fix: use Calendar.UTCOnlyTimeZoneDatabase instead of project configured tz database (#324)

Fix #317 

The Engine node inherits the configuration of the user's project, so if
the user configures a different timezone database, like
`Tzdata.TimeZoneDatabase`, the engine may crash when trying to use a
database that is not really available:

```elixir
%ArgumentError{message: "cannot add 10 second to ~U[2026-01-20 07:44:55.937263Z] (with time zone database Tzdata.TimeZoneDatabase), reason: :time_zone_not_found"}

```

The fix here is to use the builtin `Calendar.UTCOnlyTimeZoneDatabase`
for the Engine node. I also added a fixture project that replicates the
scenario from the linked issue.

Author: doorgan

apps/engine/lib/engine/modules.ex

@@ -243,7 +243,7 @@ defmodule Engine.Modules do
   defp rebuild_cache do
     {amount, unit} = @cache_timeout
 
-    expires = DateTime.add(DateTime.utc_now(), amount, unit)
+    expires = DateTime.add(DateTime.utc_now(), amount, unit, Calendar.UTCOnlyTimeZoneDatabase)
 
     module_map =
       Map.new(:code.all_available(), fn {module_charlist, _path, already_loaded?} ->

apps/expert/test/engine/modules_test.exs

@@ -0,0 +1,60 @@
+defmodule Expert.Engine.ModulesTest do
+  use ExUnit.Case
+
+  alias Expert.EngineApi
+  alias Expert.EngineNode
+  alias Expert.EngineSupervisor
+  alias Forge.Project
+
+  import Forge.EngineApi.Messages
+  import Forge.Test.Fixtures
+
+  describe "Engine.Modules with custom time zone database config" do
+    @tag timeout: :timer.seconds(60)
+    test "with_prefix/1 works when project configures custom time_zone_database" do
+      # Regression test for https://github.com/elixir-lang/expert/issues/317
+      #
+      # When a project configures a custom time_zone_database (e.g. Tzdata),
+      # the engine node inherits this config when Mix.Task.run(:loadconfig)
+      # is called during project compilation (in Engine.Build.Project).
+      # However, the tzdata application itself is not started on the engine node.
+      #
+      # This caused DateTime.add/3 to fail in Engine.Modules.rebuild_cache/0
+      # because it would use the globally configured Tzdata.TimeZoneDatabase
+      # which couldn't resolve time zones without the tzdata app running.
+      #
+      # The fix is to explicitly use Calendar.UTCOnlyTimeZoneDatabase when
+      # calling DateTime.add/4 in rebuild_cache/0.
+
+      # The :project_config fixture has:
+      #   config :elixir, :time_zone_database, Tzdata.TimeZoneDatabase
+      project = project(:project_config)
+
+      # Clean workspace to ensure fresh state
+      project |> Project.workspace_path() |> File.rm_rf()
+
+      {:ok, _} = start_supervised(Forge.NodePortMapper)
+      {:ok, _} = start_supervised({EngineSupervisor, project})
+      {:ok, _, _} = EngineNode.start(project)
+
+      EngineApi.register_listener(project, self(), [:all])
+
+      # Trigger initial compile which runs `mix loadconfig`
+      EngineApi.schedule_compile(project, true)
+      assert_receive project_compiled(), :timer.seconds(30)
+
+      # Verify the time_zone_database config was loaded on the engine node
+      tz_db = EngineApi.call(project, Application, :get_env, [:elixir, :time_zone_database])
+      assert tz_db == Tzdata.TimeZoneDatabase
+
+      # Clear the module cache to force rebuild_cache/0 to be called next
+      EngineApi.call(project, :persistent_term, :erase, [Engine.Modules])
+
+      # This triggers Engine.Modules.rebuild_cache/0 which uses DateTime.add
+      result = EngineApi.call(project, Engine.Modules, :with_prefix, ["Enum"])
+
+      assert is_list(result)
+      assert Enum in result
+    end
+  end
+end

apps/forge/test/fixtures/project_config/.formatter.exs

@@ -0,0 +1,4 @@
+# Used by "mix format"
+[
+  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
+]

apps/forge/test/fixtures/project_config/.gitignore

@@ -0,0 +1,26 @@
+# The directory Mix will write compiled artifacts to.
+/_build/
+
+# If you run "mix test --cover", coverage assets end up here.
+/cover/
+
+# The directory Mix downloads your dependencies sources to.
+/deps/
+
+# Where third-party dependencies like ExDoc output generated docs.
+/doc/
+
+# Ignore .fetch files in case you like to edit your project deps locally.
+/.fetch
+
+# If the VM crashes, it generates a dump, let's ignore it too.
+erl_crash.dump
+
+# Also ignore archive artifacts (built via "mix archive.build").
+*.ez
+
+# Ignore package tarball (built via "mix hex.build").
+configuration-*.tar
+
+# Temporary files, for example, from tests.
+/tmp/

apps/forge/test/fixtures/project_config/README.md

@@ -0,0 +1,21 @@
+# Configuration
+
+**TODO: Add description**
+
+## Installation
+
+If [available in Hex](https://hex.pm/docs/publish), the package can be installed
+by adding `configuration` to your list of dependencies in `mix.exs`:
+
+```elixir
+def deps do
+  [
+    {:configuration, "~> 0.1.0"}
+  ]
+end
+```
+
+Documentation can be generated with [ExDoc](https://github.com/elixir-lang/ex_doc)
+and published on [HexDocs](https://hexdocs.pm). Once published, the docs can
+be found at <https://hexdocs.pm/configuration>.
+

apps/forge/test/fixtures/project_config/config/config.exs

@@ -0,0 +1,3 @@
+import Config
+
+config :elixir, :time_zone_database, Tzdata.TimeZoneDatabase

apps/forge/test/fixtures/project_config/lib/project_config.ex

@@ -0,0 +1,18 @@
+defmodule ProjectConfig do
+  @moduledoc """
+  Documentation for `ProjectConfig`.
+  """
+
+  @doc """
+  Hello world.
+
+  ## Examples
+
+      iex> ProjectConfig.hello()
+      :world
+
+  """
+  def hello do
+    :world
+  end
+end

apps/forge/test/fixtures/project_config/mix.exs

@@ -0,0 +1,25 @@
+defmodule ProjectConfig.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :project_config,
+      version: "0.1.0",
+      elixir: "~> 1.17",
+      start_permanent: Mix.env() == :prod,
+      deps: deps()
+    ]
+  end
+
+  def application do
+    [
+      extra_applications: [:logger]
+    ]
+  end
+
+  defp deps do
+    [
+      {:tzdata, "~> 1.1"}
+    ]
+  end
+end

apps/forge/test/fixtures/project_config/mix.lock

@@ -0,0 +1,11 @@
+%{
+  "certifi": {:hex, :certifi, "2.15.0", "0e6e882fcdaaa0a5a9f2b3db55b1394dba07e8d6d9bcad08318fb604c6839712", [:rebar3], [], "hexpm", "b147ed22ce71d72eafdad94f055165c1c182f61a2ff49df28bcc71d1d5b94a60"},
+  "hackney": {:hex, :hackney, "1.25.0", "390e9b83f31e5b325b9f43b76e1a785cbdb69b5b6cd4e079aa67835ded046867", [:rebar3], [{:certifi, "~> 2.15.0", [hex: :certifi, repo: "hexpm", optional: false]}, {:idna, "~> 6.1.0", [hex: :idna, repo: "hexpm", optional: false]}, {:metrics, "~> 1.0.0", [hex: :metrics, repo: "hexpm", optional: false]}, {:mimerl, "~> 1.4", [hex: :mimerl, repo: "hexpm", optional: false]}, {:parse_trans, "3.4.1", [hex: :parse_trans, repo: "hexpm", optional: false]}, {:ssl_verify_fun, "~> 1.1.0", [hex: :ssl_verify_fun, repo: "hexpm", optional: false]}, {:unicode_util_compat, "~> 0.7.1", [hex: :unicode_util_compat, repo: "hexpm", optional: false]}], "hexpm", "7209bfd75fd1f42467211ff8f59ea74d6f2a9e81cbcee95a56711ee79fd6b1d4"},
+  "idna": {:hex, :idna, "6.1.1", "8a63070e9f7d0c62eb9d9fcb360a7de382448200fbbd1b106cc96d3d8099df8d", [:rebar3], [{:unicode_util_compat, "~> 0.7.0", [hex: :unicode_util_compat, repo: "hexpm", optional: false]}], "hexpm", "92376eb7894412ed19ac475e4a86f7b413c1b9fbb5bd16dccd57934157944cea"},
+  "metrics": {:hex, :metrics, "1.0.1", "25f094dea2cda98213cecc3aeff09e940299d950904393b2a29d191c346a8486", [:rebar3], [], "hexpm", "69b09adddc4f74a40716ae54d140f93beb0fb8978d8636eaded0c31b6f099f16"},
+  "mimerl": {:hex, :mimerl, "1.4.0", "3882a5ca67fbbe7117ba8947f27643557adec38fa2307490c4c4207624cb213b", [:rebar3], [], "hexpm", "13af15f9f68c65884ecca3a3891d50a7b57d82152792f3e19d88650aa126b144"},
+  "parse_trans": {:hex, :parse_trans, "3.4.1", "6e6aa8167cb44cc8f39441d05193be6e6f4e7c2946cb2759f015f8c56b76e5ff", [:rebar3], [], "hexpm", "620a406ce75dada827b82e453c19cf06776be266f5a67cff34e1ef2cbb60e49a"},
+  "ssl_verify_fun": {:hex, :ssl_verify_fun, "1.1.7", "354c321cf377240c7b8716899e182ce4890c5938111a1296add3ec74cf1715df", [:make, :mix, :rebar3], [], "hexpm", "fe4c190e8f37401d30167c8c405eda19469f34577987c76dde613e838bbc67f8"},
+  "tzdata": {:hex, :tzdata, "1.1.3", "b1cef7bb6de1de90d4ddc25d33892b32830f907e7fc2fccd1e7e22778ab7dfbc", [:mix], [{:hackney, "~> 1.17", [hex: :hackney, repo: "hexpm", optional: false]}], "hexpm", "d4ca85575a064d29d4e94253ee95912edfb165938743dbf002acdf0dcecb0c28"},
+  "unicode_util_compat": {:hex, :unicode_util_compat, "0.7.1", "a48703a25c170eedadca83b11e88985af08d35f37c6f664d6dcfb106a97782fc", [:rebar3], [], "hexpm", "b3a917854ce3ae233619744ad1e0102e05673136776fb2fa76234f3e03b23642"},
+}