From a9a8dab0cc3cbd05dca37650fc392717b9fe5bbf Mon Sep 17 00:00:00 2001
From: Marcelo Trylesinski <marcelotryle@gmail.com>
Date: Sat, 22 Feb 2025 09:54:55 -0300
Subject: [PATCH] Don't compress on server sent events (#2871)

---
 starlette/middleware/gzip.py  |  6 +++++-
 tests/middleware/test_gzip.py | 22 ++++++++++++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/starlette/middleware/gzip.py b/starlette/middleware/gzip.py
index b677063d..fc63e91b 100644
--- a/starlette/middleware/gzip.py
+++ b/starlette/middleware/gzip.py
@@ -5,6 +5,8 @@ import typing
 from starlette.datastructures import Headers, MutableHeaders
 from starlette.types import ASGIApp, Message, Receive, Scope, Send
 
+DEFAULT_EXCLUDED_CONTENT_TYPES = ("text/event-stream",)
+
 
 class GZipMiddleware:
     def __init__(self, app: ASGIApp, minimum_size: int = 500, compresslevel: int = 9) -> None:
@@ -30,6 +32,7 @@ class GZipResponder:
         self.initial_message: Message = {}
         self.started = False
         self.content_encoding_set = False
+        self.content_type_is_excluded = False
         self.gzip_buffer = io.BytesIO()
         self.gzip_file = gzip.GzipFile(mode="wb", fileobj=self.gzip_buffer, compresslevel=compresslevel)
 
@@ -46,7 +49,8 @@ class GZipResponder:
             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:
+            self.content_type_is_excluded = headers.get("content-type", "").startswith(DEFAULT_EXCLUDED_CONTENT_TYPES)
+        elif message_type == "http.response.body" and (self.content_encoding_set or self.content_type_is_excluded):
             if not self.started:
                 self.started = True
                 await self.send(self.initial_message)
diff --git a/tests/middleware/test_gzip.py b/tests/middleware/test_gzip.py
index b20a7cb8..38a4e1e3 100644
--- a/tests/middleware/test_gzip.py
+++ b/tests/middleware/test_gzip.py
@@ -104,3 +104,25 @@ def test_gzip_ignored_for_responses_with_encoding_set(
     assert response.text == "x" * 4000
     assert response.headers["Content-Encoding"] == "text"
     assert "Content-Length" not in response.headers
+
+
+def test_gzip_ignored_on_server_sent_events(test_client_factory: TestClientFactory) -> None:
+    def homepage(request: Request) -> StreamingResponse:
+        async def generator(bytes: bytes, count: int) -> ContentStream:
+            for _ in range(count):
+                yield bytes
+
+        streaming = generator(bytes=b"x" * 400, count=10)
+        return StreamingResponse(streaming, status_code=200, media_type="text/event-stream")
+
+    app = Starlette(
+        routes=[Route("/", endpoint=homepage)],
+        middleware=[Middleware(GZipMiddleware)],
+    )
+
+    client = test_client_factory(app)
+    response = client.get("/", headers={"accept-encoding": "gzip"})
+    assert response.status_code == 200
+    assert response.text == "x" * 4000
+    assert "Content-Encoding" not in response.headers
+    assert "Content-Length" not in response.headers
-- 
2.47.2