# `Tack.Credo.Check.AsyncEnvMutation`
[🔗](https://github.com/giggleton/pica/blob/v0.1.0/dev/tack/credo/check/async_env_mutation.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 `high` and works with any version of Elixir.

## Explanation

A test file declared `async: true` does not change application config.

ExUnit's scheduling is an invariant a suite comes to lean on: every `async: true` module
runs first, then the sync modules run serially — so an async test may safely *read* config
that only sync files ever mutate.

Application env is one table for the whole VM, and the database sandbox does not reach it. A
`put_env` in an async file is therefore visible to every test running beside it for as long
as the override lasts, and the test that fails is whichever one happened to read the key in
that window — never the file that set it. `SyncTestBlocker` holds the other half of this rule
(a sync file says why it is sync); this one holds the half that breaks other files.

It flags, inside a module whose `use` carries `async: true`, a call to
`Application.put_env`, `put_all_env` or `delete_env`, or to one of the `TestEnv` helpers
(`test/support/test_env.ex`: `put_app_env`, `merge_app_env`, `delete_app_env`) — qualified
or imported, since the helper restores the value afterwards but cannot make the override
private to its test. The remedy is one of two: pass the value to the code under test as an
option, or make the file `async: false` and name the key in its header.

What it cannot see is a mutation one call away: a `test/support` stub that calls
`Application.put_env` for its caller reads as an ordinary call here. A new async file that
uses one is a review finding.

## 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*