# `Mailglass.Webhook.CachingBodyReader`
[🔗](https://github.com/szTheory/mailglass/blob/v0.1.0/lib/mailglass/webhook/caching_body_reader.ex#L1)

Custom `Plug.Parsers` `:body_reader` that preserves raw request
bytes in `conn.private[:raw_body]` for HMAC verification while
still allowing JSON parsing downstream.

Accumulates iodata across `{:more, _, _}` chunks and flattens on
the final `{:ok, _, _}` — required for SendGrid batch payloads up
to 128 events (~3 MB). Configure `Plug.Parsers` with
`length: 10_000_000` (10 MB cap; ~2 MB headroom over the default).

## Footgun: `Plug.Parsers.MULTIPART` does NOT honor `:body_reader`

Plug issue #884. Mailglass providers POST JSON, so this is
irrelevant for the library — but adopters adding `:multipart` to
the same parsers config will silently bypass this reader.
Documented in `guides/webhooks.md`.

## Storage location

Bytes land in `conn.private[:raw_body]`. The `conn.private` map is
library-reserved (off the adopter `assigns` contract), matching
`LatticeStripe.Webhook.CacheBodyReader` convention. Mailglass
diverges from accrue's `conn.assigns` cons-list per CONTEXT D-09
— `private` is the right boundary for library-reserved data.

## Adopter-side wiring

    # my_app_web/endpoint.ex
    plug Plug.Parsers,
      parsers: [:json],
      pass: ["*/*"],
      json_decoder: Jason,
      body_reader: {Mailglass.Webhook.CachingBodyReader, :read_body, []},
      length: 10_000_000

# `read_body`
*since 0.1.0* 

```elixir
@spec read_body(
  Plug.Conn.t(),
  keyword()
) ::
  {:ok, binary(), Plug.Conn.t()}
  | {:more, binary(), Plug.Conn.t()}
  | {:error, term()}
```

Plug `:body_reader` MFA entry point. Wraps `Plug.Conn.read_body/2`
and accumulates iodata into `conn.private[:raw_body]`.

Returns `Plug.Conn.read_body/2`-shaped tuples:
- `{:ok, body, conn}` — final chunk; `raw_body` flattened to binary
- `{:more, body, conn}` — more chunks pending; `raw_body` is iodata
- `{:error, reason}` — propagated unchanged

---

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