Deep duplicate in deep_merge so no references remain to the original hash in the result

[MarcelEeken]

Summary

The documentation for deep_merge says that a new hash is returned. But when deep merging a Hash with a nested Hash, the nested Hash is actually a reference to the original nested Hash, only a shallow copy is created.

This can lead to unexpected modifications when the nested Hash of the merged result gets modified.

  x = { a: { b: "foo" } }
  y = { d: { e: "bar" } }

  z = x.deep_merge(y)
  # z => { a: { b: "foo" }, d: { e: "bar" } }

  z[:a][:b] = "baz"
  # z => { a: { b: "baz" }, d: { e: "bar" } }
  # x => { a: { b: "baz" } }

Other Information

Reproduce case:

# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  # Activate the gem you are reporting the issue against.
  # gem "activesupport", "6.1.0"
  gem "rails", github: "rails/rails", branch: "main"
end

require "active_support"
require "active_support/core_ext/object/blank"
require "minitest/autorun"

class BugTest < Minitest::Test
  def test_stuff
    x = { a: { b: "foo" } }
    y = { d: { e: "bar" } }

    z = x.deep_merge(y)
    z[:a][:b] = "baz"

    assert_equal "foo", x[:a][:b], "Changing 'z' should not change 'x'"
  end
end

[casperisfine]

Hum, this is breaking things in our test suite:

>> {foo: Object}.deep_merge({})
=> {:foo=>#<Class:0x00007f8f96a66da0>}

I think we should only dup hashes, not their content. I'll submit a PR.