Raise InvalidKeyError if key is unknown/invalid

lib/gitlab/application_rate_limiter.rb

@@ -11,6 +11,8 @@ module Gitlab
   #   redirect_to(edit_project_path(@project), status: :too_many_requests)
   # end
   class ApplicationRateLimiter
+    InvalidKeyError = Class.new(StandardError)
+
     def initialize(key, **options)
       @key = key
       @options = options
@@ -69,7 +71,7 @@ module Gitlab
       #
       # @return [Boolean] Whether or not a request should be throttled
       def throttled?(key, **options)
-        return unless rate_limits[key]
+        raise InvalidKeyError unless rate_limits[key]
 
         return if scoped_user_in_allowlist?(options)
 

spec/lib/gitlab/application_rate_limiter_spec.rb

@@ -50,6 +50,36 @@ RSpec.describe Gitlab::ApplicationRateLimiter do
   end
 
   describe '.throttled?' do
+    context 'when the key is invalid' do
+      context 'is provided as a Symbol' do
+        context 'but is not defined in the rate_limits Hash' do
+          it 'raises an InvalidKeyError exception' do
+            key = :key_not_in_rate_limits_hash
+
+            expect { subject.throttled?(key) }.to raise_error(Gitlab::ApplicationRateLimiter::InvalidKeyError)
+          end
+        end
+      end
+
+      context 'is provided as a String' do
+        context 'and is a String representation of an existing key in rate_limits Hash' do
+          it 'raises an InvalidKeyError exception' do
+            key = rate_limits.keys[0].to_s
+
+            expect { subject.throttled?(key) }.to raise_error(Gitlab::ApplicationRateLimiter::InvalidKeyError)
+          end
+        end
+
+        context 'but is not defined in any form the rate_limits Hash' do
+          it 'raises an InvalidKeyError exception' do
+            key = 'key_not_in_rate_limits_hash'
+
+            expect { subject.throttled?(key) }.to raise_error(Gitlab::ApplicationRateLimiter::InvalidKeyError)
+          end
+        end
+      end
+    end
+
     context 'when the key is an array of only ActiveRecord models' do
       let(:scope) { [user, project] }