FIX: stop logging every time invalid params are sent

Previously we were logging warning for invalid encoded params, this can
cause a log flood

app/controllers/application_controller.rb

@@ -252,7 +252,11 @@ class ApplicationController < ActionController::Base
     if show_json_errors
       # HACK: do not use render_json_error for topics#show
       if request.params[:controller] == 'topics' && request.params[:action] == 'show'
-        return render status: status_code, layout: false, plain: (status_code == 404 || status_code == 410) ? build_not_found_page(status_code) : message
+        return render(
+          status: status_code,
+          layout: false,
+          plain: (status_code == 404 || status_code == 410) ? build_not_found_page(status_code) : message
+        )
       end
 
       render_json_error message, type: type, status: status_code

lib/middleware/discourse_public_exceptions.rb

@@ -16,6 +16,10 @@ module Middleware
       exception = env["action_dispatch.exception"]
       response = ActionDispatch::Response.new
 
+      # Special handling for invalid params, in this case we can not re-dispatch
+      # the Request object has a "broken" .params which can not be accessed
+      exception = nil if Rack::QueryParser::InvalidParameterError === exception
+
       if exception
         begin
           fake_controller = ApplicationController.new

spec/requests/application_controller_spec.rb

@@ -15,6 +15,33 @@ RSpec.describe ApplicationController do
     end
   end
 
+  describe 'invalid request params' do
+    before do
+      @old_logger = Rails.logger
+      @logs = StringIO.new
+      Rails.logger = Logger.new(@logs)
+    end
+
+    after do
+      Rails.logger = @old_logger
+    end
+
+    it 'should not raise a 500 (nor should it log a warning) for bad params' do
+      bad_str = "d\xDE".force_encoding('utf-8')
+      expect(bad_str.valid_encoding?).to eq(false)
+
+      get "/latest.json", params: { test: bad_str }
+
+      expect(response.status).to eq(400)
+      expect(@logs.string).not_to include('exception app middleware')
+
+      expect(JSON.parse(response.body)).to eq(
+        "status" => 400,
+        "error" => "Bad Request"
+      )
+    end
+  end
+
   describe 'build_not_found_page' do
     describe 'topic not found' do