Add a Prometheus API per environment

The api will proxy requests to the environment's prometheus server.
The Prometheus::ProxyService class can be reused when we add support for
group prometheus servers.

app/controllers/projects/environments/prometheus_api_controller.rb

+# frozen_string_literal: true
+
+class Projects::Environments::PrometheusApiController < Projects::ApplicationController
+  before_action :authorize_read_prometheus!
+  before_action :environment
+
+  def proxy
+    permitted = permit_params
+
+    result = Prometheus::ProxyService.new(
+      environment,
+      request.method,
+      permitted[:proxy_path],
+      permitted.except(:proxy_path) # rubocop: disable CodeReuse/ActiveRecord
+    ).execute
+
+    if result.nil?
+      render status: :accepted, json: {
+        status: 'processing',
+        message: 'Not ready yet. Try again later.'
+      }
+      return
+    end
+
+    if result[:status] == :success
+      render status: result[:http_status], json: result[:body]
+    else
+      render status: result[:http_status] || :bad_request, json: {
+          status: result[:status],
+          message: result[:message]
+        }
+    end
+  end
+
+  private
+
+  def permit_params
+    params.permit([
+      :proxy_path, :query, :time, :timeout, :start, :end, :step, { match: [] },
+      :match_target, :metric, :limit
+    ])
+  end
+
+  def environment
+    @environment ||= project.environments.find(params[:id])
+  end
+end

app/policies/project_policy.rb

@@ -204,6 +204,7 @@ class ProjectPolicy < BasePolicy
     enable :read_merge_request
     enable :read_sentry_issue
     enable :read_release
+    enable :read_prometheus
   end
 
   # We define `:public_user_access` separately because there are cases in gitlab-ee

changelogs/unreleased/58375-api-controller.yml

+---
+title: Add a Prometheus API per environment
+merge_request: 26841
+author:
+type: added

config/routes/project.rb

@@ -219,6 +219,8 @@ constraints(::Constraints::ProjectUrlConstrainer.new) do
           get :metrics
           get :additional_metrics
           get '/terminal.ws/authorize', to: 'environments#terminal_websocket_authorize', constraints: { format: nil }
+
+          get '/prometheus/api/v1/*proxy_path', to: 'environments/prometheus_api#proxy'
         end
 
         collection do

spec/controllers/projects/environments/prometheus_api_controller_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Projects::Environments::PrometheusApiController do
+  set(:project) { create(:project) }
+  set(:environment) { create(:environment, project: project) }
+  set(:user) { create(:user) }
+
+  before do
+    project.add_reporter(user)
+    sign_in(user)
+  end
+
+  describe 'GET #proxy' do
+    let(:prometheus_proxy_service) { instance_double(Prometheus::ProxyService) }
+    let(:prometheus_response) { { status: :success, body: response_body } }
+    let(:json_response_body) { JSON.parse(response_body) }
+
+    let(:response_body) do
+      "{\"status\":\"success\",\"data\":{\"resultType\":\"scalar\",\"result\":[1553864609.117,\"1\"]}}"
+    end
+
+    before do
+      allow(Prometheus::ProxyService).to receive(:new)
+        .with(environment, 'GET', 'query', anything)
+        .and_return(prometheus_proxy_service)
+
+      allow(prometheus_proxy_service).to receive(:execute)
+        .and_return(prometheus_response)
+    end
+
+    it 'returns prometheus response' do
+      get :proxy, params: environment_params
+
+      expect(response).to have_gitlab_http_status(:ok)
+      expect(json_response).to eq(json_response_body)
+    end
+
+    it 'filters params' do
+      get :proxy, params: environment_params({ extra_param: 'dangerous value' })
+
+      expect(Prometheus::ProxyService).to have_received(:new)
+        .with(environment, 'GET', 'query', ActionController::Parameters.new({ 'query' => '1' }).permit!)
+    end
+
+    context 'Prometheus::ProxyService returns nil' do
+      before do
+        allow(prometheus_proxy_service).to receive(:execute)
+          .and_return(nil)
+      end
+
+      it 'returns 202 accepted' do
+        get :proxy, params: environment_params
+
+        expect(json_response['status']).to eq('processing')
+        expect(json_response['message']).to eq('Not ready yet. Try again later.')
+        expect(response).to have_gitlab_http_status(:accepted)
+      end
+    end
+
+    context 'Prometheus::ProxyService returns status success' do
+      let(:service_response) { { http_status: 404, status: :success, body: '{"body": "value"}' } }
+
+      before do
+        allow(prometheus_proxy_service).to receive(:execute)
+          .and_return(service_response)
+      end
+
+      it 'returns body' do
+        get :proxy, params: environment_params
+
+        expect(response).to have_gitlab_http_status(:not_found)
+        expect(json_response['body']).to eq('value')
+      end
+    end
+
+    context 'Prometheus::ProxyService returns status error' do
+      before do
+        allow(prometheus_proxy_service).to receive(:execute)
+          .and_return(service_response)
+      end
+
+      context 'with http_status' do
+        let(:service_response) do
+          { http_status: :service_unavailable, status: :error, message: 'error message' }
+        end
+
+        it 'sets the http response status code' do
+          get :proxy, params: environment_params
+
+          expect(response).to have_gitlab_http_status(:service_unavailable)
+          expect(json_response['status']).to eq('error')
+          expect(json_response['message']).to eq('error message')
+        end
+      end
+
+      context 'without http_status' do
+        let(:service_response) { { status: :error, message: 'error message' } }
+
+        it 'returns message' do
+          get :proxy, params: environment_params
+
+          expect(response).to have_gitlab_http_status(:bad_request)
+          expect(json_response['status']).to eq('error')
+          expect(json_response['message']).to eq('error message')
+        end
+      end
+    end
+
+    context 'with anonymous user' do
+      before do
+        sign_out(user)
+      end
+
+      it 'redirects to signin page' do
+        get :proxy, params: environment_params
+
+        expect(response).to redirect_to(new_user_session_path)
+      end
+    end
+
+    context 'without correct permissions' do
+      before do
+        project.team.truncate
+      end
+
+      it 'returns 404' do
+        get :proxy, params: environment_params
+
+        expect(response).to have_gitlab_http_status(:not_found)
+      end
+    end
+  end
+
+  private
+
+  def environment_params(params = {})
+    {
+      id: environment.id,
+      namespace_id: project.namespace,
+      project_id: project,
+      proxy_path: 'query',
+      query: '1'
+    }.merge(params)
+  end
+end

spec/support/shared_context/policies/project_policy_shared_context.rb

@@ -25,6 +25,7 @@ RSpec.shared_context 'ProjectPolicy context' do
       admin_issue admin_label admin_list read_commit_status read_build
       read_container_image read_pipeline read_environment read_deployment
       read_merge_request download_wiki_code read_sentry_issue read_release
+      read_prometheus
     ]
   end