Fix non-string "_method" in Plug.MethodOverride

lib/plug/method_override.ex

@@ -9,14 +9,33 @@ defmodule Plug.MethodOverride do
     * `PATCH`
     * `DELETE`
 
-  This plug expects the body parameters to be already parsed and
-  fetched. Those can be fetched with `Plug.Parsers`.
+  This plug only replaces the request method if the `_method` request
+  parameter is a string. If the `_method` request parameter is not a string,
+  the request method is not changed.
+
+  > #### Parse Body Parameters First {: .info}
+  >
+  > This plug expects the body parameters to be **already fetched and
+  > parsed**. Those can be fetched with `Plug.Parsers`.
 
   This plug doesn't accept any options.
 
-  ## Examples
+  To recap, here are all the conditions that the request must meet in order
+  for this plug to replace the `:method` field in the `Plug.Conn`:
+
+    1. The conn's request `:method` must be `POST`.
+    1. The conn's `:body_params` must have been fetched already (for example,
+       with `Plug.Parsers`).
+    1. The conn's `:body_params` must have a `_method` field that is a string
+       and whose value is `"PUT"`, `"PATCH"`, or `"DELETE"` (case insensitive).
+
+  ## Usage
+
+      # You'll need to fetch and parse parameters first, for example:
+      # plug Plug.Parsers, parsers: [:urlencoded, :multipart, :json]
 
       plug Plug.MethodOverride
+
   """
 
   @behaviour Plug
@@ -39,12 +58,14 @@ defmodule Plug.MethodOverride do
   end
 
   defp override_method(conn, body_params) do
-    method = String.upcase(body_params["_method"] || "", :ascii)
+    # dbg(String.upcase(body_params["_method"] || "", :ascii))
 
-    if method in @allowed_methods do
-      %{conn | method: method}
+    with method when is_binary(method) <- body_params["_method"] || "",
+         method = String.upcase(method, :ascii),
+         true <- method in @allowed_methods do
+      %Plug.Conn{conn | method: method}
     else
-      conn
+      _ -> conn
     end
   end
 end

test/plug/method_override_test.exs

@@ -18,6 +18,18 @@ defmodule Plug.MethodOverrideTest do
     assert conn.method == "POST"
   end
 
+  # This can happen with JSON bodies that have _method as something other than a string.
+  test "ignores non-string _method in the body" do
+    # We don't depend on any JSON library here, so we just shove the "parsed" JSON
+    # in the :body_params directly.
+    conn =
+      conn(:post, "/", "")
+      |> Map.put(:body_params, %{"_method" => ["put"]})
+      |> Plug.run([{Plug.MethodOverride, []}])
+
+    assert conn.method == "POST"
+  end
+
   test "converts POST to DELETE when _method=DELETE param is specified" do
     conn = call(urlencoded_conn(:post, "_method=DELETE"))
     assert conn.method == "DELETE"