Keep the HTTP/2 receive window credit in sync when the window shrinks

lib/mint/http2.ex

@@ -1911,8 +1911,9 @@ defmodule Mint.HTTP2 do
   end
 
   defp maybe_refill_conn(frames, conn) do
-    if conn.receive_window_remaining <= conn.receive_window_update_threshold do
-      increment = conn.receive_window_size - conn.receive_window_remaining
+    increment = conn.receive_window_size - conn.receive_window_remaining
+
+    if conn.receive_window_remaining <= conn.receive_window_update_threshold and increment > 0 do
       [window_update(stream_id: 0, window_size_increment: increment) | frames]
     else
       frames
@@ -1922,9 +1923,10 @@ defmodule Mint.HTTP2 do
   defp maybe_refill_stream(frames, conn, stream_id) do
     case Map.fetch(conn.streams, stream_id) do
       {:ok, stream} ->
-        if stream.receive_window_remaining <= conn.receive_window_update_threshold do
-          increment = stream.receive_window_size - stream.receive_window_remaining
+        increment = stream.receive_window_size - stream.receive_window_remaining
 
+        if stream.receive_window_remaining <= conn.receive_window_update_threshold and
+             increment > 0 do
           [
             window_update(stream_id: stream_id, window_size_increment: increment) | frames
           ]
@@ -2255,9 +2257,12 @@ defmodule Mint.HTTP2 do
         for {stream_id, stream} <- streams,
             stream.state in [:open, :half_closed_local, :reserved_remote],
             into: streams do
-          receive_window_size = stream.receive_window_size + diff
-
-          {stream_id, %{stream | receive_window_size: receive_window_size}}
+          {stream_id,
+           %{
+             stream
+             | receive_window_size: stream.receive_window_size + diff,
+               receive_window_remaining: stream.receive_window_remaining + diff
+           }}
         end
       end)
 

test/mint/http2/conn_test.exs

@@ -2247,6 +2247,40 @@ defmodule Mint.HTTP2Test do
   end
 
   describe "settings" do
+    @tag connect_options: [
+           receive_window_update_threshold: 8,
+           client_settings: [initial_window_size: 16]
+         ]
+    test "shrinking the initial window size keeps the remaining credit in sync",
+         %{conn: conn} do
+      {conn, ref} = open_request(conn)
+
+      assert_recv_frames [headers(stream_id: stream_id)]
+
+      assert {:ok, %HTTP2{} = conn, _responses} =
+               stream_frames(conn, [
+                 {:headers, stream_id, [{":status", "200"}], [:end_headers]},
+                 data(stream_id: stream_id, data: "123456")
+               ])
+
+      assert conn.streams[stream_id].receive_window_remaining == 10
+
+      {:ok, conn} = HTTP2.put_settings(conn, initial_window_size: 8)
+      assert_recv_frames [settings(params: [initial_window_size: 8])]
+
+      assert {:ok, %HTTP2{} = conn, []} =
+               stream_frames(conn, [settings(flags: set_flags(:settings, [:ack]), params: [])])
+
+      assert conn.streams[stream_id].receive_window_size == 8
+      assert conn.streams[stream_id].receive_window_remaining == 2
+
+      assert {:ok, %HTTP2{} = conn, [{:data, ^ref, "12"}]} =
+               stream_frames(conn, [data(stream_id: stream_id, data: "12")])
+
+      assert conn.streams[stream_id].receive_window_remaining == 8
+      assert_recv_frames [window_update(stream_id: ^stream_id, window_size_increment: 8)]
+    end
+
     test "put_settings/2 can be used to send settings to server", %{conn: conn} do
       {:ok, conn} =
         HTTP2.put_settings(conn, max_concurrent_streams: 123, initial_window_size: 1_000)