Add Gitaly diff stats RPC client

This adds a basic interface to fetch diff statistics given two SHAs.
It's a requirement for #49399 #20282 and #19232.

changelogs/unreleased/osw-gitaly-diff-stats-client.yml

+---
+title: Add Gitaly diff stats RPC client
+merge_request: 21732
+author:
+type: changed

lib/gitlab/git/diff_stats_collection.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Git
+    class DiffStatsCollection
+      include Enumerable
+
+      def initialize(diff_stats)
+        @collection = diff_stats
+      end
+
+      def each(&block)
+        @collection.each(&block)
+      end
+    end
+  end
+end

lib/gitlab/git/repository.rb

@@ -463,6 +463,16 @@ module Gitlab
         Gitlab::Git::DiffCollection.new(iterator, options)
       end
 
+      def diff_stats(left_id, right_id)
+        stats = wrapped_gitaly_errors do
+          gitaly_commit_client.diff_stats(left_id, right_id)
+        end
+
+        Gitlab::Git::DiffStatsCollection.new(stats)
+      rescue CommandError
+        Gitlab::Git::DiffStatsCollection.new([])
+      end
+
       # Returns a RefName for a given SHA
       def ref_name_for_sha(ref_path, sha)
         raise ArgumentError, "sha can't be empty" unless sha.present?

lib/gitlab/gitaly_client/commit_service.rb

@@ -172,6 +172,17 @@ module Gitlab
         consume_commits_response(response)
       end
 
+      def diff_stats(left_commit_sha, right_commit_sha)
+        request = Gitaly::DiffStatsRequest.new(
+          repository: @gitaly_repo,
+          left_commit_id: left_commit_sha,
+          right_commit_id: right_commit_sha
+        )
+
+        response = GitalyClient.call(@repository.storage, :diff_service, :diff_stats, request, timeout: GitalyClient.medium_timeout)
+        response.flat_map(&:stats)
+      end
+
       def find_all_commits(opts = {})
         request = Gitaly::FindAllCommitsRequest.new(
           repository: @gitaly_repo,

spec/lib/gitlab/git/repository_spec.rb

@@ -1229,6 +1229,32 @@ describe Gitlab::Git::Repository, :seed_helper do
     end
   end
 
+  describe '#diff_stats' do
+    let(:left_commit_id) { 'feature' }
+    let(:right_commit_id) { 'master' }
+
+    it 'returns a DiffStatsCollection' do
+      collection = repository.diff_stats(left_commit_id, right_commit_id)
+
+      expect(collection).to be_a(Gitlab::Git::DiffStatsCollection)
+      expect(collection).to be_a(Enumerable)
+    end
+
+    it 'yields Gitaly::DiffStats objects' do
+      collection = repository.diff_stats(left_commit_id, right_commit_id)
+
+      expect(collection.to_a).to all(be_a(Gitaly::DiffStats))
+    end
+
+    it 'returns no Gitaly::DiffStats when SHAs are invalid' do
+      collection = repository.diff_stats('foo', 'bar')
+
+      expect(collection).to be_a(Gitlab::Git::DiffStatsCollection)
+      expect(collection).to be_a(Enumerable)
+      expect(collection.to_a).to be_empty
+    end
+  end
+
   describe "#ls_files" do
     let(:master_file_paths) { repository.ls_files("master") }
     let(:utf8_file_paths) { repository.ls_files("ls-files-utf8") }

spec/lib/gitlab/gitaly_client/commit_service_spec.rb

@@ -118,6 +118,22 @@ describe Gitlab::GitalyClient::CommitService do
     end
   end
 
+  describe '#diff_stats' do
+    let(:left_commit_id) { 'master' }
+    let(:right_commit_id) { 'cfe32cf61b73a0d5e9f13e774abde7ff789b1660' }
+
+    it 'sends an RPC request' do
+      request = Gitaly::DiffStatsRequest.new(repository: repository_message,
+                                             left_commit_id: left_commit_id,
+                                             right_commit_id: right_commit_id)
+
+      expect_any_instance_of(Gitaly::DiffService::Stub).to receive(:diff_stats)
+        .with(request, kind_of(Hash)).and_return([])
+
+      described_class.new(repository).diff_stats(left_commit_id, right_commit_id)
+    end
+  end
+
   describe '#tree_entries' do
     let(:path) { '/' }