Content type check

lib/srh/http/base_router.ex

@@ -5,6 +5,7 @@ defmodule Srh.Http.BaseRouter do
   alias Srh.Http.ResultEncoder
 
   plug(:match)
+  plug(Srh.Http.ContentTypeCheckPlug)
   plug(Plug.Parsers, parsers: [:json], pass: ["application/json"], json_decoder: Jason)
   plug(:dispatch)
 

lib/srh/http/content_type_check_plug.ex

@@ -0,0 +1,42 @@
+defmodule Srh.Http.ContentTypeCheckPlug do
+  import Plug.Conn
+
+  def init(opts), do: opts
+
+  def call(conn, _opts) do
+    # Only parse for POST, PUT, PATCH, and DELETE requests, which is what Plug.Parsers does
+    case conn.method do
+      "POST" ->
+        check_content_type(conn)
+
+      "PUT" ->
+        check_content_type(conn)
+
+      "PATCH" ->
+        check_content_type(conn)
+
+      "DELETE" ->
+        check_content_type(conn)
+
+      # All other methods can proceed
+      _ ->
+        conn
+    end
+  end
+
+  defp check_content_type(conn) do
+    case get_req_header(conn, "content-type") do
+      ["application/json"] ->
+        # Proceed, this is the valid content type for SRH
+        conn
+
+      # Either missing, or a type that we don't support
+      other ->
+        # Return a custom error, ensuring the same format as the other errors
+        conn
+        |> put_resp_content_type("application/json")
+        |> send_resp(400, Jason.encode!(%{error: "Invalid content type. Expected application/json."}))
+        |> halt()
+    end
+  end
+end