Avoid Gitaly RPC errors when fetching diff stats

changelogs/unreleased/osw-fallback-on-blank-refs.yml

+---
+title: Avoid Gitaly RPC errors when fetching diff stats
+merge_request: 22995
+author:
+type: fixed

lib/gitlab/git/repository.rb

@@ -419,13 +419,17 @@ module Gitlab
       end
 
       def diff_stats(left_id, right_id)
+        if [left_id, right_id].any? { |ref| ref.blank? || Gitlab::Git.blank_ref?(ref) }
+          return empty_diff_stats
+        end
+
         stats = wrapped_gitaly_errors do
           gitaly_commit_client.diff_stats(left_id, right_id)
         end
 
         Gitlab::Git::DiffStatsCollection.new(stats)
       rescue CommandError, TypeError
-        Gitlab::Git::DiffStatsCollection.new([])
+        empty_diff_stats
       end
 
       # Returns a RefName for a given SHA
@@ -962,6 +966,10 @@ module Gitlab
 
       private
 
+      def empty_diff_stats
+        Gitlab::Git::DiffStatsCollection.new([])
+      end
+
       def uncached_has_local_branches?
         wrapped_gitaly_errors do
           gitaly_repository_client.has_local_branches?

spec/lib/gitlab/git/repository_spec.rb

@@ -1095,12 +1095,26 @@ describe Gitlab::Git::Repository, :seed_helper do
     end
 
     it 'returns no Gitaly::DiffStats when there is a nil SHA' do
+      expect_any_instance_of(Gitlab::GitalyClient::CommitService)
+        .not_to receive(:diff_stats)
+
       collection = repository.diff_stats(nil, 'master')
 
       expect(collection).to be_a(Gitlab::Git::DiffStatsCollection)
       expect(collection).to be_a(Enumerable)
       expect(collection.to_a).to be_empty
     end
+
+    it 'returns no Gitaly::DiffStats when there is a BLANK_SHA' do
+      expect_any_instance_of(Gitlab::GitalyClient::CommitService)
+        .not_to receive(:diff_stats)
+
+      collection = repository.diff_stats(Gitlab::Git::BLANK_SHA, 'master')
+
+      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