Add GitDeduplicationService for deduplication housekeeping

GitDeduplicationService performs idempotent operations on deduplicated
projects.

app/services/projects/git_deduplication_service.rb

@@ -0,0 +1,64 @@
+# frozen_string_literal: true
+
+module Projects
+  class GitDeduplicationService < BaseService
+    include ExclusiveLeaseGuard
+
+    LEASE_TIMEOUT = 86400
+
+    delegate :pool_repository, to: :project
+    attr_reader :project
+
+    def initialize(project)
+      @project = project
+    end
+
+    def execute
+      try_obtain_lease do
+        unless project.has_pool_repository?
+          disconnect_git_alternates
+          break
+        end
+
+        if source_project? && pool_can_fetch_from_source?
+          fetch_from_source
+        end
+
+        project.link_pool_repository if same_storage_as_pool?(project.repository)
+      end
+    end
+
+    private
+
+    def disconnect_git_alternates
+      project.repository.disconnect_alternates
+    end
+
+    def pool_can_fetch_from_source?
+      project.git_objects_poolable? &&
+        same_storage_as_pool?(pool_repository.source_project.repository)
+    end
+
+    def same_storage_as_pool?(repository)
+      pool_repository.object_pool.repository.storage == repository.storage
+    end
+
+    def fetch_from_source
+      project.pool_repository.object_pool.fetch
+    end
+
+    def source_project?
+      return unless project.has_pool_repository?
+
+      project.pool_repository.source_project == project
+    end
+
+    def lease_timeout
+      LEASE_TIMEOUT
+    end
+
+    def lease_key
+      "git_deduplication:#{project.id}"
+    end
+  end
+end

app/workers/git_garbage_collect_worker.rb

@@ -23,7 +23,9 @@ class GitGarbageCollectWorker
     end
 
     task = task.to_sym
-    project.link_pool_repository
+
+    ::Projects::GitDeduplicationService.new(project).execute
+
     gitaly_call(task, project.repository.raw_repository)
 
     # Refresh the branch cache in case garbage collection caused a ref lookup to fail

spec/services/projects/git_deduplication_service_spec.rb

@@ -0,0 +1,90 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Projects::GitDeduplicationService do
+  include ExclusiveLeaseHelpers
+
+  let(:pool) { create(:pool_repository, :ready) }
+  let(:project) { create(:project, :repository) }
+  let(:lease_key) { "git_deduplication:#{project.id}" }
+  let(:lease_timeout) { Projects::GitDeduplicationService::LEASE_TIMEOUT }
+
+  subject(:service) { described_class.new(project) }
+
+  describe '#execute' do
+    context 'when there is not already a lease' do
+      context 'when the project does not have a pool repository' do
+        it 'calls disconnect_git_alternates' do
+          stub_exclusive_lease(lease_key, timeout: lease_timeout)
+
+          expect(project.repository).to receive(:disconnect_alternates)
+
+          service.execute
+        end
+      end
+
+      context 'when the project has a pool repository' do
+        let(:project) { create(:project, :repository, pool_repository: pool) }
+
+        context 'when the project is a source project' do
+          let(:lease_key) { "git_deduplication:#{pool.source_project.id}" }
+
+          subject(:service) { described_class.new(pool.source_project) }
+
+          it 'calls fetch' do
+            stub_exclusive_lease(lease_key, timeout: lease_timeout)
+            allow(pool.source_project).to receive(:git_objects_poolable?).and_return(true)
+
+            expect(pool.object_pool).to receive(:fetch)
+
+            service.execute
+          end
+
+          it 'does not call fetch if git objects are not poolable' do
+            stub_exclusive_lease(lease_key, timeout: lease_timeout)
+            allow(pool.source_project).to receive(:git_objects_poolable?).and_return(false)
+
+            expect(pool.object_pool).not_to receive(:fetch)
+
+            service.execute
+          end
+
+          it 'does not call fetch if pool and project are not on the same storage' do
+            stub_exclusive_lease(lease_key, timeout: lease_timeout)
+            allow(pool.source_project.repository).to receive(:storage).and_return('special_storage_001')
+
+            expect(pool.object_pool).not_to receive(:fetch)
+
+            service.execute
+          end
+        end
+
+        it 'links the repository to the object pool' do
+          expect(project).to receive(:link_pool_repository)
+
+          service.execute
+        end
+
+        it 'does not link the repository to the object pool if they are not on the same storage' do
+          allow(project.repository).to receive(:storage).and_return('special_storage_001')
+          expect(project).not_to receive(:link_pool_repository)
+
+          service.execute
+        end
+      end
+
+      context 'when a lease is already out' do
+        before do
+          stub_exclusive_lease_taken(lease_key, timeout: lease_timeout)
+        end
+
+        it 'fails when a lease is already out' do
+          expect(service).to receive(:log_error).with('Cannot obtain an exclusive lease. There must be another instance already in execution.')
+
+          service.execute
+        end
+      end
+    end
+  end
+end