Merge branch 'fix-mr-source-sha' into 'master'

Fix MergeRequest#source_sha when there is no diff

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/14170

### Overview

This MR fixes an unhandled Exception when visiting the page of an open Merge Request  without diff.

### Description

`MergeRequest#source_sha` is expected to return the sha of the source branch last commit. But when an open Merge Request has no diff (e.g. all commits have already been merged to the target branch), `merge_request.source_sha` incorrectly returns `nil`.

This was without consequences before – but since !2217 was merged (a few days ago), it makes `Gitlab::Git::Commit.between` raise an "Unexpected nil argument" exception. This can be reproduced when visiting the http://localhost:3000/gitlab-org/gitlab-test/merge_requests/2 page on a fresh local Gitlab setup.

This MR fixes the crash, by making sure that `source_sha` returns a
correct result even when there is no diff available. I also added tests.

@DouweM I believe you wrote most of this code in the first place ; does this looks correct to you, or is there a better way to resolve this issue maybe?

See merge request !3135

CHANGELOG

@@ -39,6 +39,7 @@ v 8.6.0 (unreleased)
   - Fix bug where Bitbucket `closed` issues were imported as `opened` (Iuri de Silvio)
   - Don't show Issues/MRs from archived projects in Groups view
   - Fix wrong "iid of max iid" in Issuable sidebar for some merged MRs
+  - Fix empty source_sha on Merge Request when there is no diff (Pierre de La Morinerie)
   - 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

app/models/merge_request.rb

@@ -520,7 +520,11 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def source_sha
-    last_commit.try(:sha)
+    last_commit.try(:sha) || source_tip.try(:sha)
+  end
+
+  def source_tip
+    source_branch && source_project.repository.commit(source_branch)
   end
 
   def fetch_ref

spec/factories/merge_requests.rb

@@ -51,6 +51,11 @@ FactoryGirl.define do
     trait :with_diffs do
     end
 
+    trait :without_diffs do
+      source_branch "improve/awesome"
+      target_branch "master"
+    end
+
     trait :conflict do
       source_branch "feature_conflict"
       target_branch "feature"

spec/models/merge_request_spec.rb

@@ -86,6 +86,31 @@ describe MergeRequest, models: true do
     end
   end
 
+  describe '#source_sha' do
+    let(:last_branch_commit) { subject.source_project.repository.commit(subject.source_branch) }
+
+    context 'with diffs' do
+      subject { create(:merge_request, :with_diffs) }
+      it 'returns the sha of the source branch last commit' do
+        expect(subject.source_sha).to eq(last_branch_commit.sha)
+      end
+    end
+
+    context 'without diffs' do
+      subject { create(:merge_request, :without_diffs) }
+      it 'returns the sha of the source branch last commit' do
+        expect(subject.source_sha).to eq(last_branch_commit.sha)
+      end
+    end
+
+    context 'when the merge request is being created' do
+      subject { build(:merge_request, source_branch: nil, compare_commits: []) }
+      it 'returns nil' do
+        expect(subject.source_sha).to be_nil
+      end
+    end
+  end
+
   describe '#to_reference' do
     it 'returns a String reference to the object' do
       expect(subject.to_reference).to eq "!#{subject.iid}"