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.

26 files changed, 703 insertions, 6 deletions

diff --git a/app/models/pool_repository.rb b/app/models/pool_repository.rb
index bad0e30ceb5..dbde00b5584 100644
--- a/app/models/pool_repository.rb
+++ b/app/models/pool_repository.rb
@@ -1,12 +1,89 @@
 # frozen_string_literal: true
 
+# The PoolRepository model is the database equivalent of an ObjectPool for Gitaly
+# That is; PoolRepository is the record in the database, ObjectPool is the
+# repository on disk
 class PoolRepository < ActiveRecord::Base
   include Shardable
+  include AfterCommitQueue
+
+  has_one :source_project, class_name: 'Project'
+  validates :source_project, presence: true
 
   has_many :member_projects, class_name: 'Project'
 
   after_create :correct_disk_path
 
+  state_machine :state, initial: :none do
+    state :scheduled
+    state :ready
+    state :failed
+
+    event :schedule do
+      transition none: :scheduled
+    end
+
+    event :mark_ready do
+      transition [:scheduled, :failed] => :ready
+    end
+
+    event :mark_failed do
+      transition all => :failed
+    end
+
+    state all - [:ready] do
+      def joinable?
+        false
+      end
+    end
+
+    state :ready do
+      def joinable?
+        true
+      end
+    end
+
+    after_transition none: :scheduled do |pool, _|
+      pool.run_after_commit do
+        ::ObjectPool::CreateWorker.perform_async(pool.id)
+      end
+    end
+
+    after_transition scheduled: :ready do |pool, _|
+      pool.run_after_commit do
+        ::ObjectPool::ScheduleJoinWorker.perform_async(pool.id)
+      end
+    end
+  end
+
+  def create_object_pool
+    object_pool.create
+  end
+
+  # The members of the pool should have fetched the missing objects to their own
+  # objects directory. If the caller fails to do so, data loss might occur
+  def delete_object_pool
+    object_pool.delete
+  end
+
+  def link_repository(repository)
+    object_pool.link(repository.raw)
+  end
+
+  # This RPC can cause data loss, as not all objects are present the local repository
+  # No execution path yet, will be added through:
+  # https://gitlab.com/gitlab-org/gitaly/issues/1415
+  def delete_repository_alternate(repository)
+    object_pool.unlink_repository(repository.raw)
+  end
+
+  def object_pool
+    @object_pool ||= Gitlab::Git::ObjectPool.new(
+      shard.name,
+      disk_path + '.git',
+      source_project.repository.raw)
+  end
+
   private
 
   def correct_disk_path
diff --git a/app/models/project.rb b/app/models/project.rb
index 9e736a3b03c..f5dc58cd67f 100644
--- a/app/models/project.rb
+++ b/app/models/project.rb
@@ -1585,6 +1585,7 @@ class Project < ActiveRecord::Base
     import_state.remove_jid
     update_project_counter_caches
     after_create_default_branch
+    join_pool_repository
     refresh_markdown_cache!
   end
 
@@ -1981,8 +1982,48 @@ class Project < ActiveRecord::Base
     Gitlab::CurrentSettings.max_attachment_size.megabytes.to_i
   end
 
+  def object_pool_params
+    return {} unless !forked? && git_objects_poolable?
+
+    {
+      repository_storage: repository_storage,
+      pool_repository:    pool_repository || create_new_pool_repository
+    }
+  end
+
+  # Git objects are only poolable when the project is or has:
+  # - Hashed storage -> The object pool will have a remote to its members, using relative paths.
+  #                     If the repository path changes we would have to update the remote.
+  # - Public         -> User will be able to fetch Git objects that might not exist
+  #                     in their own repository.
+  # - Repository     -> Else the disk path will be empty, and there's nothing to pool
+  def git_objects_poolable?
+    hashed_storage?(:repository) &&
+      public? &&
+      repository_exists? &&
+      Gitlab::CurrentSettings.hashed_storage_enabled &&
+      Feature.enabled?(:object_pools, self)
+  end
+
   private
 
+  def create_new_pool_repository
+    pool = begin
+             create_or_find_pool_repository!(shard: Shard.by_name(repository_storage), source_project: self)
+           rescue ActiveRecord::RecordNotUnique
+             retry
+           end
+
+    pool.schedule
+    pool
+  end
+
+  def join_pool_repository
+    return unless pool_repository
+
+    ObjectPool::JoinWorker.perform_async(pool_repository.id, self.id)
+  end
+
   def use_hashed_storage
     if self.new_record? && Gitlab::CurrentSettings.hashed_storage_enabled
       self.storage_version = LATEST_STORAGE_VERSION
diff --git a/app/services/projects/fork_service.rb b/app/services/projects/fork_service.rb
index 8dc0e044875..91091c4393d 100644
--- a/app/services/projects/fork_service.rb
+++ b/app/services/projects/fork_service.rb
@@ -54,6 +54,8 @@ module Projects
         new_params[:avatar] = @project.avatar
       end
 
+      new_params.merge!(@project.object_pool_params)
+
       new_project = CreateService.new(current_user, new_params).execute
       return new_project unless new_project.persisted?
 
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
diff --git a/changelogs/unreleased/zj-pool-repository-creation.yml b/changelogs/unreleased/zj-pool-repository-creation.yml
new file mode 100644
index 00000000000..a24b96e4924
--- /dev/null
+++ b/changelogs/unreleased/zj-pool-repository-creation.yml
@@ -0,0 +1,5 @@
+---
+title: Allow public forks to be deduplicated
+merge_request: 23508
+author:
+type: added
diff --git a/config/sidekiq_queues.yml b/config/sidekiq_queues.yml
index 4782a223561..5985569bef4 100644
--- a/config/sidekiq_queues.yml
+++ b/config/sidekiq_queues.yml
@@ -81,5 +81,6 @@
   - [delete_diff_files, 1]
   - [detect_repository_languages, 1]
   - [auto_devops, 2]
+  - [object_pool, 1]
   - [repository_cleanup, 1]
   - [delete_stored_files, 1]
diff --git a/db/migrate/20181128123704_add_state_to_pool_repository.rb b/db/migrate/20181128123704_add_state_to_pool_repository.rb
new file mode 100644
index 00000000000..714232ede56
--- /dev/null
+++ b/db/migrate/20181128123704_add_state_to_pool_repository.rb
@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+class AddStateToPoolRepository < ActiveRecord::Migration[5.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  # Given the table is empty, and the non concurrent methods are chosen so
+  # the transactions don't have to be disabled
+  # rubocop: disable Migration/AddConcurrentForeignKey, Migration/AddIndex
+  def change
+    add_column(:pool_repositories, :state, :string, null: true)
+
+    add_column :pool_repositories, :source_project_id, :integer
+    add_index :pool_repositories, :source_project_id, unique: true
+    add_foreign_key :pool_repositories, :projects, column: :source_project_id, on_delete: :nullify
+  end
+  # rubocop: enable Migration/AddConcurrentForeignKey, Migration/AddIndex
+end
diff --git a/db/schema.rb b/db/schema.rb
index d7124100621..ff2dde3243c 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -1512,8 +1512,11 @@ ActiveRecord::Schema.define(version: 20181203002526) do
   create_table "pool_repositories", id: :bigserial, force: :cascade do |t|
     t.integer "shard_id", null: false
     t.string "disk_path"
+    t.string "state"
+    t.integer "source_project_id"
     t.index ["disk_path"], name: "index_pool_repositories_on_disk_path", unique: true, using: :btree
     t.index ["shard_id"], name: "index_pool_repositories_on_shard_id", using: :btree
+    t.index ["source_project_id"], name: "index_pool_repositories_on_source_project_id", unique: true, using: :btree
   end
 
   create_table "programming_languages", force: :cascade do |t|
@@ -2393,6 +2396,7 @@ ActiveRecord::Schema.define(version: 20181203002526) do
   add_foreign_key "oauth_openid_requests", "oauth_access_grants", column: "access_grant_id", name: "fk_oauth_openid_requests_oauth_access_grants_access_grant_id"
   add_foreign_key "pages_domains", "projects", name: "fk_ea2f6dfc6f", on_delete: :cascade
   add_foreign_key "personal_access_tokens", "users"
+  add_foreign_key "pool_repositories", "projects", column: "source_project_id", on_delete: :nullify
   add_foreign_key "pool_repositories", "shards", on_delete: :restrict
   add_foreign_key "project_authorizations", "projects", on_delete: :cascade
   add_foreign_key "project_authorizations", "users", on_delete: :cascade
diff --git a/doc/administration/repository_storage_types.md b/doc/administration/repository_storage_types.md
index 9379944b250..12238ba7b32 100644
--- a/doc/administration/repository_storage_types.md
+++ b/doc/administration/repository_storage_types.md
@@ -94,6 +94,23 @@ need to be performed on these nodes as well. Database changes will propagate wit
 You must make sure the migration event was already processed or otherwise it may migrate
 the files back to Hashed state again.
 
+#### Hashed object pools
+
+For deduplication of public forks and their parent repository, objects are pooled
+in an object pool. These object pools are a third repository where shared objects
+are stored.
+
+```ruby
+# object pool paths
+"@pools/#{hash[0..1]}/#{hash[2..3]}/#{hash}.git"
+```
+
+The object pool feature is behind the `object_pools` feature flag, and can be
+enabled for individual projects by executing
+`Feature.enable(:object_pools, Project.find(<id>))`. Note that the project has to
+be on hashed storage, should not be a fork itself, and hashed storage should be
+enabled for all new projects.
+
 ##### Attachments
 
 To rollback single Attachment migration, rename `aa/bb/abcdef1234567890...` folder back to `namespace/project`.
diff --git a/lib/gitlab/git/object_pool.rb b/lib/gitlab/git/object_pool.rb
new file mode 100644
index 00000000000..558699a6318
--- /dev/null
+++ b/lib/gitlab/git/object_pool.rb
@@ -0,0 +1,62 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Git
+    class ObjectPool
+      # GL_REPOSITORY has to be passed for Gitlab::Git::Repositories, but not
+      # used for ObjectPools.
+      GL_REPOSITORY = ""
+
+      delegate :exists?, :size, to: :repository
+      delegate :delete, to: :object_pool_service
+
+      attr_reader :storage, :relative_path, :source_repository
+
+      def initialize(storage, relative_path, source_repository)
+        @storage = storage
+        @relative_path = relative_path
+        @source_repository = source_repository
+      end
+
+      def create
+        object_pool_service.create(source_repository)
+      end
+
+      def link(to_link_repo)
+        remote_name = to_link_repo.object_pool_remote_name
+        repository.set_config(
+          "remote.#{remote_name}.url" => relative_path_to(to_link_repo.relative_path),
+          "remote.#{remote_name}.tagOpt" => "--no-tags",
+          "remote.#{remote_name}.fetch" => "+refs/*:refs/remotes/#{remote_name}/*"
+        )
+
+        object_pool_service.link_repository(to_link_repo)
+      end
+
+      def gitaly_object_pool
+        Gitaly::ObjectPool.new(repository: to_gitaly_repository)
+      end
+
+      def to_gitaly_repository
+        Gitlab::GitalyClient::Util.repository(storage, relative_path, GL_REPOSITORY)
+      end
+
+      # Allows for reusing other RPCs by 'tricking' Gitaly to think its a repository
+      def repository
+        @repository ||= Gitlab::Git::Repository.new(storage, relative_path, GL_REPOSITORY)
+      end
+
+      private
+
+      def object_pool_service
+        @object_pool_service ||= Gitlab::GitalyClient::ObjectPoolService.new(self)
+      end
+
+      def relative_path_to(pool_member_path)
+        pool_path = Pathname.new("#{relative_path}#{File::SEPARATOR}")
+
+        Pathname.new(pool_member_path).relative_path_from(pool_path).to_s
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/git/repository.rb b/lib/gitlab/git/repository.rb
index 0a541031884..5bbedc9d5e3 100644
--- a/lib/gitlab/git/repository.rb
+++ b/lib/gitlab/git/repository.rb
@@ -69,6 +69,13 @@ module Gitlab
 
       attr_reader :storage, :gl_repository, :relative_path
 
+      # This remote name has to be stable for all types of repositories that
+      # can join an object pool. If it's structure ever changes, a migration
+      # has to be performed on the object pools to update the remote names.
+      # Else the pool can't be updated anymore and is left in an inconsistent
+      # state.
+      alias_method :object_pool_remote_name, :gl_repository
+
       # This initializer method is only used on the client side (gitlab-ce).
       # Gitaly-ruby uses a different initializer.
       def initialize(storage, relative_path, gl_repository)
diff --git a/lib/gitlab/gitaly_client/object_pool_service.rb b/lib/gitlab/gitaly_client/object_pool_service.rb
new file mode 100644
index 00000000000..272ce73ad64
--- /dev/null
+++ b/lib/gitlab/gitaly_client/object_pool_service.rb
@@ -0,0 +1,45 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module GitalyClient
+    class ObjectPoolService
+      attr_reader :object_pool, :storage
+
+      def initialize(object_pool)
+        @object_pool = object_pool.gitaly_object_pool
+        @storage = object_pool.storage
+      end
+
+      def create(repository)
+        request = Gitaly::CreateObjectPoolRequest.new(
+          object_pool: object_pool,
+          origin: repository.gitaly_repository)
+
+        GitalyClient.call(storage, :object_pool_service, :create_object_pool, request)
+      end
+
+      def delete
+        request = Gitaly::DeleteObjectPoolRequest.new(object_pool: object_pool)
+
+        GitalyClient.call(storage, :object_pool_service, :delete_object_pool, request)
+      end
+
+      def link_repository(repository)
+        request = Gitaly::LinkRepositoryToObjectPoolRequest.new(
+          object_pool: object_pool,
+          repository: repository.gitaly_repository
+        )
+
+        GitalyClient.call(storage, :object_pool_service, :link_repository_to_object_pool,
+                          request, timeout: GitalyClient.fast_timeout)
+      end
+
+      def unlink_repository(repository)
+        request = Gitaly::UnlinkRepositoryFromObjectPoolRequest.new(repository: repository.gitaly_repository)
+
+        GitalyClient.call(storage, :object_pool_service, :unlink_repository_from_object_pool,
+                          request, timeout: GitalyClient.fast_timeout)
+      end
+    end
+  end
+end
diff --git a/spec/controllers/projects_controller_spec.rb b/spec/controllers/projects_controller_spec.rb
index 7849bec4762..576191a5788 100644
--- a/spec/controllers/projects_controller_spec.rb
+++ b/spec/controllers/projects_controller_spec.rb
@@ -279,7 +279,7 @@ describe ProjectsController do
         expected_query = /#{public_project.fork_network.find_forks_in(other_user.namespace).to_sql}/
 
         expect { get(:show, namespace_id: public_project.namespace, id: public_project) }
-          .not_to exceed_query_limit(1).for_query(expected_query)
+          .not_to exceed_query_limit(2).for_query(expected_query)
       end
     end
   end
diff --git a/spec/factories/pool_repositories.rb b/spec/factories/pool_repositories.rb
index 2ed0844ed47..265a4643f46 100644
--- a/spec/factories/pool_repositories.rb
+++ b/spec/factories/pool_repositories.rb
@@ -1,5 +1,26 @@
 FactoryBot.define do
   factory :pool_repository do
-    shard
+    shard { Shard.by_name("default") }
+    state :none
+
+    before(:create) do |pool|
+      pool.source_project = create(:project, :repository)
+    end
+
+    trait :scheduled do
+      state :scheduled
+    end
+
+    trait :failed do
+      state :failed
+    end
+
+    trait :ready do
+      state :ready
+
+      after(:create) do |pool|
+        pool.create_object_pool
+      end
+    end
   end
 end
diff --git a/spec/lib/gitlab/git/object_pool_spec.rb b/spec/lib/gitlab/git/object_pool_spec.rb
new file mode 100644
index 00000000000..363c2aa67af
--- /dev/null
+++ b/spec/lib/gitlab/git/object_pool_spec.rb
@@ -0,0 +1,89 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Gitlab::Git::ObjectPool do
+  let(:pool_repository) { create(:pool_repository) }
+  let(:source_repository) { pool_repository.source_project.repository }
+
+  subject { pool_repository.object_pool }
+
+  describe '#storage' do
+    it "equals the pool repository's shard name" do
+      expect(subject.storage).not_to be_nil
+      expect(subject.storage).to eq(pool_repository.shard_name)
+    end
+  end
+
+  describe '#create' do
+    before do
+      subject.create
+    end
+
+    context "when the pool doesn't exist yet" do
+      it 'creates the pool' do
+        expect(subject.exists?).to be(true)
+      end
+    end
+
+    context 'when the pool already exists' do
+      it 'raises an FailedPrecondition' do
+        expect do
+          subject.create
+        end.to raise_error(GRPC::FailedPrecondition)
+      end
+    end
+  end
+
+  describe '#exists?' do
+    context "when the object pool doesn't exist" do
+      it 'returns false' do
+        expect(subject.exists?).to be(false)
+      end
+    end
+
+    context 'when the object pool exists' do
+      let(:pool) { create(:pool_repository, :ready) }
+
+      subject { pool.object_pool }
+
+      it 'returns true' do
+        expect(subject.exists?).to be(true)
+      end
+    end
+  end
+
+  describe '#link' do
+    let!(:pool_repository) { create(:pool_repository, :ready) }
+
+    context 'when no remotes are set' do
+      it 'sets a remote' do
+        subject.link(source_repository)
+
+        repo = Gitlab::GitalyClient::StorageSettings.allow_disk_access do
+          Rugged::Repository.new(subject.repository.path)
+        end
+
+        expect(repo.remotes.count).to be(1)
+        expect(repo.remotes.first.name).to eq(source_repository.object_pool_remote_name)
+      end
+    end
+
+    context 'when the remote is already set' do
+      before do
+        subject.link(source_repository)
+      end
+
+      it "doesn't raise an error" do
+        subject.link(source_repository)
+
+        repo = Gitlab::GitalyClient::StorageSettings.allow_disk_access do
+          Rugged::Repository.new(subject.repository.path)
+        end
+
+        expect(repo.remotes.count).to be(1)
+        expect(repo.remotes.first.name).to eq(source_repository.object_pool_remote_name)
+      end
+    end
+  end
+end
diff --git a/spec/lib/gitlab/gitaly_client/object_pool_service_spec.rb b/spec/lib/gitlab/gitaly_client/object_pool_service_spec.rb
new file mode 100644
index 00000000000..149b7ec5bb0
--- /dev/null
+++ b/spec/lib/gitlab/gitaly_client/object_pool_service_spec.rb
@@ -0,0 +1,46 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Gitlab::GitalyClient::ObjectPoolService do
+  let(:pool_repository) { create(:pool_repository) }
+  let(:project) { create(:project, :repository) }
+  let(:raw_repository) { project.repository.raw }
+  let(:object_pool) { pool_repository.object_pool }
+
+  subject { described_class.new(object_pool) }
+
+  before do
+    subject.create(raw_repository)
+  end
+
+  describe '#create' do
+    it 'exists on disk' do
+      expect(object_pool.repository.exists?).to be(true)
+    end
+
+    context 'when the pool already exists' do
+      it 'returns an error' do
+        expect do
+          subject.create(raw_repository)
+        end.to raise_error(GRPC::FailedPrecondition)
+      end
+    end
+  end
+
+  describe '#delete' do
+    it 'removes the repository from disk' do
+      subject.delete
+
+      expect(object_pool.repository.exists?).to be(false)
+    end
+
+    context 'when called twice' do
+      it "doesn't raise an error" do
+        subject.delete
+
+        expect { object_pool.delete }.not_to raise_error
+      end
+    end
+  end
+end
diff --git a/spec/models/pool_repository_spec.rb b/spec/models/pool_repository_spec.rb
index 541e78507e5..3d3878b8c39 100644
--- a/spec/models/pool_repository_spec.rb
+++ b/spec/models/pool_repository_spec.rb
@@ -5,6 +5,7 @@ require 'spec_helper'
 describe PoolRepository do
   describe 'associations' do
     it { is_expected.to belong_to(:shard) }
+    it { is_expected.to have_one(:source_project) }
     it { is_expected.to have_many(:member_projects) }
   end
 
@@ -12,15 +13,14 @@ describe PoolRepository do
     let!(:pool_repository) { create(:pool_repository) }
 
     it { is_expected.to validate_presence_of(:shard) }
+    it { is_expected.to validate_presence_of(:source_project) }
   end
 
   describe '#disk_path' do
     it 'sets the hashed disk_path' do
       pool = create(:pool_repository)
 
-      elements = File.split(pool.disk_path)
-
-      expect(elements).to all( match(/\d{2,}/) )
+      expect(pool.disk_path).to match(%r{\A@pools/\h{2}/\h{2}/\h{64}})
     end
   end
 end
diff --git a/spec/models/project_spec.rb b/spec/models/project_spec.rb
index 93c83fd21fd..1c85411dc3b 100644
--- a/spec/models/project_spec.rb
+++ b/spec/models/project_spec.rb
@@ -4092,6 +4092,44 @@ describe Project do
     end
   end
 
+  describe '#git_objects_poolable?' do
+    subject { project }
+
+    context 'when the feature flag is turned off' do
+      before do
+        stub_feature_flags(object_pools: false)
+      end
+
+      let(:project) { create(:project, :repository, :public) }
+
+      it { is_expected.not_to be_git_objects_poolable }
+    end
+
+    context 'when the feature flag is enabled' do
+      context 'when not using hashed storage' do
+        let(:project) { create(:project, :legacy_storage, :public, :repository) }
+
+        it { is_expected.not_to be_git_objects_poolable }
+      end
+
+      context 'when the project is not public' do
+        let(:project) { create(:project, :private) }
+
+        it { is_expected.not_to be_git_objects_poolable }
+      end
+
+      context 'when objects are poolable' do
+        let(:project) { create(:project, :repository, :public) }
+
+        before do
+          stub_application_setting(hashed_storage_enabled: true)
+        end
+
+        it { is_expected.to be_git_objects_poolable }
+      end
+    end
+  end
+
   def rugged_config
     rugged_repo(project.repository).config
   end
diff --git a/spec/services/projects/fork_service_spec.rb b/spec/services/projects/fork_service_spec.rb
index a3d24ae312a..26e8d829345 100644
--- a/spec/services/projects/fork_service_spec.rb
+++ b/spec/services/projects/fork_service_spec.rb
@@ -2,7 +2,8 @@ require 'spec_helper'
 
 describe Projects::ForkService do
   include ProjectForksHelper
-  let(:gitlab_shell) { Gitlab::Shell.new }
+  include Gitlab::ShellAdapter
+
   context 'when forking a new project' do
     describe 'fork by user' do
       before do
@@ -235,6 +236,33 @@ describe Projects::ForkService do
     end
   end
 
+  context 'when forking with object pools' do
+    let(:fork_from_project) { create(:project, :public) }
+    let(:forker) { create(:user) }
+
+    before do
+      stub_feature_flags(object_pools: true)
+    end
+
+    context 'when no pool exists' do
+      it 'creates a new object pool' do
+        forked_project = fork_project(fork_from_project, forker)
+
+        expect(forked_project.pool_repository).to eq(fork_from_project.pool_repository)
+      end
+    end
+
+    context 'when a pool already exists' do
+      let!(:pool_repository) { create(:pool_repository, source_project: fork_from_project) }
+
+      it 'joins the object pool' do
+        forked_project = fork_project(fork_from_project, forker)
+
+        expect(forked_project.pool_repository).to eq(fork_from_project.pool_repository)
+      end
+    end
+  end
+
   context 'when linking fork to an existing project' do
     let(:fork_from_project) { create(:project, :public) }
     let(:fork_to_project) { create(:project, :public) }
diff --git a/spec/workers/object_pool/create_worker_spec.rb b/spec/workers/object_pool/create_worker_spec.rb
new file mode 100644
index 00000000000..06416489472
--- /dev/null
+++ b/spec/workers/object_pool/create_worker_spec.rb
@@ -0,0 +1,59 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe ObjectPool::CreateWorker do
+  let(:pool) { create(:pool_repository, :scheduled) }
+
+  subject { described_class.new }
+
+  describe '#perform' do
+    context 'when the pool creation is successful' do
+      it 'marks the pool as ready' do
+        subject.perform(pool.id)
+
+        expect(pool.reload).to be_ready
+      end
+    end
+
+    context 'when a the pool already exists' do
+      before do
+        pool.create_object_pool
+      end
+
+      it 'cleans up the pool' do
+        expect do
+          subject.perform(pool.id)
+        end.to raise_error(GRPC::FailedPrecondition)
+
+        expect(pool.reload.failed?).to be(true)
+      end
+    end
+
+    context 'when the server raises an unknown error' do
+      before do
+        allow_any_instance_of(PoolRepository).to receive(:create_object_pool).and_raise(GRPC::Internal)
+      end
+
+      it 'marks the pool as failed' do
+        expect do
+          subject.perform(pool.id)
+        end.to raise_error(GRPC::Internal)
+
+        expect(pool.reload.failed?).to be(true)
+      end
+    end
+
+    context 'when the pool creation failed before' do
+      let(:pool) { create(:pool_repository, :failed) }
+
+      it 'deletes the pool first' do
+        expect_any_instance_of(PoolRepository).to receive(:delete_object_pool)
+
+        subject.perform(pool.id)
+
+        expect(pool.reload).to be_ready
+      end
+    end
+  end
+end
diff --git a/spec/workers/object_pool/join_worker_spec.rb b/spec/workers/object_pool/join_worker_spec.rb
new file mode 100644
index 00000000000..906bc22c8d2
--- /dev/null
+++ b/spec/workers/object_pool/join_worker_spec.rb
@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe ObjectPool::JoinWorker do
+  let(:pool) { create(:pool_repository, :ready) }
+  let(:project) { pool.source_project }
+  let(:repository) { project.repository }
+
+  subject { described_class.new }
+
+  describe '#perform' do
+    context "when the pool is not joinable" do
+      let(:pool) { create(:pool_repository, :scheduled) }
+
+      it "doesn't raise an error" do
+        expect do
+          subject.perform(pool.id, project.id)
+        end.not_to raise_error
+      end
+    end
+
+    context 'when the pool has been joined before' do
+      before do
+        pool.link_repository(repository)
+      end
+
+      it 'succeeds in joining' do
+        expect do
+          subject.perform(pool.id, project.id)
+        end.not_to raise_error
+      end
+    end
+  end
+end