Track and act upon the number of executed queries

This ensures that we have more visibility in the number of SQL queries
that are executed in web requests. The current threshold is hardcoded to
100 as we will rarely (maybe once or twice) change it.

In production and development we use Sentry if enabled, in the test
environment we raise an error. This feature is also only enabled in
production/staging when running on GitLab.com as it's not very useful to
other users.

app/controllers/admin/services_controller.rb

@@ -1,6 +1,7 @@
 class Admin::ServicesController < Admin::ApplicationController
   include ServiceParams
 
+  before_action :whitelist_query_limiting, only: [:index]
   before_action :service, only: [:edit, :update]
 
   def index
@@ -37,4 +38,8 @@ class Admin::ServicesController < Admin::ApplicationController
   def service
     @service ||= Service.where(id: params[:id], template: true).first
   end
+
+  def whitelist_query_limiting
+    Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42430')
+  end
 end

app/controllers/boards/issues_controller.rb

@@ -2,6 +2,7 @@ module Boards
   class IssuesController < Boards::ApplicationController
     include BoardsResponses
 
+    before_action :whitelist_query_limiting, only: [:index, :update]
     before_action :authorize_read_issue, only: [:index]
     before_action :authorize_create_issue, only: [:create]
     before_action :authorize_update_issue, only: [:update]
@@ -92,5 +93,10 @@ module Boards
         }
       )
     end
+
+    def whitelist_query_limiting
+      # Also see https://gitlab.com/gitlab-org/gitlab-ce/issues/42439
+      Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42428')
+    end
   end
 end

app/controllers/import/gitlab_projects_controller.rb

@@ -1,4 +1,5 @@
 class Import::GitlabProjectsController < Import::BaseController
+  before_action :whitelist_query_limiting, only: [:create]
   before_action :verify_gitlab_project_import_enabled
 
   def new
@@ -40,4 +41,8 @@ class Import::GitlabProjectsController < Import::BaseController
       :path, :namespace_id, :file
     )
   end
+
+  def whitelist_query_limiting
+    Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42437')
+  end
 end

app/controllers/projects/commits_controller.rb

@@ -4,6 +4,7 @@ class Projects::CommitsController < Projects::ApplicationController
   include ExtractsPath
   include RendersCommits
 
+  before_action :whitelist_query_limiting
   before_action :require_non_empty_project
   before_action :assign_ref_vars
   before_action :authorize_download_code!
@@ -65,4 +66,8 @@ class Projects::CommitsController < Projects::ApplicationController
     @commits = @commits.with_pipeline_status
     @commits = prepare_commits_for_rendering(@commits)
   end
+
+  def whitelist_query_limiting
+    Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42330')
+  end
 end

app/controllers/projects/cycle_analytics_controller.rb

@@ -3,6 +3,7 @@ class Projects::CycleAnalyticsController < Projects::ApplicationController
   include ActionView::Helpers::TextHelper
   include CycleAnalyticsParams
 
+  before_action :whitelist_query_limiting, only: [:show]
   before_action :authorize_read_cycle_analytics!
 
   def show
@@ -31,4 +32,8 @@ class Projects::CycleAnalyticsController < Projects::ApplicationController
       permissions: @cycle_analytics.permissions(user: current_user)
     }
   end
+
+  def whitelist_query_limiting
+    Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42671')
+  end
 end

app/controllers/projects/forks_controller.rb

@@ -2,6 +2,7 @@ class Projects::ForksController < Projects::ApplicationController
   include ContinueParams
 
   # Authorize
+  before_action :whitelist_query_limiting, only: [:create]
   before_action :require_non_empty_project
   before_action :authorize_download_code!
   before_action :authenticate_user!, only: [:new, :create]
@@ -54,4 +55,8 @@ class Projects::ForksController < Projects::ApplicationController
       render :error
     end
   end
+
+  def whitelist_query_limiting
+    Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42335')
+  end
 end

app/controllers/projects/issues_controller.rb

@@ -8,6 +8,7 @@ class Projects::IssuesController < Projects::ApplicationController
 
   prepend_before_action :authenticate_user!, only: [:new]
 
+  before_action :whitelist_query_limiting, only: [:create, :create_merge_request, :move, :bulk_update]
   before_action :check_issues_available!
   before_action :issue, except: [:index, :new, :create, :bulk_update]
   before_action :set_issuables_index, only: [:index]
@@ -247,4 +248,13 @@ class Projects::IssuesController < Projects::ApplicationController
     @finder_type = IssuesFinder
     super
   end
+
+  def whitelist_query_limiting
+    # Also see the following issues:
+    #
+    # 1. https://gitlab.com/gitlab-org/gitlab-ce/issues/42423
+    # 2. https://gitlab.com/gitlab-org/gitlab-ce/issues/42424
+    # 3. https://gitlab.com/gitlab-org/gitlab-ce/issues/42426
+    Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42422')
+  end
 end

app/controllers/projects/merge_requests/creations_controller.rb

@@ -4,6 +4,7 @@ class Projects::MergeRequests::CreationsController < Projects::MergeRequests::Ap
   include RendersCommits
 
   skip_before_action :merge_request
+  before_action :whitelist_query_limiting, only: [:create]
   before_action :authorize_create_merge_request!
   before_action :apply_diff_view_cookie!, only: [:diffs, :diff_for_path]
   before_action :build_merge_request, except: [:create]
@@ -125,4 +126,8 @@ class Projects::MergeRequests::CreationsController < Projects::MergeRequests::Ap
       @project.forked_from_project
     end
   end
+
+  def whitelist_query_limiting
+    Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42384')
+  end
 end

app/controllers/projects/merge_requests_controller.rb

@@ -7,6 +7,7 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
   include IssuableCollections
 
   skip_before_action :merge_request, only: [:index, :bulk_update]
+  before_action :whitelist_query_limiting, only: [:assign_related_issues, :update]
   before_action :authorize_update_issuable!, only: [:close, :edit, :update, :remove_wip, :sort]
   before_action :set_issuables_index, only: [:index]
   before_action :authenticate_user!, only: [:assign_related_issues]
@@ -339,4 +340,9 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
 
     access_denied! unless access_check
   end
+
+  def whitelist_query_limiting
+    # Also see https://gitlab.com/gitlab-org/gitlab-ce/issues/42441
+    Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42438')
+  end
 end

app/controllers/projects/network_controller.rb

@@ -2,6 +2,7 @@ class Projects::NetworkController < Projects::ApplicationController
   include ExtractsPath
   include ApplicationHelper
 
+  before_action :whitelist_query_limiting
   before_action :require_non_empty_project
   before_action :assign_ref_vars
   before_action :authorize_download_code!
@@ -35,4 +36,8 @@ class Projects::NetworkController < Projects::ApplicationController
     @options[:extended_sha1] = params[:extended_sha1]
     @commit = @repo.commit(@options[:extended_sha1])
   end
+
+  def whitelist_query_limiting
+    Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42333')
+  end
 end

app/controllers/projects/notes_controller.rb

@@ -2,6 +2,7 @@ class Projects::NotesController < Projects::ApplicationController
   include NotesActions
   include ToggleAwardEmoji
 
+  before_action :whitelist_query_limiting, only: [:create]
   before_action :authorize_read_note!
   before_action :authorize_create_note!, only: [:create]
   before_action :authorize_resolve_note!, only: [:resolve, :unresolve]
@@ -79,4 +80,8 @@ class Projects::NotesController < Projects::ApplicationController
 
     access_denied! unless can?(current_user, :create_note, noteable)
   end
+
+  def whitelist_query_limiting
+    Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42383')
+  end
 end

app/controllers/projects/pipelines_controller.rb

@@ -1,4 +1,5 @@
 class Projects::PipelinesController < Projects::ApplicationController
+  before_action :whitelist_query_limiting, only: [:create, :retry]
   before_action :pipeline, except: [:index, :new, :create, :charts]
   before_action :commit, only: [:show, :builds, :failures]
   before_action :authorize_read_pipeline!
@@ -166,4 +167,9 @@ class Projects::PipelinesController < Projects::ApplicationController
   def commit
     @commit ||= @pipeline.commit
   end
+
+  def whitelist_query_limiting
+    # Also see https://gitlab.com/gitlab-org/gitlab-ce/issues/42343
+    Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42339')
+  end
 end

app/controllers/projects_controller.rb

@@ -3,6 +3,7 @@ class ProjectsController < Projects::ApplicationController
   include ExtractsPath
   include PreviewMarkdown
 
+  before_action :whitelist_query_limiting, only: [:create]
   before_action :authenticate_user!, except: [:index, :show, :activity, :refs]
   before_action :redirect_git_extension, only: [:show]
   before_action :project, except: [:index, :new, :create]
@@ -405,4 +406,8 @@ class ProjectsController < Projects::ApplicationController
     #
     redirect_to request.original_url.sub(%r{\.git/?\Z}, '') if params[:format] == 'git'
   end
+
+  def whitelist_query_limiting
+    Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42440')
+  end
 end

app/controllers/registrations_controller.rb

@@ -1,6 +1,8 @@
 class RegistrationsController < Devise::RegistrationsController
   include Recaptcha::Verify
 
+  before_action :whitelist_query_limiting, only: [:destroy]
+
   def new
     redirect_to(new_user_session_path)
   end
@@ -83,4 +85,8 @@ class RegistrationsController < Devise::RegistrationsController
   def devise_mapping
     @devise_mapping ||= Devise.mappings[:user]
   end
+
+  def whitelist_query_limiting
+    Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42380')
+  end
 end

changelogs/unreleased/query-counts.yml

@@ -0,0 +1,5 @@
+---
+title: Track and act upon the number of executed queries
+merge_request:
+author:
+type: added

config/initializers/query_limiting.rb

@@ -0,0 +1,9 @@
+if Gitlab::QueryLimiting.enable?
+  require_dependency 'gitlab/query_limiting/active_support_subscriber'
+  require_dependency 'gitlab/query_limiting/transaction'
+  require_dependency 'gitlab/query_limiting/middleware'
+
+  Gitlab::Application.configure do |config|
+    config.middleware.use(Gitlab::QueryLimiting::Middleware)
+  end
+end

doc/development/README.md

@@ -75,6 +75,7 @@ comments: false
 - [Ordering table columns](ordering_table_columns.md)
 - [Verifying database capabilities](verifying_database_capabilities.md)
 - [Database Debugging and Troubleshooting](database_debugging.md)
+- [Query Count Limits](query_count_limits.md)
 
 ## Testing guides
 

doc/development/query_count_limits.md

@@ -0,0 +1,65 @@
+# Query Count Limits
+
+Each controller or API endpoint is allowed to execute up to 100 SQL queries. In
+a production environment we'll only log an error in case this threshold is
+exceeded, but in a test environment we'll raise an error instead.
+
+## Solving Failing Tests
+
+When a test fails because it executes more than 100 SQL queries there are two
+solutions to this problem:
+
+1. Reduce the number of SQL queries that are executed.
+2. Whitelist the controller or API endpoint.
+
+You should only resort to whitelisting when an existing controller or endpoint
+is to blame as in this case reducing the number of SQL queries can take a lot of
+effort. Newly added controllers and endpoints are not allowed to execute more
+than 100 SQL queries and no exceptions will be made for this rule. _If_ a large
+number of SQL queries is necessary to perform certain work it's best to have
+this work performed by Sidekiq instead of doing this directly in a web request.
+
+## Whitelisting
+
+In the event that you _have_ to whitelist a controller you'll first need to
+create an issue. This issue should (preferably in the title) mention the
+controller or endpoint and include the appropriate labels (`database`,
+`performance`, and at least a team specific label such as `Discussion`).
+
+Once the issue has been created you can whitelist the code in question. For
+Rails controllers it's best to create a `before_action` hook that runs as early
+as possible. The called method in turn should call
+`Gitlab::QueryLimiting.whitelist('issue URL here')`. For example:
+
+```ruby
+class MyController < ApplicationController
+  before_action :whitelist_query_limiting, only: [:show]
+
+  def index
+    # ...
+  end
+
+  def show
+    # ...
+  end
+
+  def whitelist_query_limiting
+    Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/...')
+  end
+end
+```
+
+By using a `before_action` you don't have to modify the controller method in
+question, reducing the likelihood of merge conflicts.
+
+For Grape API endpoints there unfortunately is not a reliable way of running a
+hook before a specific endpoint. This means that you have to add the whitelist
+call directly into the endpoint like so:
+
+```ruby
+get '/projects/:id/foo' do
+  Gitlab::QueryLimiting.whitelist('...')
+
+  # ...
+end
+```

lib/api/branches.rb

@@ -29,6 +29,8 @@ module API
         use :pagination
       end
       get ':id/repository/branches' do
+        Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42329')
+
         repository = user_project.repository
         branches = ::Kaminari.paginate_array(repository.branches.sort_by(&:name))
         merged_branch_names = repository.merged_branch_names(branches.map(&:name))

lib/api/issues.rb

@@ -161,6 +161,8 @@ module API
         use :issue_params
       end
       post ':id/issues' do
+        Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42320')
+
         authorize! :create_issue, user_project
 
         # Setting created_at time only allowed for admins and project owners
@@ -201,6 +203,8 @@ module API
                         :labels, :created_at, :due_date, :confidential, :state_event
       end
       put ':id/issues/:issue_iid' do
+        Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42322')
+
         issue = user_project.issues.find_by!(iid: params.delete(:issue_iid))
         authorize! :update_issue, issue
 
@@ -234,6 +238,8 @@ module API
         requires :to_project_id, type: Integer, desc: 'The ID of the new project'
       end
       post ':id/issues/:issue_iid/move' do
+        Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42323')
+
         issue = user_project.issues.find_by(iid: params[:issue_iid])
         not_found!('Issue') unless issue
 

lib/api/merge_requests.rb

@@ -152,6 +152,8 @@ module API
         use :optional_params
       end
       post ":id/merge_requests" do
+        Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42316')
+
         authorize! :create_merge_request, user_project
 
         mr_params = declared_params(include_missing: false)
@@ -256,6 +258,8 @@ module API
         at_least_one_of(*at_least_one_of_ce)
       end
       put ':id/merge_requests/:merge_request_iid' do
+        Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42318')
+
         merge_request = find_merge_request_with_access(params.delete(:merge_request_iid), :update_merge_request)
 
         mr_params = declared_params(include_missing: false)
@@ -283,6 +287,8 @@ module API
         optional :sha, type: String, desc: 'When present, must have the HEAD SHA of the source branch'
       end
       put ':id/merge_requests/:merge_request_iid/merge' do
+        Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42317')
+
         merge_request = find_project_merge_request(params[:merge_request_iid])
         merge_when_pipeline_succeeds = to_boolean(params[:merge_when_pipeline_succeeds])
 

lib/api/pipelines.rb

@@ -42,6 +42,8 @@ module API
         requires :ref, type: String,  desc: 'Reference'
       end
       post ':id/pipeline' do
+        Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42124')
+
         authorize! :create_pipeline, user_project
 
         new_pipeline = Ci::CreatePipelineService.new(user_project,

lib/api/projects.rb

@@ -210,6 +210,8 @@ module API
         optional :namespace, type: String, desc: 'The ID or name of the namespace that the project will be forked into'
       end
       post ':id/fork' do
+        Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42284')
+
         fork_params = declared_params(include_missing: false)
         namespace_id = fork_params[:namespace]
 

lib/api/triggers.rb

@@ -15,6 +15,8 @@ module API
         optional :variables, type: Hash, desc: 'The list of variables to be injected into build'
       end
       post ":id/(ref/:ref/)trigger/pipeline", requirements: { ref: /.+/ } do
+        Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42283')
+
         # validate variables
         params[:variables] = params[:variables].to_h
         unless params[:variables].all? { |key, value| key.is_a?(String) && value.is_a?(String) }

lib/api/users.rb

@@ -383,6 +383,8 @@ module API
         optional :hard_delete, type: Boolean, desc: "Whether to remove a user's contributions"
       end
       delete ":id" do
+        Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42279')
+
         authenticated_as_admin!
 
         user = User.find_by(id: params[:id])

lib/api/v3/branches.rb

@@ -14,6 +14,8 @@ module API
           success ::API::Entities::Branch
         end
         get ":id/repository/branches" do
+          Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42276')
+
           repository = user_project.repository
           branches = repository.branches.sort_by(&:name)
           merged_branch_names = repository.merged_branch_names(branches.map(&:name))

lib/api/v3/issues.rb

@@ -134,6 +134,8 @@ module API
           use :issue_params
         end
         post ':id/issues' do
+          Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42131')
+
           # Setting created_at time only allowed for admins and project owners
           unless current_user.admin? || user_project.owner == current_user
             params.delete(:created_at)
@@ -169,6 +171,8 @@ module API
                           :labels, :created_at, :due_date, :confidential, :state_event
         end
         put ':id/issues/:issue_id' do
+          Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42132')
+
           issue = user_project.issues.find(params.delete(:issue_id))
           authorize! :update_issue, issue
 
@@ -201,6 +205,8 @@ module API
           requires :to_project_id, type: Integer, desc: 'The ID of the new project'
         end
         post ':id/issues/:issue_id/move' do
+          Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42133')
+
           issue = user_project.issues.find_by(id: params[:issue_id])
           not_found!('Issue') unless issue
 

lib/api/v3/merge_requests.rb

@@ -91,6 +91,8 @@ module API
           use :optional_params
         end
         post ":id/merge_requests" do
+          Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42126')
+
           authorize! :create_merge_request, user_project
 
           mr_params = declared_params(include_missing: false)
@@ -167,6 +169,8 @@ module API
                             :remove_source_branch
           end
           put path do
+            Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42127')
+
             merge_request = find_merge_request_with_access(params.delete(:merge_request_id), :update_merge_request)
 
             mr_params = declared_params(include_missing: false)

lib/api/v3/pipelines.rb

@@ -19,6 +19,8 @@ module API
                               desc: 'Either running, branches, or tags'
         end
         get ':id/pipelines' do
+          Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42123')
+
           authorize! :read_pipeline, user_project
 
           pipelines = PipelinesFinder.new(user_project, scope: params[:scope]).execute

lib/api/v3/triggers.rb

@@ -16,6 +16,8 @@ module API
           optional :variables, type: Hash, desc: 'The list of variables to be injected into build'
         end
         post ":id/(ref/:ref/)trigger/builds", requirements: { ref: /.+/ } do
+          Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42121')
+
           # validate variables
           params[:variables] = params[:variables].to_h
           unless params[:variables].all? { |key, value| key.is_a?(String) && value.is_a?(String) }

lib/gitlab/query_limiting.rb

@@ -0,0 +1,36 @@
+module Gitlab
+  module QueryLimiting
+    # Returns true if we should enable tracking of query counts.
+    #
+    # This is only enabled in production/staging if we're running on GitLab.com.
+    # This ensures we don't produce any errors that users can't do anything
+    # about themselves.
+    def self.enable?
+      Gitlab.com? || Rails.env.development? || Rails.env.test?
+    end
+
+    # Allows the current request to execute any number of SQL queries.
+    #
+    # This method should _only_ be used when there's a corresponding issue to
+    # reduce the number of queries.
+    #
+    # The issue URL is only meant to push developers into creating an issue
+    # instead of blindly whitelisting offending blocks of code.
+    def self.whitelist(issue_url)
+      return unless enable_whitelist?
+
+      unless issue_url.start_with?('https://')
+        raise(
+          ArgumentError,
+          'You must provide a valid issue URL in order to whitelist a block of code'
+        )
+      end
+
+      Transaction&.current&.whitelisted = true
+    end
+
+    def self.enable_whitelist?
+      Rails.env.development? || Rails.env.test?
+    end
+  end
+end

lib/gitlab/query_limiting/active_support_subscriber.rb

@@ -0,0 +1,11 @@
+module Gitlab
+  module QueryLimiting
+    class ActiveSupportSubscriber < ActiveSupport::Subscriber
+      attach_to :active_record
+
+      def sql(*)
+        Transaction.current&.increment
+      end
+    end
+  end
+end

lib/gitlab/query_limiting/middleware.rb

@@ -0,0 +1,55 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module QueryLimiting
+    # Middleware for reporting (or raising) when a request performs more than a
+    # certain amount of database queries.
+    class Middleware
+      CONTROLLER_KEY = 'action_controller.instance'.freeze
+      ENDPOINT_KEY = 'api.endpoint'.freeze
+
+      def initialize(app)
+        @app = app
+      end
+
+      def call(env)
+        transaction, retval = Transaction.run do
+          @app.call(env)
+        end
+
+        transaction.action = action_name(env)
+        transaction.act_upon_results
+
+        retval
+      end
+
+      def action_name(env)
+        if env[CONTROLLER_KEY]
+          action_for_rails(env)
+        elsif env[ENDPOINT_KEY]
+          action_for_grape(env)
+        end
+      end
+
+      private
+
+      def action_for_rails(env)
+        controller = env[CONTROLLER_KEY]
+        action = "#{controller.class.name}##{controller.action_name}"
+
+        if controller.content_type == 'text/html'
+          action
+        else
+          "#{action} (#{controller.content_type})"
+        end
+      end
+
+      def action_for_grape(env)
+        endpoint = env[ENDPOINT_KEY]
+        route = endpoint.route rescue nil
+
+        "#{route.request_method} #{route.path}" if route
+      end
+    end
+  end
+end

lib/gitlab/query_limiting/transaction.rb

@@ -0,0 +1,83 @@
+module Gitlab
+  module QueryLimiting
+    class Transaction
+      THREAD_KEY = :__gitlab_query_counts_transaction
+
+      attr_accessor :count, :whitelisted
+
+      # The name of the action (e.g. `UsersController#show`) that is being
+      # executed.
+      attr_accessor :action
+
+      # The maximum number of SQL queries that can be executed in a request. For
+      # the sake of keeping things simple we hardcode this value here, it's not
+      # supposed to be changed very often anyway.
+      THRESHOLD = 100
+
+      # Error that is raised whenever exceeding the maximum number of queries.
+      ThresholdExceededError = Class.new(StandardError)
+
+      def self.current
+        Thread.current[THREAD_KEY]
+      end
+
+      # Starts a new transaction and returns it and the blocks' return value.
+      #
+      # Example:
+      #
+      #     transaction, retval = Transaction.run do
+      #       10
+      #     end
+      #
+      #     retval # => 10
+      def self.run
+        transaction = new
+        Thread.current[THREAD_KEY] = transaction
+
+        [transaction, yield]
+      ensure
+        Thread.current[THREAD_KEY] = nil
+      end
+
+      def initialize
+        @action = nil
+        @count = 0
+        @whitelisted = false
+      end
+
+      # Sends a notification based on the number of executed SQL queries.
+      def act_upon_results
+        return unless threshold_exceeded?
+
+        error = ThresholdExceededError.new(error_message)
+
+        if raise_error?
+          raise(error)
+        else
+          # Raven automatically logs to the Rails log if disabled, thus we don't
+          # need to manually log anything in case Sentry support is not enabled.
+          Raven.capture_exception(error)
+        end
+      end
+
+      def increment
+        @count += 1 unless whitelisted
+      end
+
+      def raise_error?
+        Rails.env.test?
+      end
+
+      def threshold_exceeded?
+        count > THRESHOLD
+      end
+
+      def error_message
+        header = 'Too many SQL queries were executed'
+        header += " in #{action}" if action
+
+        "#{header}: a maximum of #{THRESHOLD} is allowed but #{count} SQL queries were executed"
+      end
+    end
+  end
+end

spec/lib/gitlab/query_limiting/active_support_subscriber_spec.rb

@@ -0,0 +1,19 @@
+require 'spec_helper'
+
+describe Gitlab::QueryLimiting::ActiveSupportSubscriber do
+  describe '#sql' do
+    it 'increments the number of executed SQL queries' do
+      transaction = double(:transaction)
+
+      allow(Gitlab::QueryLimiting::Transaction)
+        .to receive(:current)
+        .and_return(transaction)
+
+      expect(transaction)
+        .to receive(:increment)
+        .at_least(:once)
+
+      User.count
+    end
+  end
+end

spec/lib/gitlab/query_limiting/middleware_spec.rb

@@ -0,0 +1,72 @@
+require 'spec_helper'
+
+describe Gitlab::QueryLimiting::Middleware do
+  describe '#call' do
+    it 'runs the application with query limiting in place' do
+      middleware = described_class.new(-> (env) { env })
+
+      expect_any_instance_of(Gitlab::QueryLimiting::Transaction)
+        .to receive(:act_upon_results)
+
+      expect(middleware.call({ number: 10 }))
+        .to eq({ number: 10 })
+    end
+  end
+
+  describe '#action_name' do
+    let(:middleware) { described_class.new(-> (env) { env }) }
+
+    context 'using a Rails request' do
+      it 'returns the name of the controller and action' do
+        env = {
+          described_class::CONTROLLER_KEY => double(
+            :controller,
+            action_name: 'show',
+            class: double(:class, name: 'UsersController'),
+            content_type: 'text/html'
+          )
+        }
+
+        expect(middleware.action_name(env)).to eq('UsersController#show')
+      end
+
+      it 'includes the content type if this is not text/html' do
+        env = {
+          described_class::CONTROLLER_KEY => double(
+            :controller,
+            action_name: 'show',
+            class: double(:class, name: 'UsersController'),
+            content_type: 'application/json'
+          )
+        }
+
+        expect(middleware.action_name(env))
+          .to eq('UsersController#show (application/json)')
+      end
+    end
+
+    context 'using a Grape API request' do
+      it 'returns the name of the request method and endpoint path' do
+        env = {
+          described_class::ENDPOINT_KEY => double(
+            :endpoint,
+            route: double(:route, request_method: 'GET', path: '/foo')
+          )
+        }
+
+        expect(middleware.action_name(env)).to eq('GET /foo')
+      end
+
+      it 'returns nil if the route can not be retrieved' do
+        endpoint = double(:endpoint)
+        env = { described_class::ENDPOINT_KEY => endpoint }
+
+        allow(endpoint)
+          .to receive(:route)
+          .and_raise(RuntimeError)
+
+        expect(middleware.action_name(env)).to be_nil
+      end
+    end
+  end
+end

spec/lib/gitlab/query_limiting/transaction_spec.rb

@@ -0,0 +1,144 @@
+require 'spec_helper'
+
+describe Gitlab::QueryLimiting::Transaction do
+  after do
+    Thread.current[described_class::THREAD_KEY] = nil
+  end
+
+  describe '.current' do
+    it 'returns nil when there is no transaction' do
+      expect(described_class.current).to be_nil
+    end
+
+    it 'returns the transaction when present' do
+      Thread.current[described_class::THREAD_KEY] = described_class.new
+
+      expect(described_class.current).to be_an_instance_of(described_class)
+    end
+  end
+
+  describe '.run' do
+    it 'runs a transaction and returns it and its return value' do
+      trans, ret = described_class.run do
+        10
+      end
+
+      expect(trans).to be_an_instance_of(described_class)
+      expect(ret).to eq(10)
+    end
+
+    it 'removes the transaction from the current thread upon completion' do
+      described_class.run do
+        10
+      end
+
+      expect(Thread.current[described_class::THREAD_KEY]).to be_nil
+    end
+  end
+
+  describe '#act_upon_results' do
+    context 'when the query threshold is not exceeded' do
+      it 'does nothing' do
+        trans = described_class.new
+
+        expect(trans).not_to receive(:raise)
+
+        trans.act_upon_results
+      end
+    end
+
+    context 'when the query threshold is exceeded' do
+      let(:transaction) do
+        trans = described_class.new
+        trans.count = described_class::THRESHOLD + 1
+
+        trans
+      end
+
+      it 'raises an error when this is enabled' do
+        expect { transaction.act_upon_results }
+          .to raise_error(described_class::ThresholdExceededError)
+      end
+
+      it 'reports the error in Sentry if raising an error is disabled' do
+        expect(transaction)
+          .to receive(:raise_error?)
+          .and_return(false)
+
+        expect(Raven)
+          .to receive(:capture_exception)
+          .with(an_instance_of(described_class::ThresholdExceededError))
+
+        transaction.act_upon_results
+      end
+    end
+  end
+
+  describe '#increment' do
+    it 'increments the number of executed queries' do
+      transaction = described_class.new
+
+      expect(transaction.count).to be_zero
+
+      transaction.increment
+
+      expect(transaction.count).to eq(1)
+    end
+  end
+
+  describe '#raise_error?' do
+    it 'returns true in a test environment' do
+      transaction = described_class.new
+
+      expect(transaction.raise_error?).to eq(true)
+    end
+
+    it 'returns false in a production environment' do
+      transaction = described_class.new
+
+      expect(Rails.env)
+        .to receive(:test?)
+        .and_return(false)
+
+      expect(transaction.raise_error?).to eq(false)
+    end
+  end
+
+  describe '#threshold_exceeded?' do
+    it 'returns false when the threshold is not exceeded' do
+      transaction = described_class.new
+
+      expect(transaction.threshold_exceeded?).to eq(false)
+    end
+
+    it 'returns true when the threshold is exceeded' do
+      transaction = described_class.new
+      transaction.count = described_class::THRESHOLD + 1
+
+      expect(transaction.threshold_exceeded?).to eq(true)
+    end
+  end
+
+  describe '#error_message' do
+    it 'returns the error message to display when the threshold is exceeded' do
+      transaction = described_class.new
+      transaction.count = max = described_class::THRESHOLD
+
+      expect(transaction.error_message).to eq(
+        "Too many SQL queries were executed: a maximum of #{max} " \
+        "is allowed but #{max} SQL queries were executed"
+      )
+    end
+
+    it 'includes the action name in the error message when present' do
+      transaction = described_class.new
+      transaction.count = max = described_class::THRESHOLD
+      transaction.action = 'UsersController#show'
+
+      expect(transaction.error_message).to eq(
+        "Too many SQL queries were executed in UsersController#show: " \
+        "a maximum of #{max} is allowed but #{max} SQL queries were executed"
+      )
+    end
+  end
+end

spec/lib/gitlab/query_limiting_spec.rb

@@ -0,0 +1,65 @@
+require 'spec_helper'
+
+describe Gitlab::QueryLimiting do
+  describe '.enable?' do
+    it 'returns true in a test environment' do
+      expect(described_class.enable?).to eq(true)
+    end
+
+    it 'returns true in a development environment' do
+      allow(Rails.env).to receive(:development?).and_return(true)
+
+      expect(described_class.enable?).to eq(true)
+    end
+
+    it 'returns true on GitLab.com' do
+      allow(Gitlab).to receive(:com?).and_return(true)
+
+      expect(described_class.enable?).to eq(true)
+    end
+
+    it 'returns true in a non GitLab.com' do
+      expect(Gitlab).to receive(:com?).and_return(false)
+      expect(Rails.env).to receive(:development?).and_return(false)
+      expect(Rails.env).to receive(:test?).and_return(false)
+
+      expect(described_class.enable?).to eq(false)
+    end
+  end
+
+  describe '.whitelist' do
+    it 'raises ArgumentError when an invalid issue URL is given' do
+      expect { described_class.whitelist('foo') }
+        .to raise_error(ArgumentError)
+    end
+
+    context 'without a transaction' do
+      it 'does nothing' do
+        expect { described_class.whitelist('https://example.com') }
+          .not_to raise_error
+      end
+    end
+
+    context 'with a transaction' do
+      let(:transaction) { Gitlab::QueryLimiting::Transaction.new }
+
+      before do
+        allow(Gitlab::QueryLimiting::Transaction)
+          .to receive(:current)
+          .and_return(transaction)
+      end
+
+      it 'does not increment the number of SQL queries executed in the block' do
+        before = transaction.count
+
+        described_class.whitelist('https://example.com')
+
+        2.times do
+          User.count
+        end
+
+        expect(transaction.count).to eq(before)
+      end
+    end
+  end
+end