Document workers that need their own queue

Some Sidekiq workers cannot share a queue with other Sidekiq workers,
and must continue to have their own named queue. We can't fix them all
at once, but we can document this and link to the correct issues.

app/workers/email_receiver_worker.rb

@@ -11,7 +11,7 @@ class EmailReceiverWorker # rubocop:disable Scalability/IdempotentWorker
   urgency :high
   weight 2
 
-  # https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/1087#jobs-written-to-redis-without-passing-through-the-application
+  # https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/1263
   tags :needs_own_queue
 
   attr_accessor :raw

app/workers/hashed_storage/migrator_worker.rb

@@ -11,8 +11,7 @@ module HashedStorage
     queue_namespace :hashed_storage
     feature_category :source_code_management
 
-    # Gitlab::HashedStorage::Migrator#migration_pending? depends on the
-    # queue size of this worker.
+    # https://gitlab.com/gitlab-org/gitlab/-/issues/340629
     tags :needs_own_queue
 
     # @param [Integer] start initial ID of the batch

app/workers/hashed_storage/project_migrate_worker.rb

@@ -11,8 +11,7 @@ module HashedStorage
     queue_namespace :hashed_storage
     loggable_arguments 1
 
-    # Gitlab::HashedStorage::Migrator#migration_pending? depends on the
-    # queue size of this worker.
+    # https://gitlab.com/gitlab-org/gitlab/-/issues/340629
     tags :needs_own_queue
 
     attr_reader :project_id

app/workers/hashed_storage/project_rollback_worker.rb

@@ -11,8 +11,7 @@ module HashedStorage
     queue_namespace :hashed_storage
     loggable_arguments 1
 
-    # Gitlab::HashedStorage::Migrator#rollback_pending? depends on the
-    # queue size of this worker.
+    # https://gitlab.com/gitlab-org/gitlab/-/issues/340629
     tags :needs_own_queue
 
     attr_reader :project_id

app/workers/hashed_storage/rollbacker_worker.rb

@@ -11,8 +11,7 @@ module HashedStorage
     queue_namespace :hashed_storage
     feature_category :source_code_management
 
-    # Gitlab::HashedStorage::Migrator#rollback_pending? depends on the
-    # queue size of this worker.
+    # https://gitlab.com/gitlab-org/gitlab/-/issues/340629
     tags :needs_own_queue
 
     # @param [Integer] start initial ID of the batch

app/workers/service_desk_email_receiver_worker.rb

@@ -9,7 +9,7 @@ class ServiceDeskEmailReceiverWorker < EmailReceiverWorker # rubocop:disable Sca
   urgency :high
   sidekiq_options retry: 3
 
-  # https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/1087#jobs-written-to-redis-without-passing-through-the-application
+  # https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/1263
   tags :needs_own_queue
 
   def should_perform?

doc/administration/operations/extra_sidekiq_routing.md

@@ -40,6 +40,8 @@ In `/etc/gitlab/gitlab.rb`:
 
 ```ruby
 sidekiq['routing_rules'] = [
+  # Do not re-route workers that require their own queue
+  ['tags=needs_own_queue', nil],
   # Route all non-CPU-bound workers that are high urgency to `high-urgency` queue
   ['resource_boundary!=cpu&urgency=high', 'high-urgency'],
   # Route all database, gitaly and global search workers that are throttled to `throttled` queue
@@ -164,3 +166,34 @@ with the migration to avoid losing jobs entirely, especially in a system with
 long queues of jobs. The migration can be done by following the migration steps
 mentioned in [Sidekiq job
 migration](../../raketasks/sidekiq_job_migration.md)
+
+### Workers that cannot be migrated
+
+Some workers cannot share a queue with other workers - typically because
+they check the size of their own queue - and so must be excluded from
+this process. We recommend excluding these from any further worker
+routing by adding a rule to keep them in their own queue, for example:
+
+```ruby
+sidekiq['routing_rules'] = [
+  ['tags=needs_own_queue', nil],
+  # ...
+]
+```
+
+These queues will also need to be included in at least one [Sidekiq
+queue group](extra_sidekiq_processes.md#start-multiple-processes).
+
+The following table shows the workers that should have their own queue:
+
+<!-- markdownlint-disable MD044 -->
+| Worker name | Queue name | GitLab issue |
+| --- | --- | --- |
+| EmailReceiverWorker | `email_receiver` | [gitlab-com/gl-infra/scalability#1263](https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/1263) |
+| ServiceDeskEmailReceiverWorker | `service_desk_email_receiver` | [gitlab-com/gl-infra/scalability#1263](https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/1263) |
+| ProjectImportScheduleWorker | `project_import_schedule` | [gitlab-org/gitlab#340630](https://gitlab.com/gitlab-org/gitlab/-/issues/340630) |
+| HashedStorage::MigratorWorker | `hashed_storage:hashed_storage_migrator` | [gitlab-org/gitlab#340629](https://gitlab.com/gitlab-org/gitlab/-/issues/340629) |
+| HashedStorage::ProjectMigrateWorker | `hashed_storage:hashed_storage_project_migrate` | [gitlab-org/gitlab#340629](https://gitlab.com/gitlab-org/gitlab/-/issues/340629) |
+| HashedStorage::ProjectRollbackWorker | `hashed_storage:hashed_storage_project_rollback` | [gitlab-org/gitlab#340629](https://gitlab.com/gitlab-org/gitlab/-/issues/340629) |
+| HashedStorage::RollbackerWorker | `hashed_storage:hashed_storage_rollbacker` | [gitlab-org/gitlab#340629](https://gitlab.com/gitlab-org/gitlab/-/issues/340629) |
+<!-- markdownlint-disable MD044 -->

ee/app/workers/project_import_schedule_worker.rb

@@ -14,7 +14,7 @@ class ProjectImportScheduleWorker
   loggable_arguments 1 # For the job waiter key
 
   # UpdateAllMirrorsWorker depends on the queue size of this worker:
-  # https://gitlab.com/gitlab-org/gitlab/-/issues/325496#note_550866012
+  # https://gitlab.com/gitlab-org/gitlab/-/issues/340630
   tags :needs_own_queue
 
   def perform(project_id)