Perform FindTag RPC request for a single tag

Currently, we perform FindAllTags RPC even for a single tag
Using FindTag for single cases would be more efficient

Changelog: performance

app/models/repository.rb

@@ -191,7 +191,11 @@ class Repository
   end
 
   def find_tag(name)
-    tags.find { |tag| tag.name == name }
+    if @tags.blank? && Feature.enabled?(:find_tag_via_gitaly, project, default_enabled: :yaml)
+      raw_repository.find_tag(name)
+    else
+      tags.find { |tag| tag.name == name }
+    end
   end
 
   def ambiguous_ref?(ref)

config/feature_flags/development/find_tag_via_gitaly.yml

+---
+name: find_tag_via_gitaly
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/70181
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/340899
+milestone: '14.3'
+type: development
+group: group::source code
+default_enabled: false

lib/gitlab/git/repository.rb

@@ -127,6 +127,13 @@ module Gitlab
         end
       end
 
+      def find_tag(name)
+        wrapped_gitaly_errors do
+          gitaly_ref_client.find_tag(name)
+        end
+      rescue CommandError
+      end
+
       def local_branches(sort_by: nil, pagination_params: nil)
         wrapped_gitaly_errors do
           gitaly_ref_client.local_branches(sort_by: sort_by, pagination_params: pagination_params)
@@ -604,10 +611,6 @@ module Gitlab
         end
       end
 
-      def find_tag(name)
-        tags.find { |tag| tag.name == name }
-      end
-
       def merge_to_ref(user, **kwargs)
         wrapped_gitaly_errors do
           gitaly_operation_client.user_merge_to_ref(user, **kwargs)

lib/gitlab/gitaly_client/ref_service.rb

@@ -129,6 +129,21 @@ module Gitlab
         Gitlab::Git::Branch.new(@repository, encode!(branch.name.dup), branch.target_commit.id, target_commit)
       end
 
+      def find_tag(tag_name)
+        return if tag_name.blank?
+
+        request = Gitaly::FindTagRequest.new(
+          repository: @gitaly_repo,
+          tag_name: encode_binary(tag_name)
+        )
+
+        response = GitalyClient.call(@repository.storage, :ref_service, :find_tag, request, timeout: GitalyClient.medium_timeout)
+        tag = response.tag
+        return unless tag
+
+        Gitlab::Git::Tag.new(@repository, tag)
+      end
+
       def delete_refs(refs: [], except_with_prefixes: [])
         request = Gitaly::DeleteRefsRequest.new(
           repository: @gitaly_repo,

spec/lib/gitlab/gitaly_client/ref_service_spec.rb

@@ -92,6 +92,36 @@ RSpec.describe Gitlab::GitalyClient::RefService do
     end
   end
 
+  describe '#find_branch' do
+    it 'sends a find_branch message' do
+      expect_any_instance_of(Gitaly::RefService::Stub)
+        .to receive(:find_branch)
+        .with(gitaly_request_with_path(storage_name, relative_path), kind_of(Hash))
+        .and_return(double(branch: Gitaly::Branch.new(name: 'name', target_commit: build(:gitaly_commit))))
+
+      client.find_branch('name')
+    end
+  end
+
+  describe '#find_tag' do
+    it 'sends a find_tag message' do
+      expect_any_instance_of(Gitaly::RefService::Stub)
+        .to receive(:find_tag)
+        .with(gitaly_request_with_path(storage_name, relative_path), kind_of(Hash))
+        .and_return(double(tag: Gitaly::Tag.new))
+
+      client.find_tag('name')
+    end
+
+    context 'when tag is empty' do
+      it 'does not send a fing_tag message' do
+        expect_any_instance_of(Gitaly::RefService::Stub).not_to receive(:find_tag)
+
+        expect(client.find_tag('')).to be_nil
+      end
+    end
+  end
+
   describe '#default_branch_name' do
     it 'sends a find_default_branch_name message' do
       expect_any_instance_of(Gitaly::RefService::Stub)

spec/models/repository_spec.rb

@@ -2366,6 +2366,42 @@ RSpec.describe Repository do
     end
   end
 
+  describe '#find_tag' do
+    before do
+      allow(Gitlab::GitalyClient).to receive(:call).and_call_original
+    end
+
+    it 'finds a tag with specified name by performing FindTag request' do
+      expect(Gitlab::GitalyClient)
+        .to receive(:call).with(anything, :ref_service, :find_tag, anything, anything).and_call_original
+
+      expect(repository.find_tag('v1.1.0').name).to eq('v1.1.0')
+    end
+
+    it 'does not perform Gitaly call when tags are preloaded' do
+      repository.tags
+
+      expect(Gitlab::GitalyClient).not_to receive(:call)
+
+      expect(repository.find_tag('v1.1.0').name).to eq('v1.1.0')
+    end
+
+    it 'returns nil when tag does not exists' do
+      expect(repository.find_tag('does-not-exist')).to be_nil
+    end
+
+    context 'when find_tag_via_gitaly is disabled' do
+      it 'fetches all tags' do
+        stub_feature_flags(find_tag_via_gitaly: false)
+
+        expect(Gitlab::GitalyClient)
+          .to receive(:call).with(anything, :ref_service, :find_all_tags, anything, anything).and_call_original
+
+        expect(repository.find_tag('v1.1.0').name).to eq('v1.1.0')
+      end
+    end
+  end
+
   describe '#avatar' do
     it 'returns nil if repo does not exist' do
       allow(repository).to receive(:root_ref).and_raise(Gitlab::Git::Repository::NoRepository)