Only use async: true for ConnCase error tests on Postgres/no-Ecto

[rhcarvalho]

Fixes a long-standing flaky test issue in generated SQLite applications where running mix test would intermittently fail with:

** (Exqlite.Error) Database busy

Recent failure in CI: #6781 (comment), https://github.com/phoenixframework/phoenix/actions/runs/30761860392/job/91533719530?pr=6781 which motivated me to go down the rabbit hole. I've ran into similar flakes in a production app generated years ago, but never bothered to root cause it until now.

Root Cause

phx.new generated error_html_test.exs and error_json_test.exs with hardcoded use ConnCase, async: true.

In apps with Ecto enabled, ConnCase includes a setup block that calls DataCase.setup_sandbox(tags). When ExUnit runs an async: true test module concurrently with async: false test modules (such as tests generated by phx.gen.auth), setup_sandbox sees tags[:async] == true and checks out a non-shared secondary connection handle (shared: false) from the Repo pool.

Because SQLite uses single-file database locking, having two active database connections opening sandbox transactions concurrently causes file lock contention whenever a synchronous test inserts records (e.g. user_fixture()). Depending on ExUnit test seeding and process scheduling, the secondary connection checkout fails with (Exqlite.Error) Database busy.

Background

When --database sqlite3 support was added in 5143696 (#4268, April 2021), error controller test templates retained hardcoded use ConnCase, async: true. Later, when phx.gen.auth introduced support for async: true in 3d609e0 (#5689, January 2024), it added a helper to conditionally append async: true ONLY for PostgreSQL adapters while leaving non-Postgres adapters as async: false. However, phx.new was not updated at the time to use the same logic for its generated error test templates.

Fix

1. Add test_case_options/1 in Phx.New.Generator matching the behavior of phx.gen.auth:

   • Ecto.Adapters.Postgres -> ", async: true"
   • nil (no Ecto) -> ", async: true"
   • non-Postgres adapters (SQLite, MySQL, TDS) -> ""

2. Update error_html_test.exs.eex and error_json_test.exs.eex templates to use <%= @test_case_options %>.

3. Added test coverage across single app and umbrella project generators for Postgres, --no-ecto, SQLite3, MySQL, and MSSQL.

[SteffenDE]

Let's gooo 🙌🏻