# `Tack.Credo.Check.SyncTestBlocker`
[🔗](https://github.com/giggleton/pica/blob/v0.1.0/dev/tack/credo/check/sync_test_blocker.ex#L1)

## Basics

> #### This check is disabled by default. {: .neutral}
>
> [Learn how to enable it](`e:credo:config_file.html#checks`) via `.credo.exs`.

This check has a base priority of `normal` and works with any version of Elixir.

## Explanation

A test file that opts out of `async: true` says in its header what stops it.

`elixir-code-review`, `references/test-quality.md`:

> `async:` is explicit in every file, and **`async: true` is the default everywhere** — the
> database sandbox isolates DB tests. A file is `async: false` only for a blocker the sandbox
> can't isolate, named in a header comment: `Application.put_env`, an app-wide singleton or
> whole-VM absence assertion, or a process whose start-up reads the database outside the
> test's sandbox ownership. A new sync file without a stated blocker is a finding.

Sync is not free, and that is why the reason is worth writing down. Every `async: true`
module runs before the sync ones, and a suite comes to lean on that: an async test may read
config that only sync files mutate. So a file that goes sync without a blocker slows the
suite for nothing, and one that goes async while mutating config breaks readers somewhere
else entirely — the two mistakes are not symmetric, and neither is visible at the file you
are reading.

The check reads the lines between `defmodule` and the `use` line and asks only whether a
comment or a `@moduledoc` is there. Whether the stated blocker is the *real* one is a review
judgment (and an empirical one — flip the file to `async: true` and see what fails).

## Check-Specific Parameters

*There are no specific parameters for this check.*

## General Parameters

Like with all checks, [general params](`e:credo:check_params.html`) can be applied.

Parameters can be configured via the [`.credo.exs` config file](`e:credo:config_file.html`).

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*Consult [api-reference.md](api-reference.md) for complete listing*