Omit max-count for diverging_commit_counts behind feature flag

We want to optimize the query for the CountDivergingCommits rpc by
removing the --max-count argument now that we have commit graphs
enabled for all repositories during housekeeping. However, we want to
test this first behind a feature flag.

app/models/repository.rb

@@ -283,14 +283,19 @@ class Repository
   end
 
   def diverging_commit_counts(branch)
+    return diverging_commit_counts_without_max(branch) if Feature.enabled?('gitaly_count_diverging_commits_no_max')
+
+    ## TODO: deprecate the below code after 12.0
     @root_ref_hash ||= raw_repository.commit(root_ref).id
     cache.fetch(:"diverging_commit_counts_#{branch.name}") do
       # Rugged seems to throw a `ReferenceError` when given branch_names rather
       # than SHA-1 hashes
+      branch_sha = branch.dereferenced_target.sha
+
       number_commits_behind, number_commits_ahead =
         raw_repository.diverging_commit_count(
           @root_ref_hash,
-          branch.dereferenced_target.sha,
+          branch_sha,
           max_count: MAX_DIVERGING_COUNT)
 
       if number_commits_behind + number_commits_ahead >= MAX_DIVERGING_COUNT
@@ -301,6 +306,22 @@ class Repository
     end
   end
 
+  def diverging_commit_counts_without_max(branch)
+    @root_ref_hash ||= raw_repository.commit(root_ref).id
+    cache.fetch(:"diverging_commit_counts_without_max_#{branch.name}") do
+      # Rugged seems to throw a `ReferenceError` when given branch_names rather
+      # than SHA-1 hashes
+      branch_sha = branch.dereferenced_target.sha
+
+      number_commits_behind, number_commits_ahead =
+        raw_repository.diverging_commit_count(
+          @root_ref_hash,
+          branch_sha)
+
+      { behind: number_commits_behind, ahead: number_commits_ahead }
+    end
+  end
+
   def archive_metadata(ref, storage_path, format = "tar.gz", append_sha:, path: nil)
     raw_repository.archive_metadata(
       ref,

changelogs/unreleased/jc-omit-count-diverging-commits-max.yml

@@ -0,0 +1,5 @@
+---
+title: Omit max-count for diverging_commit_counts behind feature flag
+merge_request: 28157
+author:
+type: other

lib/gitlab/git/repository.rb

@@ -500,7 +500,7 @@ module Gitlab
       end
 
       # Return total diverging commits count
-      def diverging_commit_count(from, to, max_count:)
+      def diverging_commit_count(from, to, max_count: 0)
         wrapped_gitaly_errors do
           gitaly_commit_client.diverging_commit_count(from, to, max_count: max_count)
         end

spec/models/repository_spec.rb

@@ -2286,12 +2286,45 @@ describe Repository do
   end
 
   describe '#diverging_commit_counts' do
+    let(:diverged_branch) { repository.find_branch('fix') }
+    let(:root_ref_sha) { repository.raw_repository.commit(repository.root_ref).id }
+    let(:diverged_branch_sha) { diverged_branch.dereferenced_target.sha }
+
     it 'returns the commit counts behind and ahead of default branch' do
-      result = repository.diverging_commit_counts(
-        repository.find_branch('fix'))
+      result = repository.diverging_commit_counts(diverged_branch)
 
       expect(result).to eq(behind: 29, ahead: 2)
     end
+
+    context 'when gitaly_count_diverging_commits_no_max is enabled' do
+      before do
+        stub_feature_flags(gitaly_count_diverging_commits_no_max: true)
+      end
+
+      it 'calls diverging_commit_count without max count' do
+        expect(repository.raw_repository)
+          .to receive(:diverging_commit_count)
+          .with(root_ref_sha, diverged_branch_sha)
+          .and_return([29, 2])
+
+        repository.diverging_commit_counts(diverged_branch)
+      end
+    end
+
+    context 'when gitaly_count_diverging_commits_no_max is disabled' do
+      before do
+        stub_feature_flags(gitaly_count_diverging_commits_no_max: false)
+      end
+
+      it 'calls diverging_commit_count with max count' do
+        expect(repository.raw_repository)
+          .to receive(:diverging_commit_count)
+          .with(root_ref_sha, diverged_branch_sha, max_count: Repository::MAX_DIVERGING_COUNT)
+          .and_return([29, 2])
+
+        repository.diverging_commit_counts(diverged_branch)
+      end
+    end
   end
 
   describe '#refresh_method_caches' do