Write Malformed JSON in the body Plug

tanweerdev

I am trying to write a plug which will generate a custom error if the request has malformed JSON which is quite often the case in our scenarios(as we use variables in postman. eg sometimes there is no quote outside the value and it results in malformed JSON). The only help I got is Redirecting to Google Groups which isnt working of course.

defmodule PogioApi.Plug.PrepareParse do
  import Plug.Conn
  @env Application.get_env(:application_api, :env)

  def init(opts) do
    opts
  end

  def call(conn, opts) do
    %{method: method} = conn
    # TODO: check for PUT aswell
    if method in ["POST"] and not(@env in [:test]) do
      {:ok, body, _conn} = Plug.Conn.read_body(conn)
      case Jason.decode(body) do
        {:ok, _result} -> conn
        {:error, _reason} ->
          error = %{message: "Malformed JSON in the body"}
          conn
          |> put_resp_header("content-type", "application/json; charset=utf-8")
          |> send_resp(400, Jason.encode!(error))
          |> halt
      end
    else
      conn
    end
  end
end

13 comments

/phoenix #plug #json #endpoint

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2020-04-13 09:19:54 UTC

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tanweerdev

How to read and parse body properly. I know in POST, we will always get format=JSON request

Post #1

NobbZ

That will read the body, if it hasn’t been read before, eg by Plug.Parsers.

After you have read the body, you should put the parsed data somewhere in the conn, or it will be lost.

Post #2

tanweerdev

my current state of plug

defmodule PogioApi.Plug.PrepareParse do
  import Plug.Conn
  @env Application.get_env(:application_api, :env)

  def init(opts) do
    opts
  end

  def call(conn, _opts) do
    %{method: method} = conn
    if method in ["POST"] and not(@env in [:test]) do
      {:ok, body, _conn} = Plug.Conn.read_body(conn)
      case Jason.decode(body) do
        {:ok, _result} ->
          private = Map.put(conn.private, :body, body)
          %{conn | private: private}
        {:error, _reason} ->
          error = %{message: "Malformed JSON in the body"}
          conn
          |> put_resp_header("content-type", "application/json; charset=utf-8")
          |> send_resp(400, Jason.encode!(error))
          |> halt
      end
    else
      conn
    end
  end
end

I am trying to put back the body I read but not successful. Reference: How do you put a request body in a Plug.Conn? - #2 by idi527

Post #3

tanweerdev

I am calling parser after my custom plug. So I need to put back body which I have read so that nothing else got effected

Post #4

NobbZ

What do you mean by “not successful”?

Do you get an error? Is your plug not called at all? Can you give a repo with a minified, but mix phx.serverable project?

Post #5

tanweerdev

means request body is not passed to Plug.Parsers as its already been read by my custom plug

Post #6

tanweerdev

Maybe this will clear my question a little bit more

Post #7

NobbZ

Correct. Thats impossible.

There can only be one plug that calls Plug.Conn.read_body/2:

Because the request body can be of any size, reading the body will only work once, as Plug will not cache the result of these operations. If you need to access the body multiple times, it is your responsibility to store it. Finally keep in mind some plugs like Plug.Parsers may read the body, so the body may be unavailable after being accessed by such plugs.

Post #8