fix: Local evaluation silently drops identity overrides on large environments

lib/flagsmith_client.ex

@@ -1,4 +1,6 @@
 defmodule Flagsmith.Client do
+  require Logger
+
   alias Flagsmith.Schemas
   alias Flagsmith.Configuration
 
@@ -74,18 +76,65 @@ defmodule Flagsmith.Client do
 
   @doc false
   def get_environment_request(%Configuration{} = config) do
-    case Tesla.get(http_client(config), @api_paths.environment) do
-      {:ok, %{status: status, body: body}} when status >= 200 and status < 300 ->
-        {:ok,
-         body
-         |> Schemas.Environment.from_response()
-         |> Schemas.Environment.add_client_config(config)}
+    started_at = System.monotonic_time(:millisecond)
+
+    with {:ok, body} <- get_environment_document(http_client(config)) do
+      warn_if_slower_than_refresh_interval(started_at, config)
+
+      {:ok,
+       body
+       |> Schemas.Environment.from_response()
+       |> Schemas.Environment.add_client_config(config)}
+    end
+  end
+
+  defp get_environment_document(client, query \\ [], document \\ nil) do
+    case Tesla.get(client, @api_paths.environment, query: query) do
+      {:ok, %{status: status, body: body} = response} when status >= 200 and status < 300 ->
+        document = append_identity_overrides(document, body)
+
+        case next_page_id(response) do
+          nil -> {:ok, document}
+          page_id -> get_environment_document(client, [page_id: page_id], document)
+        end
 
       error_resp ->
         return_error(error_resp)
     end
   end
 
+  defp append_identity_overrides(nil, page), do: page
+
+  defp append_identity_overrides(document, page) do
+    Map.put(
+      document,
+      "identity_overrides",
+      (document["identity_overrides"] || []) ++ (page["identity_overrides"] || [])
+    )
+  end
+
+  defp next_page_id(response) do
+    with link when is_binary(link) <- Tesla.get_header(response, "link"),
+         [_, next_url] <- Regex.run(~r/<([^>]+)>;\s*rel="next"/, link),
+         %URI{query: query} when is_binary(query) <- URI.parse(next_url) do
+      query |> URI.decode_query() |> Map.get("page_id")
+    else
+      _ -> nil
+    end
+  end
+
+  defp warn_if_slower_than_refresh_interval(started_at, %Configuration{
+         environment_refresh_interval_milliseconds: interval
+       }) do
+    elapsed = System.monotonic_time(:millisecond) - started_at
+
+    if elapsed > interval do
+      Logger.warning(
+        "Fetching the environment document took #{elapsed}ms, longer than the environment refresh interval of #{interval}ms; raise the refresh interval or reduce the environment size."
+      )
+    end
+  end
+
   @doc """
   Returns an `:ok` tuple containing a list of `t:Flagsmith.Schemas.Flag.t/0` structs,
   either from the local evaluation or API depending on the configuration used, or an

test/flagsmith_client_test.exs

@@ -1,7 +1,7 @@
 defmodule Flagsmith.Client.Test do
   use ExUnit.Case
 
-  import Mox, only: [verify_on_exit!: 1, expect: 3]
+  import Mox, only: [verify_on_exit!: 1, expect: 3, expect: 4]
   import Flagsmith.Test.Helpers, only: [assert_request: 2]
 
   alias Flagsmith.Engine.Test
@@ -461,4 +461,73 @@ defmodule Flagsmith.Client.Test do
       Flagsmith.Client.get_environment(config)
     end
   end
+
+  describe "paginated environment document" do
+    test "get_environment follows the next page links", %{config: config} do
+      first_page = Test.Generators.map_env()
+      [identity_override] = first_page["identity_overrides"]
+
+      override_page = fn identifier ->
+        %{"identity_overrides" => [Map.put(identity_override, "identifier", identifier)]}
+      end
+
+      next_link = fn page_id ->
+        [
+          {"link",
+           "</api/v1/environment-document/?page_id=#{URI.encode_www_form(page_id)}>; rel=\"next\""}
+        ]
+      end
+
+      expect(Tesla.Adapter.Mock, :call, 3, fn tesla_env, _options ->
+        case tesla_env.query do
+          [] ->
+            {:ok,
+             %Tesla.Env{
+               status: 200,
+               body: first_page,
+               headers: next_link.("identity_override:1:page-2")
+             }}
+
+          [page_id: "identity_override:1:page-2"] ->
+            {:ok,
+             %Tesla.Env{
+               status: 200,
+               body: override_page.("page-2-id"),
+               headers: next_link.("identity_override:1:page-3")
+             }}
+
+          [page_id: "identity_override:1:page-3"] ->
+            {:ok, %Tesla.Env{status: 200, body: override_page.("page-3-id")}}
+        end
+      end)
+
+      assert {:ok, %Schemas.Environment{} = env} = Flagsmith.Client.get_environment(config)
+
+      assert ["overridden-id", "page-2-id", "page-3-id"] =
+               Enum.map(env.identity_overrides, & &1.identifier)
+
+      assert length(env.feature_states) == length(first_page["feature_states"])
+    end
+
+    test "get_environment warns when fetching is slower than the refresh interval" do
+      config =
+        Flagsmith.Client.new(
+          environment_key: "client_test_key",
+          environment_refresh_interval_milliseconds: 1
+        )
+
+      expect(Tesla.Adapter.Mock, :call, fn _tesla_env, _options ->
+        Process.sleep(5)
+        {:ok, %Tesla.Env{status: 200, body: Test.Generators.map_env()}}
+      end)
+
+      log =
+        ExUnit.CaptureLog.capture_log(fn ->
+          assert {:ok, %Schemas.Environment{}} = Flagsmith.Client.get_environment(config)
+        end)
+
+      assert log =~ "Fetching the environment document took"
+      assert log =~ "longer than the environment refresh interval of 1ms"
+    end
+  end
 end