DEV: Use rspec mocks to properly verify a race condition

This is a small followup of https://github.com/discourse/discourse/pull/28124.

Gemfile

@@ -158,6 +158,8 @@ group :test, :development do
 
   gem "syntax_tree"
   gem "syntax_tree-disable_ternary"
+
+  gem "rspec-multi-mock"
 end
 
 group :development do

Gemfile.lock

@@ -426,6 +426,8 @@ GEM
     rspec-mocks (3.13.1)
       diff-lcs (>= 1.2.0, < 2.0)
       rspec-support (~> 3.13.0)
+    rspec-multi-mock (0.3.1)
+      rspec (>= 3.7.0)
     rspec-rails (6.1.3)
       actionpack (>= 6.1)
       activesupport (>= 6.1)
@@ -699,6 +701,7 @@ DEPENDENCIES
   rrule
   rspec
   rspec-html-matchers
+  rspec-multi-mock
   rspec-rails
   rss
   rswag-specs

spec/lib/cache_spec.rb

@@ -122,14 +122,16 @@ RSpec.describe Cache do
       end
     end
 
-    it "isn't prone to a race condition due to key expiring between GET calls" do
-      key = cache.normalize_key("my_key")
+    context "when there is a race condition due to key expiring between GET calls" do
+      before do
+        allow(Discourse.redis).to receive(:get).and_wrap_original do |original_method, *args|
+          original_method.call(*args).tap { Discourse.redis.del(*args) }
+        end
+      end
 
-      # while this is not technically testing the race condition, it's
-      # ensuring we're only calling redis.get once, which is a good enough proxy
-      Discourse.redis.stubs(:get).with(key).returns(Marshal.dump("bob")).once
-
-      expect(fetch_value).to eq("bob")
+      it "isn't prone to that race condition" do
+        expect(fetch_value).to eq("bob")
+      end
     end
   end
 end

spec/rails_helper.rb

@@ -186,7 +186,18 @@ end
 PER_SPEC_TIMEOUT_SECONDS = 45
 BROWSER_READ_TIMEOUT = 30
 
+# To avoid erasing `any_instance` from Mocha
+require "rspec/mocks/syntax"
+RSpec::Mocks::Syntax.singleton_class.define_method(:enable_should) { |*| nil }
+RSpec::Mocks::Syntax.singleton_class.define_method(:disable_should) { |*| nil }
+
+RSpec::Mocks::ArgumentMatchers.remove_method(:hash_including) # We’re currently relying on the version from Webmock
+
 RSpec.configure do |config|
+  config.expect_with :rspec do |c|
+    c.syntax = :expect
+  end
+
   config.fail_fast = ENV["RSPEC_FAIL_FAST"] == "1"
   config.silence_filter_announcements = ENV["RSPEC_SILENCE_FILTER_ANNOUNCEMENTS"] == "1"
   config.extend RedisSnapshotHelper
@@ -206,10 +217,18 @@ RSpec.configure do |config|
   config.include ServiceMatchers
   config.include I18nHelpers
 
-  config.mock_framework = :mocha
   config.order = "random"
   config.infer_spec_type_from_file_location!
 
+  config.mock_with :rspec do |mocks|
+    mocks.verify_partial_doubles = true
+    mocks.verify_doubled_constant_names = true
+    mocks.syntax = :expect
+  end
+  config.mock_with MultiMock::Adapter.for(:mocha, :rspec)
+
+  config.include Mocha::API
+
   if ENV["GITHUB_ACTIONS"]
     # Enable color output in GitHub Actions
     # This eventually will be `config.color_mode = :on` in RSpec 4?