Add support for Workhorse notifications

app/models/ci/runner.rb

@@ -127,17 +127,16 @@ module Ci
 
     def tick_runner_queue
       SecureRandom.hex.tap do |new_update|
-        Gitlab::Redis.with do |redis|
-          redis.set(runner_queue_key, new_update, ex: RUNNER_QUEUE_EXPIRY_TIME)
-        end
+        ::Gitlab::Workhorse.ensure_and_notify(runner_queue_key, new_update,
+          expire: RUNNER_QUEUE_EXPIRY_TIME, overwrite: true)
       end
     end
 
     def ensure_runner_queue_value
       Gitlab::Redis.with do |redis|
-        value = SecureRandom.hex
-        redis.set(runner_queue_key, value, ex: RUNNER_QUEUE_EXPIRY_TIME, nx: true)
-        redis.get(runner_queue_key)
+        new_value = SecureRandom.hex
+        ::Gitlab::Workhorse.ensure_and_notify(runner_queue_key, new_value,
+          expire: RUNNER_QUEUE_EXPIRY_TIME, overwrite: false)
       end
     end
 

changelogs/unreleased/use-redis-channel-to-post-runner-notifcations.yml

@@ -0,0 +1,4 @@
+---
+title: Use redis channel to post notifications
+merge_request:
+author:

lib/gitlab/workhorse.rb

@@ -8,6 +8,7 @@ module Gitlab
     VERSION_FILE = 'GITLAB_WORKHORSE_VERSION'.freeze
     INTERNAL_API_CONTENT_TYPE = 'application/vnd.gitlab-workhorse+json'.freeze
     INTERNAL_API_REQUEST_HEADER = 'Gitlab-Workhorse-Api-Request'.freeze
+    NOTIFICATION_CHANNEL = 'workhorse:notifications'.freeze
 
     # Supposedly the effective key size for HMAC-SHA256 is 256 bits, i.e. 32
     # bytes https://tools.ietf.org/html/rfc4868#section-2.6
@@ -154,6 +155,19 @@ module Gitlab
         Rails.root.join('.gitlab_workhorse_secret')
       end
 
+      def ensure_and_notify(key, value, expire: nil, overwrite: true)
+        Gitlab::Redis.with do |redis|
+          result = redis.set(key, value, ex: expire, nx: !overwrite)
+          if result
+            payload = "#{key}=#{value}"
+            redis.publish(RUNNER_NOTIFICATION_CHANNEL, payload)
+            value
+          else
+            redis.get(key)
+          end
+        end
+      end
+
       protected
 
       def encode(hash)