Allow public forks to be deduplicated

When a project is forked, the new repository used to be a deep copy of everything
stored on disk by leveraging `git clone`. This works well, and makes isolation
between repository easy. However, the clone is at the start 100% the same as the
origin repository. And in the case of the objects in the object directory, this
is almost always going to be a lot of duplication.

Object Pools are a way to create a third repository that essentially only exists
for its 'objects' subdirectory. This third repository's object directory will be
set as alternate location for objects. This means that in the case an object is
missing in the local repository, git will look in another location. This other
location is the object pool repository.

When Git performs garbage collection, it's smart enough to check the
alternate location. When objects are duplicated, it will allow git to
throw one copy away. This copy is on the local repository, where to pool
remains as is.

These pools have an origin location, which for now will always be a
repository that itself is not a fork. When the root of a fork network is
forked by a user, the fork still clones the full repository. Async, the
pool repository will be created.

Either one of these processes can be done earlier than the other. To
handle this race condition, the Join ObjectPool operation is
idempotent. Given its idempotent, we can schedule it twice, with the
same effect.

To accommodate the holding of state two migrations have been added.
1. Added a state column to the pool_repositories column. This column is
managed by the state machine, allowing for hooks on transitions.
2. pool_repositories now has a source_project_id. This column in
convenient to have for multiple reasons: it has a unique index allowing
the database to handle race conditions when creating a new record. Also,
it's nice to know who the host is. As that's a short link to the fork
networks root.

Object pools are only available for public project, which use hashed
storage and when forking from the root of the fork network. (That is,
the project being forked from itself isn't a fork)

In this commit message I use both ObjectPool and Pool repositories,
which are alike, but different from each other. ObjectPool refers to
whatever is on the disk stored and managed by Gitaly. PoolRepository is
the record in the database.

6 files changed, 101 insertions, 0 deletions

diff --git a/app/workers/all_queues.yml b/app/workers/all_queues.yml
index 672c77539af..dfce00a10a1 100644
--- a/app/workers/all_queues.yml
+++ b/app/workers/all_queues.yml
@@ -85,6 +85,10 @@
 - todos_destroyer:todos_destroyer_project_private
 - todos_destroyer:todos_destroyer_private_features
 
+- object_pool:object_pool_create
+- object_pool:object_pool_schedule_join
+- object_pool:object_pool_join
+
 - default
 - mailers # ActionMailer::DeliveryJob.queue_name
 
diff --git a/app/workers/concerns/object_pool_queue.rb b/app/workers/concerns/object_pool_queue.rb
new file mode 100644
index 00000000000..5b648df9c72
--- /dev/null
+++ b/app/workers/concerns/object_pool_queue.rb
@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+##
+# Concern for setting Sidekiq settings for the various ObjectPool queues
+#
+module ObjectPoolQueue
+  extend ActiveSupport::Concern
+
+  included do
+    queue_namespace :object_pool
+  end
+end
diff --git a/app/workers/git_garbage_collect_worker.rb b/app/workers/git_garbage_collect_worker.rb
index 2d381c6fd6c..d3628b23189 100644
--- a/app/workers/git_garbage_collect_worker.rb
+++ b/app/workers/git_garbage_collect_worker.rb
@@ -28,6 +28,8 @@ class GitGarbageCollectWorker
     # Refresh the branch cache in case garbage collection caused a ref lookup to fail
     flush_ref_caches(project) if task == :gc
 
+    project.repository.expire_statistics_caches
+
     # In case pack files are deleted, release libgit2 cache and open file
     # descriptors ASAP instead of waiting for Ruby garbage collection
     project.cleanup
diff --git a/app/workers/object_pool/create_worker.rb b/app/workers/object_pool/create_worker.rb
new file mode 100644
index 00000000000..135b99886dc
--- /dev/null
+++ b/app/workers/object_pool/create_worker.rb
@@ -0,0 +1,44 @@
+# frozen_string_literal: true
+
+module ObjectPool
+  class CreateWorker
+    include ApplicationWorker
+    include ObjectPoolQueue
+    include ExclusiveLeaseGuard
+
+    attr_reader :pool
+
+    def perform(pool_id)
+      @pool = PoolRepository.find_by_id(pool_id)
+      return unless pool
+
+      try_obtain_lease do
+        perform_pool_creation
+      end
+    end
+
+    private
+
+    def perform_pool_creation
+      return unless pool.failed? || pool.scheduled?
+
+      # If this is a retry and the previous execution failed, deletion will
+      # bring the pool back to a pristine state
+      pool.delete_object_pool if pool.failed?
+
+      pool.create_object_pool
+      pool.mark_ready
+    rescue => e
+      pool.mark_failed
+      raise e
+    end
+
+    def lease_key
+      "object_pool:create:#{pool.id}"
+    end
+
+    def lease_timeout
+      1.hour
+    end
+  end
+end
diff --git a/app/workers/object_pool/join_worker.rb b/app/workers/object_pool/join_worker.rb
new file mode 100644
index 00000000000..07676011b2a
--- /dev/null
+++ b/app/workers/object_pool/join_worker.rb
@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+module ObjectPool
+  class JoinWorker
+    include ApplicationWorker
+    include ObjectPoolQueue
+
+    def perform(pool_id, project_id)
+      pool = PoolRepository.find_by_id(pool_id)
+      return unless pool&.joinable?
+
+      project = Project.find_by_id(project_id)
+      return unless project
+
+      pool.link_repository(project.repository)
+
+      Projects::HousekeepingService.new(project).execute
+    end
+  end
+end
diff --git a/app/workers/object_pool/schedule_join_worker.rb b/app/workers/object_pool/schedule_join_worker.rb
new file mode 100644
index 00000000000..647a8b72435
--- /dev/null
+++ b/app/workers/object_pool/schedule_join_worker.rb
@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+module ObjectPool
+  class ScheduleJoinWorker
+    include ApplicationWorker
+    include ObjectPoolQueue
+
+    def perform(pool_id)
+      pool = PoolRepository.find_by_id(pool_id)
+      return unless pool&.joinable?
+
+      pool.member_projects.find_each do |project|
+        next if project.forked? && !project.import_finished?
+
+        ObjectPool::JoinWorker.perform_async(pool.id, project.id)
+      end
+    end
+  end
+end