Ensure MRs always use branch refs for comparison

app/controllers/projects/merge_requests/creations_controller.rb

@@ -65,7 +65,7 @@ class Projects::MergeRequests::CreationsController < Projects::MergeRequests::Ap
 
     if params[:ref].present?
       @ref = params[:ref]
-      @commit = @repository.commit("refs/heads/#{@ref}")
+      @commit = @repository.commit(Gitlab::Git::BRANCH_REF_PREFIX + @ref)
     end
 
     render layout: false
@@ -76,7 +76,7 @@ class Projects::MergeRequests::CreationsController < Projects::MergeRequests::Ap
 
     if params[:ref].present?
       @ref = params[:ref]
-      @commit = @target_project.commit("refs/heads/#{@ref}")
+      @commit = @target_project.commit(Gitlab::Git::BRANCH_REF_PREFIX + @ref)
     end
 
     render layout: false

app/models/merge_request.rb

@@ -365,16 +365,28 @@ class MergeRequest < ActiveRecord::Base
   # We use these attributes to force these to the intended values.
   attr_writer :target_branch_sha, :source_branch_sha
 
+  def source_branch_ref
+    return @source_branch_sha if @source_branch_sha
+    return unless source_branch
+
+    Gitlab::Git::BRANCH_REF_PREFIX + source_branch
+  end
+
+  def target_branch_ref
+    return @target_branch_sha if @target_branch_sha
+    return unless target_branch
+
+    Gitlab::Git::BRANCH_REF_PREFIX + target_branch
+  end
+
   def source_branch_head
     return unless source_project
 
-    source_branch_ref = @source_branch_sha || source_branch
     source_project.repository.commit(source_branch_ref) if source_branch_ref
   end
 
   def target_branch_head
-    target_branch_ref = @target_branch_sha || target_branch
-    target_project.repository.commit(target_branch_ref) if target_branch_ref
+    target_project.repository.commit(target_branch_ref)
   end
 
   def branch_merge_base_commit

app/services/compare_service.rb

 require 'securerandom'
 
-# Compare 2 branches for one repo or between repositories
+# Compare 2 refs for one repo or between repositories
 # and return Gitlab::Git::Compare object that responds to commits and diffs
 class CompareService
-  attr_reader :start_project, :start_branch_name
+  attr_reader :start_project, :start_ref_name
 
-  def initialize(new_start_project, new_start_branch_name)
+  def initialize(new_start_project, new_start_ref_name)
     @start_project = new_start_project
-    @start_branch_name = new_start_branch_name
+    @start_ref_name = new_start_ref_name
   end
 
-  def execute(target_project, target_branch, straight: false)
-    raw_compare = target_project.repository.compare_source_branch(target_branch, start_project.repository, start_branch_name, straight: straight)
+  def execute(target_project, target_ref, straight: false)
+    raw_compare = target_project.repository.compare_source_branch(target_ref, start_project.repository, start_ref_name, straight: straight)
 
     Compare.new(raw_compare, target_project, straight: straight) if raw_compare
   end

app/services/merge_requests/build_service.rb

@@ -18,7 +18,17 @@ module MergeRequests
 
     attr_accessor :merge_request
 
-    delegate :target_branch, :source_branch, :source_project, :target_project, :compare_commits, :wip_title, :description, :errors, to: :merge_request
+    delegate :target_branch,
+             :target_branch_ref,
+             :target_project,
+             :source_branch,
+             :source_branch_ref,
+             :source_project,
+             :compare_commits,
+             :wip_title,
+             :description,
+             :errors,
+             to: :merge_request
 
     def find_source_project
       return source_project if source_project.present? && can?(current_user, :read_project, source_project)
@@ -54,10 +64,10 @@ module MergeRequests
     def compare_branches
       compare = CompareService.new(
         source_project,
-        source_branch
+        source_branch_ref
       ).execute(
         target_project,
-        target_branch
+        target_branch_ref
       )
 
       if compare

changelogs/unreleased/40530-merge-request-generates-wrong-diff-when-branch-and-tag-have-the-same-name.yml

+---
+title: Fix merge requests where the source or target branch name matches a tag name
+merge_request: 15591
+author:
+type: fixed

lib/gitlab/git/repository.rb

@@ -1046,9 +1046,15 @@ module Gitlab
       end
 
       def with_repo_tmp_commit(start_repository, start_branch_name, sha)
+        source_ref = start_branch_name
+
+        unless Gitlab::Git.branch_ref?(source_ref)
+          source_ref = "#{Gitlab::Git::BRANCH_REF_PREFIX}#{source_ref}"
+        end
+
         tmp_ref = fetch_ref(
           start_repository,
-          source_ref: "#{Gitlab::Git::BRANCH_REF_PREFIX}#{start_branch_name}",
+          source_ref: source_ref,
           target_ref: "refs/tmp/#{SecureRandom.hex}"
         )
 

spec/lib/gitlab/diff/position_tracer_spec.rb

@@ -1771,9 +1771,9 @@ describe Gitlab::Diff::PositionTracer do
 
     describe "merge of target branch" do
       let(:merge_commit) do
-        update_file_again_commit
+        second_create_file_commit
 
-        merge_request = create(:merge_request, source_branch: second_create_file_commit.sha, target_branch: branch_name, source_project: project)
+        merge_request = create(:merge_request, source_branch: second_branch_name, target_branch: branch_name, source_project: project)
 
         repository.merge(current_user, merge_request.diff_head_sha, merge_request, "Merge branches")
 

spec/models/merge_request_spec.rb

@@ -259,7 +259,7 @@ describe MergeRequest do
   end
 
   describe '#source_branch_sha' do
-    let(:last_branch_commit) { subject.source_project.repository.commit(subject.source_branch) }
+    let(:last_branch_commit) { subject.source_project.repository.commit(Gitlab::Git::BRANCH_REF_PREFIX + subject.source_branch) }
 
     context 'with diffs' do
       subject { create(:merge_request, :with_diffs) }
@@ -273,6 +273,21 @@ describe MergeRequest do
       it 'returns the sha of the source branch last commit' do
         expect(subject.source_branch_sha).to eq(last_branch_commit.sha)
       end
+
+      context 'when there is a tag name matching the branch name' do
+        let(:tag_name) { subject.source_branch }
+
+        it 'returns the sha of the source branch last commit' do
+          subject.source_project.repository.add_tag(subject.author,
+                                                    tag_name,
+                                                    subject.target_branch_sha,
+                                                    'Add a tag')
+
+          expect(subject.source_branch_sha).to eq(last_branch_commit.sha)
+
+          subject.source_project.repository.rm_tag(subject.author, tag_name)
+        end
+      end
     end
 
     context 'when the merge request is being created' do
@@ -933,7 +948,7 @@ describe MergeRequest do
 
       context 'with a completely different branch' do
         before do
-          subject.update(target_branch: 'v1.0.0')
+          subject.update(target_branch: 'csv')
         end
 
         it_behaves_like 'returning all SHA'
@@ -941,7 +956,7 @@ describe MergeRequest do
 
       context 'with a branch having no difference' do
         before do
-          subject.update(target_branch: 'v1.1.0')
+          subject.update(target_branch: 'branch-merged')
           subject.reload # make sure commits were not cached
         end
 

spec/services/merge_requests/build_service_spec.rb

@@ -29,13 +29,27 @@ describe MergeRequests::BuildService do
 
   before do
     project.team << [user, :guest]
+  end
 
+  def stub_compare
     allow(CompareService).to receive_message_chain(:new, :execute).and_return(compare)
     allow(project).to receive(:commit).and_return(commit_1)
     allow(project).to receive(:commit).and_return(commit_2)
   end
 
-  describe 'execute' do
+  describe '#execute' do
+    it 'calls the compare service with the correct arguments' do
+      expect(CompareService).to receive(:new)
+                                  .with(project, Gitlab::Git::BRANCH_REF_PREFIX + source_branch)
+                                  .and_call_original
+
+      expect_any_instance_of(CompareService).to receive(:execute)
+                                                  .with(project, Gitlab::Git::BRANCH_REF_PREFIX + target_branch)
+                                                  .and_call_original
+
+      merge_request
+    end
+
     context 'missing source branch' do
       let(:source_branch) { '' }
 
@@ -52,6 +66,10 @@ describe MergeRequests::BuildService do
       let(:target_branch) { nil }
       let(:commits) { Commit.decorate([commit_1], project) }
 
+      before do
+        stub_compare
+      end
+
       it 'creates compare object with target branch as default branch' do
         expect(merge_request.compare).to be_present
         expect(merge_request.target_branch).to eq(project.default_branch)
@@ -77,6 +95,10 @@ describe MergeRequests::BuildService do
     context 'no commits in the diff' do
       let(:commits) { [] }
 
+      before do
+        stub_compare
+      end
+
       it 'allows the merge request to be created' do
         expect(merge_request.can_be_created).to eq(true)
       end
@@ -89,6 +111,10 @@ describe MergeRequests::BuildService do
     context 'one commit in the diff' do
       let(:commits) { Commit.decorate([commit_1], project) }
 
+      before do
+        stub_compare
+      end
+
       it 'allows the merge request to be created' do
         expect(merge_request.can_be_created).to eq(true)
       end
@@ -149,6 +175,10 @@ describe MergeRequests::BuildService do
     context 'more than one commit in the diff' do
       let(:commits) { Commit.decorate([commit_1, commit_2], project) }
 
+      before do
+        stub_compare
+      end
+
       it 'allows the merge request to be created' do
         expect(merge_request.can_be_created).to eq(true)
       end