb", fileobj=gzip_buffer)
+        gzip_file.write(b"hello world" * 200)
+        gzip_file.close()
+        body = gzip_buffer.getvalue()
+        return Response(body, headers={"content-encoding": "gzip", "x-gzipped-content-length-in-test": str(len(body))})
+
+    client = TestClient(app)
+    response = client.get("/", headers={"accept-encoding": "br"})
+    assert response.status_code == 200
+    assert response.text == "hello world" * 200
+    assert response.headers["Content-Encoding"] == "gzip"
+    assert response.headers["Content-Length"] == response.headers["x-gzipped-content-length-in-test"]