feat(sse): wire @defer incremental delivery into Absinthe.Plug

lib/absinthe/plug.ex

@@ -305,6 +305,10 @@ defmodule Absinthe.Plug do
         conn
         |> subscribe(topic, config)
 
+      {:ok, :streaming} ->
+        # Already sent via SSE chunked transport
+        conn
+
       {:ok, %{data: _} = result} ->
         conn
         |> encode(200, result, config)
@@ -390,7 +394,7 @@ defmodule Absinthe.Plug do
         conn
     after
       30_000 ->
-        case chunk(conn, ": keep-alive\n\n") do
+        case chunk(conn, ":ping\n\n") do
           {:ok, conn} ->
             subscribe_loop(conn, topic, config)
 
@@ -542,6 +546,16 @@ defmodule Absinthe.Plug do
       Request.Query.add_pipeline(query, conn_info, config)
 
     case Absinthe.Pipeline.run(document, pipeline) do
+      {:ok, %{execution: %{incremental_delivery: true}} = bp, _} ->
+        conn = apply_before_send(conn, [bp], config)
+
+        if accepts_sse?(conn) do
+          conn = deliver_incremental_sse(conn, bp)
+          {conn, {:ok, :streaming}}
+        else
+          {conn, {:ok, bp.result}}
+        end
+
       {:ok, %{result: result} = bp, _} ->
         conn = apply_before_send(conn, [bp], config)
         {conn, {:ok, result}}
@@ -551,6 +565,115 @@ defmodule Absinthe.Plug do
     end
   end
 
+  defp accepts_sse?(conn) do
+    conn
+    |> Plug.Conn.get_req_header("accept")
+    |> Enum.any?(&String.contains?(&1, "text/event-stream"))
+  end
+
+  defp deliver_incremental_sse(conn, blueprint) do
+    alias Absinthe.Plug.Incremental.SSE.EventFormatter
+
+    streaming_ctx = get_in(blueprint.execution.context, [:__streaming__])
+    defer_info = streaming_ctx[:defer_info] || []
+    full_data = blueprint.result[:data] || %{}
+
+    # Split the resolved result: strip deferred fields from initial, collect for incremental
+    {initial_data, deferred_tasks} =
+      defer_info
+      |> Enum.with_index()
+      |> Enum.reduce({full_data, []}, fn {info, idx}, {data, tasks} ->
+        parent_obj = get_nested(data, info.parent_path) || %{}
+
+        # Extract deferred fields from parent
+        fragment_data =
+          Enum.reduce(info.field_names, %{}, fn name, acc ->
+            case Map.fetch(parent_obj, name) do
+              {:ok, value} -> Map.put(acc, name, value)
+              :error -> acc
+            end
+          end)
+
+        # Strip deferred fields from initial
+        stripped = Enum.reduce(info.field_names, parent_obj, &Map.delete(&2, &1))
+        updated_data = put_nested(data, info.parent_path, stripped)
+
+        task = %{id: to_string(idx), label: info.label, path: info.parent_path, data: fragment_data}
+        {updated_data, tasks ++ [task]}
+      end)
+
+    has_next = not Enum.empty?(deferred_tasks)
+
+    # Build initial payload with deferred fields stripped
+    initial_response =
+      %{data: initial_data}
+      |> Map.put(:hasNext, has_next)
+      |> then(fn r ->
+        if has_next do
+          pending = Enum.map(deferred_tasks, fn t ->
+            entry = %{id: t.id, path: t.path}
+            if t.label, do: Map.put(entry, :label, t.label), else: entry
+          end)
+          Map.put(r, :pending, pending)
+        else
+          r
+        end
+      end)
+
+    # Setup SSE connection — clear before_send callbacks (ETag etc.)
+    conn =
+      conn
+      |> Plug.Conn.put_resp_header("content-type", "text/event-stream")
+      |> Plug.Conn.put_resp_header("cache-control", "no-cache")
+      |> Plug.Conn.put_resp_header("x-accel-buffering", "no")
+      |> Map.update(:private, %{}, &Map.put(&1, :before_send, []))
+      |> Plug.Conn.send_chunked(200)
+
+    # Send initial event
+    initial_event = EventFormatter.format_event("next", initial_response, 0)
+
+    conn =
+      case Plug.Conn.chunk(conn, initial_event) do
+        {:ok, conn} -> conn
+        {:error, _} -> conn
+      end
+
+    # Send incremental events — data already resolved, just deliver
+    {conn, _event_id} =
+      deferred_tasks
+      |> Enum.with_index()
+      |> Enum.reduce({conn, 1}, fn {task, idx}, {conn, event_id} ->
+        incremental_payload = %{
+          incremental: [%{id: task.id, data: task.data, path: task.path}],
+          hasNext: idx < length(deferred_tasks) - 1
+        }
+
+        event = EventFormatter.format_event("next", incremental_payload, event_id)
+
+        case Plug.Conn.chunk(conn, event) do
+          {:ok, conn} -> {conn, event_id + 1}
+          {:error, _} -> {conn, event_id + 1}
+        end
+      end)
+
+    # Send complete event
+    complete_event = EventFormatter.format_event("complete", %{}, 999)
+    case Plug.Conn.chunk(conn, complete_event) do
+      {:ok, conn} -> conn
+      {:error, _} -> conn
+    end
+  end
+
+  defp get_nested(data, []), do: data
+  defp get_nested(data, [key | rest]) when is_map(data), do: get_nested(Map.get(data, key), rest)
+  defp get_nested(_, _), do: nil
+
+  defp put_nested(_data, [], value), do: value
+  defp put_nested(data, [key | rest], value) when is_map(data) do
+    Map.put(data, key, put_nested(Map.get(data, key, %{}), rest, value))
+  end
+  defp put_nested(data, _, _), do: data
+
   #
   # PIPELINE
   #