Don't compress 206 Partial Content responses (#677)

Author: tomciopp

lib/bandit/compression.ex

@@ -77,7 +77,10 @@ defmodule Bandit.Compression do
 
     headers = maybe_add_vary_header(adapter, status, headers)
 
-    if status not in [204, 304] && not is_nil(adapter.content_encoding) &&
+    # 206 responses are excluded since compressing them would change the bytes relative to the
+    # content-range the response advertises against the identity representation, corrupting
+    # range reassembly at the client
+    if status not in [204, 206, 304] && not is_nil(adapter.content_encoding) &&
          is_nil(response_content_encoding_header) &&
          !response_has_strong_etag(headers) && !response_indicates_no_transform(headers) &&
          !empty_body? do

test/bandit/http1/protocol_test.exs

@@ -1992,6 +1992,21 @@ defmodule HTTP1ProtocolTest do
       assert response.body == String.duplicate("a", 10_000)
     end
 
+    test "does no encoding for 206 responses", context do
+      response =
+        Req.get!(context.req,
+          url: "/send_206_response",
+          headers: [{"accept-encoding", "deflate"}]
+        )
+
+      # Assert that we did not try to compress the body, since doing so would
+      # invalidate the range the response advertises via content-range
+      assert response.status == 206
+      assert response.headers["content-length"] == ["10000"]
+      assert response.headers["content-encoding"] == nil
+      assert response.body == String.duplicate("a", 10_000)
+    end
+
     test "does no encoding if a malformed etag starting with W but not W/ is present", context do
       response =
         Req.get!(context.req,
@@ -2216,6 +2231,12 @@ defmodule HTTP1ProtocolTest do
       |> send_resp(200, String.duplicate("a", 10_000))
     end
 
+    def send_206_response(conn) do
+      conn
+      |> put_resp_header("content-range", "bytes 0-9999/20000")
+      |> send_resp(206, String.duplicate("a", 10_000))
+    end
+
     def send_weak_etag(conn) do
       conn
       |> put_resp_header("etag", "W/\"1234\"")