Add back Gitaly feature flag for cache invalidation

Part of https://gitlab.com/gitlab-org/gitaly/issues/2292

Related to https://gitlab.com/gitlab-org/gitaly/issues/2288

This change is essentially a revert of
https://gitlab.com/gitlab-org/gitlab/merge_requests/18766. Due to an
issue found in production (see
https://gitlab.com/gitlab-com/gl-infra/production/issues/1501#note_262058610),
the cache invalidator needs to be disabled until the cache walker is
able to remove empty directories. This feature flag will allow us to
reenable the cache invalidator when we are ready to test the updated
cache walker.

GITALY_SERVER_VERSION

-1.76.0
+1.78.0

changelogs/unreleased/gitaly-cache-invalidator-ff.yml

+---
+title: Add back feature flag for cache invalidator
+merge_request: 22106
+author:
+type: changed

lib/feature/gitaly.rb

@@ -7,6 +7,7 @@ class Feature
     # Server feature flags should use '_' to separate words.
     SERVER_FEATURE_FLAGS =
       %w[
+        cache_invalidator
         inforef_uploadpack_cache
         get_tag_messages_go
         filter_shas_with_signatures_go

spec/requests/api/internal/base_spec.rb

@@ -326,7 +326,7 @@ describe API::Internal::Base do
           expect(json_response["gitaly"]["repository"]["relative_path"]).to eq(project.repository.gitaly_repository.relative_path)
           expect(json_response["gitaly"]["address"]).to eq(Gitlab::GitalyClient.address(project.repository_storage))
           expect(json_response["gitaly"]["token"]).to eq(Gitlab::GitalyClient.token(project.repository_storage))
-          expect(json_response["gitaly"]["features"]).to eq('gitaly-feature-inforef-uploadpack-cache' => 'true', 'gitaly-feature-get-tag-messages-go' => 'true', 'gitaly-feature-filter-shas-with-signatures-go' => 'true')
+          expect(json_response["gitaly"]["features"]).to eq('gitaly-feature-inforef-uploadpack-cache' => 'true', 'gitaly-feature-get-tag-messages-go' => 'true', 'gitaly-feature-filter-shas-with-signatures-go' => 'true', 'gitaly-feature-cache-invalidator' => 'true')
           expect(user.reload.last_activity_on).to eql(Date.today)
         end
       end
@@ -346,7 +346,7 @@ describe API::Internal::Base do
             expect(json_response["gitaly"]["repository"]["relative_path"]).to eq(project.repository.gitaly_repository.relative_path)
             expect(json_response["gitaly"]["address"]).to eq(Gitlab::GitalyClient.address(project.repository_storage))
             expect(json_response["gitaly"]["token"]).to eq(Gitlab::GitalyClient.token(project.repository_storage))
-            expect(json_response["gitaly"]["features"]).to eq('gitaly-feature-inforef-uploadpack-cache' => 'true', 'gitaly-feature-get-tag-messages-go' => 'true', 'gitaly-feature-filter-shas-with-signatures-go' => 'true')
+            expect(json_response["gitaly"]["features"]).to eq('gitaly-feature-inforef-uploadpack-cache' => 'true', 'gitaly-feature-get-tag-messages-go' => 'true', 'gitaly-feature-filter-shas-with-signatures-go' => 'true', 'gitaly-feature-cache-invalidator' => 'true')
             expect(user.reload.last_activity_on).to be_nil
           end
         end
@@ -588,7 +588,7 @@ describe API::Internal::Base do
           expect(json_response["gitaly"]["repository"]["relative_path"]).to eq(project.repository.gitaly_repository.relative_path)
           expect(json_response["gitaly"]["address"]).to eq(Gitlab::GitalyClient.address(project.repository_storage))
           expect(json_response["gitaly"]["token"]).to eq(Gitlab::GitalyClient.token(project.repository_storage))
-          expect(json_response["gitaly"]["features"]).to eq('gitaly-feature-inforef-uploadpack-cache' => 'true', 'gitaly-feature-get-tag-messages-go' => 'true', 'gitaly-feature-filter-shas-with-signatures-go' => 'true')
+          expect(json_response["gitaly"]["features"]).to eq('gitaly-feature-inforef-uploadpack-cache' => 'true', 'gitaly-feature-get-tag-messages-go' => 'true', 'gitaly-feature-filter-shas-with-signatures-go' => 'true', 'gitaly-feature-cache-invalidator' => 'true')
         end
       end