Introduce Repository#with_repo_branch_commit

app/models/repository.rb

@@ -1063,19 +1063,26 @@ class Repository
     Gitlab::Popen.popen(args, path_to_repo).first.lines.map(&:strip)
   end
 
-  def with_tmp_ref(source_repository, source_branch_name)
-    tmp_ref = "refs/tmp/#{SecureRandom.hex}/head"
+  def with_repo_branch_commit(source_repository, source_branch_name)
+    branch_name_or_sha =
+      if source_repository == self
+        source_branch_name
+      else
+        tmp_ref = "refs/tmp/#{SecureRandom.hex}/head"
 
-    fetch_ref(
-      source_repository.path_to_repo,
-      "#{Gitlab::Git::BRANCH_REF_PREFIX}#{source_branch_name}",
-      tmp_ref
-    )
+        fetch_ref(
+          source_repository.path_to_repo,
+          "#{Gitlab::Git::BRANCH_REF_PREFIX}#{source_branch_name}",
+          tmp_ref
+        )
+
+        source_repository.commit(source_branch_name).sha
+      end
 
-    yield
+    yield(commit(branch_name_or_sha))
 
   ensure
-    rugged.references.delete(tmp_ref)
+    rugged.references.delete(tmp_ref) if tmp_ref
   end
 
   def fetch_ref(source_path, source_ref, target_ref)

app/services/compare_service.rb

@@ -11,19 +11,13 @@ class CompareService
   end
 
   def execute(target_project, target_branch, straight: false)
-    source_sha = source_project.repository.
-      commit(source_branch_name).try(:sha)
-
-    return unless source_sha
-
     # If compare with other project we need to fetch ref first
-    if target_project == source_project
-      compare(source_sha, target_project, target_branch, straight)
-    else
-      target_project.repository.with_tmp_ref(
-        source_project.repository, source_branch_name) do
-        compare(source_sha, target_project, target_branch, straight)
-      end
+    target_project.repository.with_repo_branch_commit(
+      source_project.repository,
+      source_branch_name) do |commit|
+      break unless commit
+
+      compare(commit.sha, target_project, target_branch, straight)
     end
   end
 

app/services/git_operation_service.rb

@@ -43,23 +43,17 @@ class GitOperationService
   def with_branch(
     branch_name,
     source_branch_name: nil,
-    source_project: repository.project)
+    source_project: repository.project,
+    &block)
 
     check_with_branch_arguments!(
       branch_name, source_branch_name, source_project)
 
-    source_commit = source_project.repository.find_branch(
-      source_branch_name || branch_name).try(:dereferenced_target)
-
     update_branch_with_hooks(branch_name) do
-      if repository.project == source_project
-        yield(source_commit)
-      else
-        repository.with_tmp_ref(
-          source_project.repository, source_branch_name) do
-            yield(source_commit)
-          end
-      end
+      repository.with_repo_branch_commit(
+        source_project.repository,
+        source_branch_name || branch_name,
+        &block)
     end
   end