credo_exception_swallow

# CredoExceptionSwallow

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A [Credo](https://github.com/rrrene/credo) check to detect silent exception swallowing in Elixir rescue blocks.

Silent exception handling is a dangerous anti-pattern that:
- Hides bugs in production
- Makes debugging extremely difficult
- Creates unpredictable system behavior

This check enforces proper error handling by requiring that every rescue block either logs, reports to error monitoring, or re-raises the exception.

## Installation

Add to your `mix.exs`:

```elixir
def deps do
  [
    {:credo_exception_swallow, "~> 0.3.0", only: [:dev, :test], runtime: false}
  ]
end
```

This package extends Credo with two focused checks:

- `CredoExceptionSwallow.Checks.Warning.SilentRescue` — exceptions that get
  caught and thrown away. Covers `try/rescue` blocks and function-level rescue
  in `def`/`defp`.
- `CredoExceptionSwallow.Checks.Warning.LoggedAndDroppedError` — failures that
  never raise anything at all: an `{:error, _}` branch that logs a warning and
  then returns a success-shaped value, so the batch reports `{:ok, stats}` while
  the row is gone. Covers `case`, `with/else`, `fn` and `receive` clauses plus
  function heads that pattern-match the failure.

## Configuration

Add to your `.credo.exs` in the `checks: %{enabled: [...]}` section:

```elixir
{CredoExceptionSwallow.Checks.Warning.SilentRescue, []},
{CredoExceptionSwallow.Checks.Warning.LoggedAndDroppedError, []}
```

### Options

#### `LoggedAndDroppedError`

```elixir
{CredoExceptionSwallow.Checks.Warning.LoggedAndDroppedError, [
  # Where a dropped row hurts most. Applies everywhere if omitted.
  files: %{included: ["lib/**/sync/**", "lib/**/workers/**"]},
  # Log calls that mark a branch as "the author said this mattered"
  log_calls: ["Logger.warning", "Logger.error"],
  # Calls that count as reporting. Bare local names work too, so a project
  # whose reporting goes through a private helper can name it.
  reporting_calls: ["ErrorReporter.report_message", "report_degraded_board_type"],
  # The project's own doors out: an HTTP error response, say. Empty by default.
  propagating_calls: ["send_error"],
  # Skip test files (default: true)
  skip_test_files: true
]}
```

Not flagged: branches that report, raise, retry (including
`Process.send_after(self(), ...)`), or propagate — an error tuple, the error
under another name, or any value that still mentions what the error pattern
bound.

Known blind spot: accumulating the failure so the *caller* reports it once per
batch is the right pattern and cannot be seen one branch at a time. Mark those
with `# credo:disable-for-next-line`, which turns the false positive into a
reviewed statement of intent.

#### `SilentRescue`

```elixir
{CredoExceptionSwallow.Checks.Warning.SilentRescue, [
  # Exclude specific files (e.g., health checks)
  files: %{excluded: ["lib/my_app_web/controllers/health_controller.ex"]},
  # Set priority (:high, :normal, :low)
  priority: :high,
  # Skip test files (default: true)
  skip_test_files: true,
  # Additional acceptable function calls beyond defaults
  acceptable_calls: [
    "MyApp.ErrorHandler.report"
  ]
]}
```

### Custom Error Reporters

If your application uses a project-specific reporter facade instead of the default
`ErrorReporter.report_exception/2` naming, add it explicitly:

```elixir
{CredoExceptionSwallow.Checks.Warning.SilentRescue, [
  acceptable_calls: [
    "MyApp.ErrorReporter.capture_exception",
    "MyApp.ErrorReporter.capture_error"
  ]
]}
```

## What It Detects

### Bad Examples (will trigger warning)

```elixir
# Silent swallow - VERY BAD
try do
  risky_operation()
rescue
  _ -> nil
end

# Silent swallow with specific exception - STILL BAD
try do
  parse_data(input)
rescue
  ArgumentError -> {:error, :invalid}
end
```

### Good Examples (acceptable patterns)

```elixir
# Log the error
try do
  risky_operation()
rescue
  e ->
    Logger.error("Operation failed: #{inspect(e)}")
    {:error, :failed}
end

# Report to error monitoring
try do
  risky_operation()
rescue
  e ->
    Sentry.capture_exception(e, stacktrace: __STACKTRACE__)
    {:error, :failed}
end

# Re-raise (let it crash philosophy)
try do
  risky_operation()
rescue
  e -> reraise e, __STACKTRACE__
end
```

```elixir
# Function-level rescue with reporting
defp load_data(id) do
  Repo.get!(Data, id)
rescue
  error ->
    ErrorReporter.report_exception(error, %{context: "load_data"})
    []
end
```

## Default Acceptable Calls

The following function calls are considered proper error handling:

- `Logger.error/1,2`
- `Logger.warning/1,2`
- `Logger.warn/1,2`
- `Logger.info/1,2`
- `Logger.debug/1,2`
- `ErrorReporter.report_exception/1,2`
- `ErrorReporter.report_message/1,2`
- `Sentry.capture_exception/1,2`
- `Sentry.capture_message/1,2`
- `reraise/2`
- `raise/1,2`

Project-specific reporters can be added through `acceptable_calls`.

## License

MIT License - see [LICENSE](LICENSE) file.