Track and act upon the number of executed queriesquery-counts

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.

38 files changed, 682 insertions, 0 deletions

diff --git a/app/controllers/admin/services_controller.rb b/app/controllers/admin/services_controller.rb
index 4c3d336b3af..a7025b62ad7 100644
--- a/app/controllers/admin/services_controller.rb
+++ b/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
diff --git a/app/controllers/boards/issues_controller.rb b/app/controllers/boards/issues_controller.rb
index f8049b20b9f..ee23ee0bcc3 100644
--- a/app/controllers/boards/issues_controller.rb
+++ b/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
diff --git a/app/controllers/import/gitlab_projects_controller.rb b/app/controllers/import/gitlab_projects_controller.rb
index 567957ba2cb..f22df992fe9 100644
--- a/app/controllers/import/gitlab_projects_controller.rb
+++ b/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
diff --git a/app/controllers/projects/commits_controller.rb b/app/controllers/projects/commits_controller.rb
index 0a40c67368f..1d910e461b1 100644
--- a/app/controllers/projects/commits_controller.rb
+++ b/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
diff --git a/app/controllers/projects/cycle_analytics_controller.rb b/app/controllers/projects/cycle_analytics_controller.rb
index 88ac3ad046b..d1b8fd80c4e 100644
--- a/app/controllers/projects/cycle_analytics_controller.rb
+++ b/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
diff --git a/app/controllers/projects/forks_controller.rb b/app/controllers/projects/forks_controller.rb
index 68978f8fdd1..f43bba18d81 100644
--- a/app/controllers/projects/forks_controller.rb
+++ b/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
diff --git a/app/controllers/projects/issues_controller.rb b/app/controllers/projects/issues_controller.rb
index 384f18b316c..515cb08f1fc 100644
--- a/app/controllers/projects/issues_controller.rb
+++ b/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
diff --git a/app/controllers/projects/merge_requests/creations_controller.rb b/app/controllers/projects/merge_requests/creations_controller.rb
index 0df80fa700f..a5a2d54ba82 100644
--- a/app/controllers/projects/merge_requests/creations_controller.rb
+++ b/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
diff --git a/app/controllers/projects/merge_requests_controller.rb b/app/controllers/projects/merge_requests_controller.rb
index 2e8a738b6d9..8af4e379f0a 100644
--- a/app/controllers/projects/merge_requests_controller.rb
+++ b/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
diff --git a/app/controllers/projects/network_controller.rb b/app/controllers/projects/network_controller.rb
index fb68dd771a1..3b10a93e97f 100644
--- a/app/controllers/projects/network_controller.rb
+++ b/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
diff --git a/app/controllers/projects/notes_controller.rb b/app/controllers/projects/notes_controller.rb
index 5940fae8dd0..4f8978c93c3 100644
--- a/app/controllers/projects/notes_controller.rb
+++ b/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
diff --git a/app/controllers/projects/pipelines_controller.rb b/app/controllers/projects/pipelines_controller.rb
index e146d0d3cd5..78d109cf33e 100644
--- a/app/controllers/projects/pipelines_controller.rb
+++ b/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
diff --git a/app/controllers/projects_controller.rb b/app/controllers/projects_controller.rb
index 8158934322d..ea13a72e151 100644
--- a/app/controllers/projects_controller.rb
+++ b/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
diff --git a/app/controllers/registrations_controller.rb b/app/controllers/registrations_controller.rb
index d9142311b6f..1848c806c41 100644
--- a/app/controllers/registrations_controller.rb
+++ b/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
diff --git a/changelogs/unreleased/query-counts.yml b/changelogs/unreleased/query-counts.yml
new file mode 100644
index 00000000000..e01ff8a4ad8
--- /dev/null
+++ b/changelogs/unreleased/query-counts.yml
@@ -0,0 +1,5 @@
+---
+title: Track and act upon the number of executed queries
+merge_request:
+author:
+type: added
diff --git a/config/initializers/query_limiting.rb b/config/initializers/query_limiting.rb
new file mode 100644
index 00000000000..66864d1898e
--- /dev/null
+++ b/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
diff --git a/doc/development/README.md b/doc/development/README.md
index 12cca9f84b7..45e9565f9a7 100644
--- a/doc/development/README.md
+++ b/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
 
diff --git a/doc/development/query_count_limits.md b/doc/development/query_count_limits.md
new file mode 100644
index 00000000000..ebb6e0c2dac
--- /dev/null
+++ b/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
+```
diff --git a/lib/api/branches.rb b/lib/api/branches.rb
index 0791a110c39..1794207e29b 100644
--- a/lib/api/branches.rb
+++ b/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))
diff --git a/lib/api/issues.rb b/lib/api/issues.rb
index c99fe3ab5b3..b6c278c89d0 100644
--- a/lib/api/issues.rb
+++ b/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
 
diff --git a/lib/api/merge_requests.rb b/lib/api/merge_requests.rb
index 420aaf1c964..719afa09295 100644
--- a/lib/api/merge_requests.rb
+++ b/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])
 
diff --git a/lib/api/pipelines.rb b/lib/api/pipelines.rb
index 675c963bae2..d2b8b832e4e 100644
--- a/lib/api/pipelines.rb
+++ b/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,
diff --git a/lib/api/projects.rb b/lib/api/projects.rb
index 8b5e4f8edcc..5b481121a10 100644
--- a/lib/api/projects.rb
+++ b/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]
 
diff --git a/lib/api/triggers.rb b/lib/api/triggers.rb
index dd6801664b1..b3709455bc3 100644
--- a/lib/api/triggers.rb
+++ b/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) }
diff --git a/lib/api/users.rb b/lib/api/users.rb
index e5de31ad51b..c7c2aa280d5 100644
--- a/lib/api/users.rb
+++ b/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])
diff --git a/lib/api/v3/branches.rb b/lib/api/v3/branches.rb
index b201bf77667..25176c5b38e 100644
--- a/lib/api/v3/branches.rb
+++ b/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))
diff --git a/lib/api/v3/issues.rb b/lib/api/v3/issues.rb
index cb371fdbab8..b59947d81d9 100644
--- a/lib/api/v3/issues.rb
+++ b/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
 
diff --git a/lib/api/v3/merge_requests.rb b/lib/api/v3/merge_requests.rb
index 0a24fea52a3..ce216497996 100644
--- a/lib/api/v3/merge_requests.rb
+++ b/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)
diff --git a/lib/api/v3/pipelines.rb b/lib/api/v3/pipelines.rb
index c48cbd2b765..6d31c12f572 100644
--- a/lib/api/v3/pipelines.rb
+++ b/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
diff --git a/lib/api/v3/triggers.rb b/lib/api/v3/triggers.rb
index 534911fde5c..34f07dfb486 100644
--- a/lib/api/v3/triggers.rb
+++ b/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) }
diff --git a/lib/gitlab/query_limiting.rb b/lib/gitlab/query_limiting.rb
new file mode 100644
index 00000000000..f64f1757144
--- /dev/null
+++ b/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
diff --git a/lib/gitlab/query_limiting/active_support_subscriber.rb b/lib/gitlab/query_limiting/active_support_subscriber.rb
new file mode 100644
index 00000000000..66049c94ec6
--- /dev/null
+++ b/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
diff --git a/lib/gitlab/query_limiting/middleware.rb b/lib/gitlab/query_limiting/middleware.rb
new file mode 100644
index 00000000000..949ae79a047
--- /dev/null
+++ b/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
diff --git a/lib/gitlab/query_limiting/transaction.rb b/lib/gitlab/query_limiting/transaction.rb
new file mode 100644
index 00000000000..3cbafadb0d0
--- /dev/null
+++ b/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
diff --git a/spec/lib/gitlab/query_limiting/active_support_subscriber_spec.rb b/spec/lib/gitlab/query_limiting/active_support_subscriber_spec.rb
new file mode 100644
index 00000000000..b49bc5c328c
--- /dev/null
+++ b/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
diff --git a/spec/lib/gitlab/query_limiting/middleware_spec.rb b/spec/lib/gitlab/query_limiting/middleware_spec.rb
new file mode 100644
index 00000000000..a04bcdecb4b
--- /dev/null
+++ b/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
diff --git a/spec/lib/gitlab/query_limiting/transaction_spec.rb b/spec/lib/gitlab/query_limiting/transaction_spec.rb
new file mode 100644
index 00000000000..b4231fcd0fa
--- /dev/null
+++ b/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
diff --git a/spec/lib/gitlab/query_limiting_spec.rb b/spec/lib/gitlab/query_limiting_spec.rb
new file mode 100644
index 00000000000..2eddab0b8c3
--- /dev/null
+++ b/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