Mailglass.Webhook.CachingBodyReader (Mailglass v0.1.0)

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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

Summary

Functions

read_body(conn, opts)

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

Functions

read_body(conn, opts)

(since 0.1.0) 
@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