DEV: Disallow the use of Rails.logger= in RSpec tests (#31920)

Setting Rails.logger after the application has been initialized does not
seem to be safe anymore and can lead to flaky tests. This commit
disallows the reassignment of `Rails.logger` going forward and updates
the affected test.

### Reviewer notes

Reassigning `Rails.logger` from within RSpec tests is causing tests
which uses `Rails.logger.broadcast_to(FakeLogger.new)` to flake.
Example:
https://github.com/discourse/discourse/actions/runs/13951116847/job/39050616967

```
  1) invalid requests handles NotFound with invalid json body
     Failure/Error: expect(fake_logger.errors).to have_attributes(size: 1)
     
       expected [] to have attributes {:size => 1} but had attributes {:size => 0}
       Diff:
       @@ -1 +1 @@
       -:size => 1,
       +:size => 0,
       
     # ./spec/integration/invalid_request_spec.rb:18:in `block (2 levels) in <main>'
     # ./spec/rails_helper.rb:619:in `block (3 levels) in <top (required)>'
     # /var/www/discourse/vendor/bundle/ruby/3.3.0/gems/benchmark-0.4.0/lib/benchmark.rb:304:in `measure'
     # ./spec/rails_helper.rb:619:in `block (2 levels) in <top (required)>'
     # ./spec/rails_helper.rb:580:in `block (3 levels) in <top (required)>'
     # /var/www/discourse/vendor/bundle/ruby/3.3.0/gems/timeout-0.4.3/lib/timeout.rb:185:in `block in timeout'
     # /var/www/discourse/vendor/bundle/ruby/3.3.0/gems/timeout-0.4.3/lib/timeout.rb:192:in `timeout'
     # ./spec/rails_helper.rb:570:in `block (2 levels) in <top (required)>'
     # ./spec/rails_helper.rb:527:in `block (2 levels) in <top (required)>'
     # /var/www/discourse/vendor/bundle/ruby/3.3.0/gems/webmock-3.25.1/lib/webmock/rspec.rb:39:in `block (2 levels) in <top (required)>'
```

spec/rails_helper.rb

@@ -1000,6 +1000,10 @@ ensure
   STDOUT.unstub(:write)
 end
 
+def Rails.logger=(logger)
+  raise "Setting Rails.logger is not allowed as it can lead to unexpected behavior in tests. Use `fake_logger = track_log_messages { ... }` instead."
+end
+
 def track_log_messages
   logger = FakeLogger.new
   Rails.logger.broadcast_to(logger)

spec/requests/application_controller_spec.rb

@@ -305,13 +305,11 @@ RSpec.describe ApplicationController do
   end
 
   describe "invalid request params" do
-    before do
-      @old_logger = Rails.logger
-      @logs = StringIO.new
-      Rails.logger = Logger.new(@logs)
-    end
+    let(:fake_logger) { FakeLogger.new }
 
-    after { Rails.logger = @old_logger }
+    before { Rails.logger.broadcast_to(fake_logger) }
+
+    after { Rails.logger.stop_broadcasting_to(fake_logger) }
 
     it "should not raise a 500 (nor should it log a warning) for bad params" do
       bad_str = (+"d\xDE").force_encoding("utf-8")
@@ -321,20 +319,7 @@ RSpec.describe ApplicationController do
 
       expect(response.status).to eq(400)
 
-      log = @logs.string
-
-      if (log.include? "exception app middleware")
-        # heisentest diagnostics
-        puts
-        puts "EXTRA DIAGNOSTICS FOR INTERMITTENT TEST FAIL"
-        puts log
-        puts ">> action_dispatch.exception"
-        ex = request.env["action_dispatch.exception"]
-        puts ">> exception class: #{ex.class} : #{ex}"
-      end
-
-      expect(log).not_to include("exception app middleware")
-
+      expect(fake_logger.warnings.length).to eq(0)
       expect(response.status).to eq(400)
     end
   end

spec/requests/default_headers_spec.rb

@@ -52,13 +52,11 @@ RSpec.describe Middleware::DefaultHeaders do
   end
 
   context "when a rescued exception is raised" do
-    before do
-      @old_logger = Rails.logger
-      @logs = StringIO.new
-      Rails.logger = Logger.new(@logs)
-    end
+    let(:fake_logger) { FakeLogger.new }
 
-    after { Rails.logger = @old_logger }
+    before { Rails.logger.broadcast_to(fake_logger) }
+
+    after { Rails.logger.stop_broadcasting_to(fake_logger) }
 
     it "adds default headers to the response" do
       bad_str = (+"d\xDE").force_encoding("utf-8")
@@ -66,9 +64,7 @@ RSpec.describe Middleware::DefaultHeaders do
 
       get "/latest", params: { test: bad_str }
 
-      log = @logs.string
-      expect(log).not_to include("exception app middleware")
-
+      expect(fake_logger.warnings.length).to eq(0)
       expect(response.status).to eq(400)
       expect(response.headers).to have_key("Cross-Origin-Opener-Policy")
       expect(response.headers["Cross-Origin-Opener-Policy"]).to eq("same-origin-allow-popups")