Allow external diffs to be used conditionally

Since external diffs are likely to be a bit slower than in-database
ones, add a mode that makes diffs external after they've been obsoleted
by events. This should strike a balance between performance and disk
space.

A background cron drives the majority of migrations, since diffs become
outdated through user actions.

2 files changed, 50 insertions, 0 deletions

diff --git a/spec/workers/migrate_external_diffs_worker_spec.rb b/spec/workers/migrate_external_diffs_worker_spec.rb
new file mode 100644
index 00000000000..88d48cad14b
--- /dev/null
+++ b/spec/workers/migrate_external_diffs_worker_spec.rb
@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe MigrateExternalDiffsWorker do
+  let(:worker) { described_class.new }
+  let(:diff) { create(:merge_request).merge_request_diff }
+
+  describe '#perform' do
+    it 'migrates the listed diff' do
+      expect_next_instance_of(MergeRequests::MigrateExternalDiffsService) do |instance|
+        expect(instance.diff).to eq(diff)
+        expect(instance).to receive(:execute)
+      end
+
+      worker.perform(diff.id)
+    end
+
+    it 'does nothing if the diff is missing' do
+      diff.destroy
+
+      worker.perform(diff.id)
+    end
+  end
+end
diff --git a/spec/workers/schedule_migrate_external_diffs_worker_spec.rb b/spec/workers/schedule_migrate_external_diffs_worker_spec.rb
new file mode 100644
index 00000000000..9d6fecc9f4e
--- /dev/null
+++ b/spec/workers/schedule_migrate_external_diffs_worker_spec.rb
@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe ScheduleMigrateExternalDiffsWorker do
+  include ExclusiveLeaseHelpers
+
+  let(:worker) { described_class.new }
+
+  describe '#perform' do
+    it 'triggers a scan for diffs to migrate' do
+      expect(MergeRequests::MigrateExternalDiffsService).to receive(:enqueue!)
+
+      worker.perform
+    end
+
+    it 'will not run if the lease is already taken' do
+      stub_exclusive_lease_taken('schedule_migrate_external_diffs_worker', timeout: 2.hours)
+
+      expect(MergeRequests::MigrateExternalDiffsService).not_to receive(:enqueue!)
+
+      worker.perform
+    end
+  end
+end