Merge branch 'fix/14388' into 'master'

Fix an issue when the target branch of a MR had been deleted

Before displaying the "diverged commits" note, we're checking if the MR
is open, but we should check if it's mergeable instead because this
check ensure the source and target branches exist.

This was introduced by !2217 and fixes #14388.

See merge request !3294

3 files changed, 27 insertions, 1 deletions

diff --git a/CHANGELOG b/CHANGELOG
index 28c1c30e208..ca360c6706d 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -43,6 +43,7 @@ v 8.6.0 (unreleased)
   - Increase the notes polling timeout over time (Roberto Dip)
   - Add shortcut to toggle markdown preview (Florent Baldino)
   - Show labels in dashboard and group milestone views
+  - Fix an issue when the target branch of a MR had been deleted
   - Add main language of a project in the list of projects (Tiago Botelho)
   - Add #upcoming filter to Milestone filter (Tiago Botelho)
   - Add ability to show archived projects on dashboard, explore and group pages
diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index 30a7bd47be7..a6140b5b04c 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -516,7 +516,7 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def target_sha
-    @target_sha ||= target_project.repository.commit(target_branch).sha
+    @target_sha ||= target_project.repository.commit(target_branch).try(:sha)
   end
 
   def source_sha
@@ -572,8 +572,11 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def compute_diverged_commits_count
+    return 0 unless source_sha && target_sha
+
     Gitlab::Git::Commit.between(target_project.repository.raw_repository, source_sha, target_sha).size
   end
+  private :compute_diverged_commits_count
 
   def diverged_from_target_branch?
     diverged_commits_count > 0
diff --git a/spec/models/merge_request_spec.rb b/spec/models/merge_request_spec.rb
index 654c71b6825..2165cfb7a32 100644
--- a/spec/models/merge_request_spec.rb
+++ b/spec/models/merge_request_spec.rb
@@ -86,6 +86,16 @@ describe MergeRequest, models: true do
     end
   end
 
+  describe '#target_sha' do
+    context 'when the target branch does not exist anymore' do
+      subject { create(:merge_request).tap { |mr| mr.update_attribute(:target_branch, 'deleted') } }
+
+      it 'returns nil' do
+        expect(subject.target_sha).to be_nil
+      end
+    end
+  end
+
   describe '#source_sha' do
     let(:last_branch_commit) { subject.source_project.repository.commit(subject.source_branch) }
 
@@ -310,6 +320,18 @@ describe MergeRequest, models: true do
     let(:project)      { create(:project) }
     let(:fork_project) { create(:project, forked_from_project: project) }
 
+    context 'when the target branch does not exist anymore' do
+      subject { create(:merge_request).tap { |mr| mr.update_attribute(:target_branch, 'deleted') } }
+
+      it 'does not crash' do
+        expect{ subject.diverged_commits_count }.not_to raise_error
+      end
+
+      it 'returns 0' do
+        expect(subject.diverged_commits_count).to eq(0)
+      end
+    end
+
     context 'diverged on same repository' do
       subject(:merge_request_with_divergence) { create(:merge_request, :diverged, source_project: project, target_project: project) }