Add a safeguard in MergeRequest#compute_diverged_commits_count

We have to ensure source_sha and target_sha are not nil before calling
Gitlab::Git::Commit.between.

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

app/views/projects/merge_requests/_show.html.haml

@@ -34,7 +34,7 @@
         %span into
         = link_to namespace_project_commits_path(@project.namespace, @project, @merge_request.target_branch), class: "label-branch" do
           = @merge_request.target_branch
-        - if @merge_request.mergeable? && @merge_request.diverged_from_target_branch?
+        - if @merge_request.open? && @merge_request.diverged_from_target_branch?
           %span (#{pluralize(@merge_request.diverged_commits_count, 'commit')} behind)
     = render "projects/merge_requests/show/how_to_merge"

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) }